On Tue, Apr 17, 2012 at 9:28 PM, Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx> wrote: > On Thu, Apr 12, 2012 at 5:48 PM, Will Drewry <wad@xxxxxxxxxxxx> wrote: >> Documents how system call filtering using Berkeley Packet >> Filter programs works and how it may be used. >> Includes an example for x86 and a semi-generic >> example using a macro-based code generator. > > Hi guys, that is quite the To/CC list. I didn't have the heart to add > yet another address (linux-next) to it... > > Can you have a look at this link? It appears to be breaking > the i386 allmodconfig builds in linux-next. > > http://kisskb.ellerman.id.au/kisskb/buildresult/6123842/ > > "samples/seccomp/bpf-direct.c:47:2: error: #error Unsupported platform" Ouch. It's checking the __i386__ and __x86_64__ macros. I'll reproduce it and see what needs to change. I suspect it'll be something dumb, but the worst case is that I drop that bit of fanciness. Thanks and sorry! will >> >> Acked-by: Eric Paris <eparis@xxxxxxxxxx> >> Signed-off-by: Will Drewry <wad@xxxxxxxxxxxx> >> >> v18: - added acked by >> - update no new privs numbers >> v17: - remove @compat note and add Pitfalls section for arch checking >> (keescook@xxxxxxxxxxxx) >> v16: - >> v15: - >> v14: - rebase/nochanges >> v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc >> v12: - comment on the ptrace_event use >> - update arch support comment >> - note the behavior of SECCOMP_RET_DATA when there are multiple filters >> (keescook@xxxxxxxxxxxx) >> - lots of samples/ clean up incl 64-bit bpf-direct support >> (markus@xxxxxxxxxxxx) >> - rebase to linux-next >> v11: - overhaul return value language, updates (keescook@xxxxxxxxxxxx) >> - comment on do_exit(SIGSYS) >> v10: - update for SIGSYS >> - update for new seccomp_data layout >> - update for ptrace option use >> v9: - updated bpf-direct.c for SIGILL >> v8: - add PR_SET_NO_NEW_PRIVS to the samples. >> v7: - updated for all the new stuff in v7: TRAP, TRACE >> - only talk about PR_SET_SECCOMP now >> - fixed bad JLE32 check (coreyb@xxxxxxxxxxxxxxxxxx) >> - adds dropper.c: a simple system call disabler >> v6: - tweak the language to note the requirement of >> PR_SET_NO_NEW_PRIVS being called prior to use. (luto@xxxxxxx) >> v5: - update sample to use system call arguments >> - adds a "fancy" example using a macro-based generator >> - cleaned up bpf in the sample >> - update docs to mention arguments >> - fix prctl value (eparis@xxxxxxxxxx) >> - language cleanup (rdunlap@xxxxxxxxxxxx) >> v4: - update for no_new_privs use >> - minor tweaks >> v3: - call out BPF <-> Berkeley Packet Filter (rdunlap@xxxxxxxxxxxx) >> - document use of tentative always-unprivileged >> - guard sample compilation for i386 and x86_64 >> v2: - move code to samples (corbet@xxxxxxx) >> --- >> Documentation/prctl/seccomp_filter.txt | 163 ++++++++++++++++++++++ >> samples/Makefile | 2 +- >> samples/seccomp/Makefile | 38 +++++ >> samples/seccomp/bpf-direct.c | 176 +++++++++++++++++++++++ >> samples/seccomp/bpf-fancy.c | 102 ++++++++++++++ >> samples/seccomp/bpf-helper.c | 89 ++++++++++++ >> samples/seccomp/bpf-helper.h | 238 ++++++++++++++++++++++++++++++++ >> samples/seccomp/dropper.c | 68 +++++++++ >> 8 files changed, 875 insertions(+), 1 deletions(-) >> create mode 100644 Documentation/prctl/seccomp_filter.txt >> create mode 100644 samples/seccomp/Makefile >> create mode 100644 samples/seccomp/bpf-direct.c >> create mode 100644 samples/seccomp/bpf-fancy.c >> create mode 100644 samples/seccomp/bpf-helper.c >> create mode 100644 samples/seccomp/bpf-helper.h >> create mode 100644 samples/seccomp/dropper.c >> >> diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt >> new file mode 100644 >> index 0000000..597c3c5 >> --- /dev/null >> +++ b/Documentation/prctl/seccomp_filter.txt >> @@ -0,0 +1,163 @@ >> + SECure COMPuting with filters >> + ============================= >> + >> +Introduction >> +------------ >> + >> +A large number of system calls are exposed to every userland process >> +with many of them going unused for the entire lifetime of the process. >> +As system calls change and mature, bugs are found and eradicated. A >> +certain subset of userland applications benefit by having a reduced set >> +of available system calls. The resulting set reduces the total kernel >> +surface exposed to the application. System call filtering is meant for >> +use with those applications. >> + >> +Seccomp filtering provides a means for a process to specify a filter for >> +incoming system calls. The filter is expressed as a Berkeley Packet >> +Filter (BPF) program, as with socket filters, except that the data >> +operated on is related to the system call being made: system call >> +number and the system call arguments. This allows for expressive >> +filtering of system calls using a filter program language with a long >> +history of being exposed to userland and a straightforward data set. >> + >> +Additionally, BPF makes it impossible for users of seccomp to fall prey >> +to time-of-check-time-of-use (TOCTOU) attacks that are common in system >> +call interposition frameworks. BPF programs may not dereference >> +pointers which constrains all filters to solely evaluating the system >> +call arguments directly. >> + >> +What it isn't >> +------------- >> + >> +System call filtering isn't a sandbox. It provides a clearly defined >> +mechanism for minimizing the exposed kernel surface. It is meant to be >> +a tool for sandbox developers to use. Beyond that, policy for logical >> +behavior and information flow should be managed with a combination of >> +other system hardening techniques and, potentially, an LSM of your >> +choosing. Expressive, dynamic filters provide further options down this >> +path (avoiding pathological sizes or selecting which of the multiplexed >> +system calls in socketcall() is allowed, for instance) which could be >> +construed, incorrectly, as a more complete sandboxing solution. >> + >> +Usage >> +----- >> + >> +An additional seccomp mode is added and is enabled using the same >> +prctl(2) call as the strict seccomp. If the architecture has >> +CONFIG_HAVE_ARCH_SECCOMP_FILTER, then filters may be added as below: >> + >> +PR_SET_SECCOMP: >> + Now takes an additional argument which specifies a new filter >> + using a BPF program. >> + The BPF program will be executed over struct seccomp_data >> + reflecting the system call number, arguments, and other >> + metadata. The BPF program must then return one of the >> + acceptable values to inform the kernel which action should be >> + taken. >> + >> + Usage: >> + prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog); >> + >> + The 'prog' argument is a pointer to a struct sock_fprog which >> + will contain the filter program. If the program is invalid, the >> + call will return -1 and set errno to EINVAL. >> + >> + If fork/clone and execve are allowed by @prog, any child >> + processes will be constrained to the same filters and system >> + call ABI as the parent. >> + >> + Prior to use, the task must call prctl(PR_SET_NO_NEW_PRIVS, 1) or >> + run with CAP_SYS_ADMIN privileges in its namespace. If these are not >> + true, -EACCES will be returned. This requirement ensures that filter >> + programs cannot be applied to child processes with greater privileges >> + than the task that installed them. >> + >> + Additionally, if prctl(2) is allowed by the attached filter, >> + additional filters may be layered on which will increase evaluation >> + time, but allow for further decreasing the attack surface during >> + execution of a process. >> + >> +The above call returns 0 on success and non-zero on error. >> + >> +Return values >> +------------- >> +A seccomp filter may return any of the following values. If multiple >> +filters exist, the return value for the evaluation of a given system >> +call will always use the highest precedent value. (For example, >> +SECCOMP_RET_KILL will always take precedence.) >> + >> +In precedence order, they are: >> + >> +SECCOMP_RET_KILL: >> + Results in the task exiting immediately without executing the >> + system call. The exit status of the task (status & 0x7f) will >> + be SIGSYS, not SIGKILL. >> + >> +SECCOMP_RET_TRAP: >> + Results in the kernel sending a SIGSYS signal to the triggering >> + task without executing the system call. The kernel will >> + rollback the register state to just before the system call >> + entry such that a signal handler in the task will be able to >> + inspect the ucontext_t->uc_mcontext registers and emulate >> + system call success or failure upon return from the signal >> + handler. >> + >> + The SECCOMP_RET_DATA portion of the return value will be passed >> + as si_errno. >> + >> + SIGSYS triggered by seccomp will have a si_code of SYS_SECCOMP. >> + >> +SECCOMP_RET_ERRNO: >> + Results in the lower 16-bits of the return value being passed >> + to userland as the errno without executing the system call. >> + >> +SECCOMP_RET_TRACE: >> + When returned, this value will cause the kernel to attempt to >> + notify a ptrace()-based tracer prior to executing the system >> + call. If there is no tracer present, -ENOSYS is returned to >> + userland and the system call is not executed. >> + >> + A tracer will be notified if it requests PTRACE_O_TRACESECCOMP >> + using ptrace(PTRACE_SETOPTIONS). The tracer will be notified >> + of a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of >> + the BPF program return value will be available to the tracer >> + via PTRACE_GETEVENTMSG. >> + >> +SECCOMP_RET_ALLOW: >> + Results in the system call being executed. >> + >> +If multiple filters exist, the return value for the evaluation of a >> +given system call will always use the highest precedent value. >> + >> +Precedence is only determined using the SECCOMP_RET_ACTION mask. When >> +multiple filters return values of the same precedence, only the >> +SECCOMP_RET_DATA from the most recently installed filter will be >> +returned. >> + >> +Pitfalls >> +-------- >> + >> +The biggest pitfall to avoid during use is filtering on system call >> +number without checking the architecture value. Why? On any >> +architecture that supports multiple system call invocation conventions, >> +the system call numbers may vary based on the specific invocation. If >> +the numbers in the different calling conventions overlap, then checks in >> +the filters may be abused. Always check the arch value! >> + >> +Example >> +------- >> + >> +The samples/seccomp/ directory contains both an x86-specific example >> +and a more generic example of a higher level macro interface for BPF >> +program generation. >> + >> + >> + >> +Adding architecture support >> +----------------------- >> + >> +See arch/Kconfig for the authoritative requirements. In general, if an >> +architecture supports both ptrace_event and seccomp, it will be able to >> +support seccomp filter with minor fixup: SIGSYS support and seccomp return >> +value checking. Then it must just add CONFIG_HAVE_ARCH_SECCOMP_FILTER >> +to its arch-specific Kconfig. >> diff --git a/samples/Makefile b/samples/Makefile >> index 2f75851..5ef08bb 100644 >> --- a/samples/Makefile >> +++ b/samples/Makefile >> @@ -1,4 +1,4 @@ >> # Makefile for Linux samples code >> >> obj-$(CONFIG_SAMPLES) += kobject/ kprobes/ tracepoints/ trace_events/ \ >> - hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ >> + hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/ >> diff --git a/samples/seccomp/Makefile b/samples/seccomp/Makefile >> new file mode 100644 >> index 0000000..e8fe0f5 >> --- /dev/null >> +++ b/samples/seccomp/Makefile >> @@ -0,0 +1,38 @@ >> +# kbuild trick to avoid linker error. Can be omitted if a module is built. >> +obj- := dummy.o >> + >> +hostprogs-$(CONFIG_SECCOMP) := bpf-fancy dropper >> +bpf-fancy-objs := bpf-fancy.o bpf-helper.o >> + >> +HOSTCFLAGS_bpf-fancy.o += -I$(objtree)/usr/include >> +HOSTCFLAGS_bpf-fancy.o += -idirafter $(objtree)/include >> +HOSTCFLAGS_bpf-helper.o += -I$(objtree)/usr/include >> +HOSTCFLAGS_bpf-helper.o += -idirafter $(objtree)/include >> + >> +HOSTCFLAGS_dropper.o += -I$(objtree)/usr/include >> +HOSTCFLAGS_dropper.o += -idirafter $(objtree)/include >> +dropper-objs := dropper.o >> + >> +# bpf-direct.c is x86-only. >> +ifeq ($(SRCARCH),x86) >> +# List of programs to build >> +hostprogs-$(CONFIG_SECCOMP) += bpf-direct >> +bpf-direct-objs := bpf-direct.o >> +endif >> + >> +HOSTCFLAGS_bpf-direct.o += -I$(objtree)/usr/include >> +HOSTCFLAGS_bpf-direct.o += -idirafter $(objtree)/include >> + >> +# Try to match the kernel target. >> +ifeq ($(CONFIG_64BIT),) >> +HOSTCFLAGS_bpf-direct.o += -m32 >> +HOSTCFLAGS_dropper.o += -m32 >> +HOSTCFLAGS_bpf-helper.o += -m32 >> +HOSTCFLAGS_bpf-fancy.o += -m32 >> +HOSTLOADLIBES_bpf-direct += -m32 >> +HOSTLOADLIBES_bpf-fancy += -m32 >> +HOSTLOADLIBES_dropper += -m32 >> +endif >> + >> +# Tell kbuild to always build the programs >> +always := $(hostprogs-y) >> diff --git a/samples/seccomp/bpf-direct.c b/samples/seccomp/bpf-direct.c >> new file mode 100644 >> index 0000000..26f523e >> --- /dev/null >> +++ b/samples/seccomp/bpf-direct.c >> @@ -0,0 +1,176 @@ >> +/* >> + * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros >> + * >> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@xxxxxxxxxxxx> >> + * Author: Will Drewry <wad@xxxxxxxxxxxx> >> + * >> + * The code may be used by anyone for any purpose, >> + * and can serve as a starting point for developing >> + * applications using prctl(PR_SET_SECCOMP, 2, ...). >> + */ >> +#define __USE_GNU 1 >> +#define _GNU_SOURCE 1 >> + >> +#include <linux/types.h> >> +#include <linux/filter.h> >> +#include <linux/seccomp.h> >> +#include <linux/unistd.h> >> +#include <signal.h> >> +#include <stdio.h> >> +#include <stddef.h> >> +#include <string.h> >> +#include <sys/prctl.h> >> +#include <unistd.h> >> + >> +#define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n])) >> +#define syscall_nr (offsetof(struct seccomp_data, nr)) >> + >> +#if defined(__i386__) >> +#define REG_RESULT REG_EAX >> +#define REG_SYSCALL REG_EAX >> +#define REG_ARG0 REG_EBX >> +#define REG_ARG1 REG_ECX >> +#define REG_ARG2 REG_EDX >> +#define REG_ARG3 REG_ESI >> +#define REG_ARG4 REG_EDI >> +#define REG_ARG5 REG_EBP >> +#elif defined(__x86_64__) >> +#define REG_RESULT REG_RAX >> +#define REG_SYSCALL REG_RAX >> +#define REG_ARG0 REG_RDI >> +#define REG_ARG1 REG_RSI >> +#define REG_ARG2 REG_RDX >> +#define REG_ARG3 REG_R10 >> +#define REG_ARG4 REG_R8 >> +#define REG_ARG5 REG_R9 >> +#else >> +#error Unsupported platform >> +#endif >> + >> +#ifndef PR_SET_NO_NEW_PRIVS >> +#define PR_SET_NO_NEW_PRIVS 38 >> +#endif >> + >> +#ifndef SYS_SECCOMP >> +#define SYS_SECCOMP 1 >> +#endif >> + >> +static void emulator(int nr, siginfo_t *info, void *void_context) >> +{ >> + ucontext_t *ctx = (ucontext_t *)(void_context); >> + int syscall; >> + char *buf; >> + ssize_t bytes; >> + size_t len; >> + if (info->si_code != SYS_SECCOMP) >> + return; >> + if (!ctx) >> + return; >> + syscall = ctx->uc_mcontext.gregs[REG_SYSCALL]; >> + buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1]; >> + len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2]; >> + >> + if (syscall != __NR_write) >> + return; >> + if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO) >> + return; >> + /* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */ >> + ctx->uc_mcontext.gregs[REG_RESULT] = -1; >> + if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) { >> + bytes = write(STDOUT_FILENO, buf, len); >> + ctx->uc_mcontext.gregs[REG_RESULT] = bytes; >> + } >> + return; >> +} >> + >> +static int install_emulator(void) >> +{ >> + struct sigaction act; >> + sigset_t mask; >> + memset(&act, 0, sizeof(act)); >> + sigemptyset(&mask); >> + sigaddset(&mask, SIGSYS); >> + >> + act.sa_sigaction = &emulator; >> + act.sa_flags = SA_SIGINFO; >> + if (sigaction(SIGSYS, &act, NULL) < 0) { >> + perror("sigaction"); >> + return -1; >> + } >> + if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) { >> + perror("sigprocmask"); >> + return -1; >> + } >> + return 0; >> +} >> + >> +static int install_filter(void) >> +{ >> + struct sock_filter filter[] = { >> + /* Grab the system call number */ >> + BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr), >> + /* Jump table for the allowed syscalls */ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1), >> + BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), >> +#ifdef __NR_sigreturn >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1), >> + BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), >> +#endif >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1), >> + BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1), >> + BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0), >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2), >> + >> + /* Check that read is only using stdin. */ >> + BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)), >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0), >> + BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL), >> + >> + /* Check that write is only using stdout */ >> + BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)), >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0), >> + /* Trap attempts to write to stderr */ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2), >> + >> + BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), >> + BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP), >> + BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL), >> + }; >> + struct sock_fprog prog = { >> + .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])), >> + .filter = filter, >> + }; >> + >> + if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) { >> + perror("prctl(NO_NEW_PRIVS)"); >> + return 1; >> + } >> + >> + >> + if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) { >> + perror("prctl"); >> + return 1; >> + } >> + return 0; >> +} >> + >> +#define payload(_c) (_c), sizeof((_c)) >> +int main(int argc, char **argv) >> +{ >> + char buf[4096]; >> + ssize_t bytes = 0; >> + if (install_emulator()) >> + return 1; >> + if (install_filter()) >> + return 1; >> + syscall(__NR_write, STDOUT_FILENO, >> + payload("OHAI! WHAT IS YOUR NAME? ")); >> + bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf)); >> + syscall(__NR_write, STDOUT_FILENO, payload("HELLO, ")); >> + syscall(__NR_write, STDOUT_FILENO, buf, bytes); >> + syscall(__NR_write, STDERR_FILENO, >> + payload("Error message going to STDERR\n")); >> + return 0; >> +} >> diff --git a/samples/seccomp/bpf-fancy.c b/samples/seccomp/bpf-fancy.c >> new file mode 100644 >> index 0000000..8eb483a >> --- /dev/null >> +++ b/samples/seccomp/bpf-fancy.c >> @@ -0,0 +1,102 @@ >> +/* >> + * Seccomp BPF example using a macro-based generator. >> + * >> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@xxxxxxxxxxxx> >> + * Author: Will Drewry <wad@xxxxxxxxxxxx> >> + * >> + * The code may be used by anyone for any purpose, >> + * and can serve as a starting point for developing >> + * applications using prctl(PR_ATTACH_SECCOMP_FILTER). >> + */ >> + >> +#include <linux/filter.h> >> +#include <linux/seccomp.h> >> +#include <linux/unistd.h> >> +#include <stdio.h> >> +#include <string.h> >> +#include <sys/prctl.h> >> +#include <unistd.h> >> + >> +#include "bpf-helper.h" >> + >> +#ifndef PR_SET_NO_NEW_PRIVS >> +#define PR_SET_NO_NEW_PRIVS 38 >> +#endif >> + >> +int main(int argc, char **argv) >> +{ >> + struct bpf_labels l; >> + static const char msg1[] = "Please type something: "; >> + static const char msg2[] = "You typed: "; >> + char buf[256]; >> + struct sock_filter filter[] = { >> + /* TODO: LOAD_SYSCALL_NR(arch) and enforce an arch */ >> + LOAD_SYSCALL_NR, >> + SYSCALL(__NR_exit, ALLOW), >> + SYSCALL(__NR_exit_group, ALLOW), >> + SYSCALL(__NR_write, JUMP(&l, write_fd)), >> + SYSCALL(__NR_read, JUMP(&l, read)), >> + DENY, /* Don't passthrough into a label */ >> + >> + LABEL(&l, read), >> + ARG(0), >> + JNE(STDIN_FILENO, DENY), >> + ARG(1), >> + JNE((unsigned long)buf, DENY), >> + ARG(2), >> + JGE(sizeof(buf), DENY), >> + ALLOW, >> + >> + LABEL(&l, write_fd), >> + ARG(0), >> + JEQ(STDOUT_FILENO, JUMP(&l, write_buf)), >> + JEQ(STDERR_FILENO, JUMP(&l, write_buf)), >> + DENY, >> + >> + LABEL(&l, write_buf), >> + ARG(1), >> + JEQ((unsigned long)msg1, JUMP(&l, msg1_len)), >> + JEQ((unsigned long)msg2, JUMP(&l, msg2_len)), >> + JEQ((unsigned long)buf, JUMP(&l, buf_len)), >> + DENY, >> + >> + LABEL(&l, msg1_len), >> + ARG(2), >> + JLT(sizeof(msg1), ALLOW), >> + DENY, >> + >> + LABEL(&l, msg2_len), >> + ARG(2), >> + JLT(sizeof(msg2), ALLOW), >> + DENY, >> + >> + LABEL(&l, buf_len), >> + ARG(2), >> + JLT(sizeof(buf), ALLOW), >> + DENY, >> + }; >> + struct sock_fprog prog = { >> + .filter = filter, >> + .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])), >> + }; >> + ssize_t bytes; >> + bpf_resolve_jumps(&l, filter, sizeof(filter)/sizeof(*filter)); >> + >> + if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) { >> + perror("prctl(NO_NEW_PRIVS)"); >> + return 1; >> + } >> + >> + if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) { >> + perror("prctl(SECCOMP)"); >> + return 1; >> + } >> + syscall(__NR_write, STDOUT_FILENO, msg1, strlen(msg1)); >> + bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf)-1); >> + bytes = (bytes > 0 ? bytes : 0); >> + syscall(__NR_write, STDERR_FILENO, msg2, strlen(msg2)); >> + syscall(__NR_write, STDERR_FILENO, buf, bytes); >> + /* Now get killed */ >> + syscall(__NR_write, STDERR_FILENO, msg2, strlen(msg2)+2); >> + return 0; >> +} >> diff --git a/samples/seccomp/bpf-helper.c b/samples/seccomp/bpf-helper.c >> new file mode 100644 >> index 0000000..579cfe3 >> --- /dev/null >> +++ b/samples/seccomp/bpf-helper.c >> @@ -0,0 +1,89 @@ >> +/* >> + * Seccomp BPF helper functions >> + * >> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@xxxxxxxxxxxx> >> + * Author: Will Drewry <wad@xxxxxxxxxxxx> >> + * >> + * The code may be used by anyone for any purpose, >> + * and can serve as a starting point for developing >> + * applications using prctl(PR_ATTACH_SECCOMP_FILTER). >> + */ >> + >> +#include <stdio.h> >> +#include <string.h> >> + >> +#include "bpf-helper.h" >> + >> +int bpf_resolve_jumps(struct bpf_labels *labels, >> + struct sock_filter *filter, size_t count) >> +{ >> + struct sock_filter *begin = filter; >> + __u8 insn = count - 1; >> + >> + if (count < 1) >> + return -1; >> + /* >> + * Walk it once, backwards, to build the label table and do fixups. >> + * Since backward jumps are disallowed by BPF, this is easy. >> + */ >> + filter += insn; >> + for (; filter >= begin; --insn, --filter) { >> + if (filter->code != (BPF_JMP+BPF_JA)) >> + continue; >> + switch ((filter->jt<<8)|filter->jf) { >> + case (JUMP_JT<<8)|JUMP_JF: >> + if (labels->labels[filter->k].location == 0xffffffff) { >> + fprintf(stderr, "Unresolved label: '%s'\n", >> + labels->labels[filter->k].label); >> + return 1; >> + } >> + filter->k = labels->labels[filter->k].location - >> + (insn + 1); >> + filter->jt = 0; >> + filter->jf = 0; >> + continue; >> + case (LABEL_JT<<8)|LABEL_JF: >> + if (labels->labels[filter->k].location != 0xffffffff) { >> + fprintf(stderr, "Duplicate label use: '%s'\n", >> + labels->labels[filter->k].label); >> + return 1; >> + } >> + labels->labels[filter->k].location = insn; >> + filter->k = 0; /* fall through */ >> + filter->jt = 0; >> + filter->jf = 0; >> + continue; >> + } >> + } >> + return 0; >> +} >> + >> +/* Simple lookup table for labels. */ >> +__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label) >> +{ >> + struct __bpf_label *begin = labels->labels, *end; >> + int id; >> + if (labels->count == 0) { >> + begin->label = label; >> + begin->location = 0xffffffff; >> + labels->count++; >> + return 0; >> + } >> + end = begin + labels->count; >> + for (id = 0; begin < end; ++begin, ++id) { >> + if (!strcmp(label, begin->label)) >> + return id; >> + } >> + begin->label = label; >> + begin->location = 0xffffffff; >> + labels->count++; >> + return id; >> +} >> + >> +void seccomp_bpf_print(struct sock_filter *filter, size_t count) >> +{ >> + struct sock_filter *end = filter + count; >> + for ( ; filter < end; ++filter) >> + printf("{ code=%u,jt=%u,jf=%u,k=%u },\n", >> + filter->code, filter->jt, filter->jf, filter->k); >> +} >> diff --git a/samples/seccomp/bpf-helper.h b/samples/seccomp/bpf-helper.h >> new file mode 100644 >> index 0000000..643279d >> --- /dev/null >> +++ b/samples/seccomp/bpf-helper.h >> @@ -0,0 +1,238 @@ >> +/* >> + * Example wrapper around BPF macros. >> + * >> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@xxxxxxxxxxxx> >> + * Author: Will Drewry <wad@xxxxxxxxxxxx> >> + * >> + * The code may be used by anyone for any purpose, >> + * and can serve as a starting point for developing >> + * applications using prctl(PR_SET_SECCOMP, 2, ...). >> + * >> + * No guarantees are provided with respect to the correctness >> + * or functionality of this code. >> + */ >> +#ifndef __BPF_HELPER_H__ >> +#define __BPF_HELPER_H__ >> + >> +#include <asm/bitsperlong.h> /* for __BITS_PER_LONG */ >> +#include <endian.h> >> +#include <linux/filter.h> >> +#include <linux/seccomp.h> /* for seccomp_data */ >> +#include <linux/types.h> >> +#include <linux/unistd.h> >> +#include <stddef.h> >> + >> +#define BPF_LABELS_MAX 256 >> +struct bpf_labels { >> + int count; >> + struct __bpf_label { >> + const char *label; >> + __u32 location; >> + } labels[BPF_LABELS_MAX]; >> +}; >> + >> +int bpf_resolve_jumps(struct bpf_labels *labels, >> + struct sock_filter *filter, size_t count); >> +__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label); >> +void seccomp_bpf_print(struct sock_filter *filter, size_t count); >> + >> +#define JUMP_JT 0xff >> +#define JUMP_JF 0xff >> +#define LABEL_JT 0xfe >> +#define LABEL_JF 0xfe >> + >> +#define ALLOW \ >> + BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW) >> +#define DENY \ >> + BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL) >> +#define JUMP(labels, label) \ >> + BPF_JUMP(BPF_JMP+BPF_JA, FIND_LABEL((labels), (label)), \ >> + JUMP_JT, JUMP_JF) >> +#define LABEL(labels, label) \ >> + BPF_JUMP(BPF_JMP+BPF_JA, FIND_LABEL((labels), (label)), \ >> + LABEL_JT, LABEL_JF) >> +#define SYSCALL(nr, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (nr), 0, 1), \ >> + jt >> + >> +/* Lame, but just an example */ >> +#define FIND_LABEL(labels, label) seccomp_bpf_label((labels), #label) >> + >> +#define EXPAND(...) __VA_ARGS__ >> +/* Map all width-sensitive operations */ >> +#if __BITS_PER_LONG == 32 >> + >> +#define JEQ(x, jt) JEQ32(x, EXPAND(jt)) >> +#define JNE(x, jt) JNE32(x, EXPAND(jt)) >> +#define JGT(x, jt) JGT32(x, EXPAND(jt)) >> +#define JLT(x, jt) JLT32(x, EXPAND(jt)) >> +#define JGE(x, jt) JGE32(x, EXPAND(jt)) >> +#define JLE(x, jt) JLE32(x, EXPAND(jt)) >> +#define JA(x, jt) JA32(x, EXPAND(jt)) >> +#define ARG(i) ARG_32(i) >> +#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) >> + >> +#elif __BITS_PER_LONG == 64 >> + >> +/* Ensure that we load the logically correct offset. */ >> +#if __BYTE_ORDER == __LITTLE_ENDIAN >> +#define ENDIAN(_lo, _hi) _lo, _hi >> +#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) >> +#define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32) >> +#elif __BYTE_ORDER == __BIG_ENDIAN >> +#define ENDIAN(_lo, _hi) _hi, _lo >> +#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32) >> +#define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) >> +#else >> +#error "Unknown endianness" >> +#endif >> + >> +union arg64 { >> + struct { >> + __u32 ENDIAN(lo32, hi32); >> + }; >> + __u64 u64; >> +}; >> + >> +#define JEQ(x, jt) \ >> + JEQ64(((union arg64){.u64 = (x)}).lo32, \ >> + ((union arg64){.u64 = (x)}).hi32, \ >> + EXPAND(jt)) >> +#define JGT(x, jt) \ >> + JGT64(((union arg64){.u64 = (x)}).lo32, \ >> + ((union arg64){.u64 = (x)}).hi32, \ >> + EXPAND(jt)) >> +#define JGE(x, jt) \ >> + JGE64(((union arg64){.u64 = (x)}).lo32, \ >> + ((union arg64){.u64 = (x)}).hi32, \ >> + EXPAND(jt)) >> +#define JNE(x, jt) \ >> + JNE64(((union arg64){.u64 = (x)}).lo32, \ >> + ((union arg64){.u64 = (x)}).hi32, \ >> + EXPAND(jt)) >> +#define JLT(x, jt) \ >> + JLT64(((union arg64){.u64 = (x)}).lo32, \ >> + ((union arg64){.u64 = (x)}).hi32, \ >> + EXPAND(jt)) >> +#define JLE(x, jt) \ >> + JLE64(((union arg64){.u64 = (x)}).lo32, \ >> + ((union arg64){.u64 = (x)}).hi32, \ >> + EXPAND(jt)) >> + >> +#define JA(x, jt) \ >> + JA64(((union arg64){.u64 = (x)}).lo32, \ >> + ((union arg64){.u64 = (x)}).hi32, \ >> + EXPAND(jt)) >> +#define ARG(i) ARG_64(i) >> + >> +#else >> +#error __BITS_PER_LONG value unusable. >> +#endif >> + >> +/* Loads the arg into A */ >> +#define ARG_32(idx) \ >> + BPF_STMT(BPF_LD+BPF_W+BPF_ABS, LO_ARG(idx)) >> + >> +/* Loads hi into A and lo in X */ >> +#define ARG_64(idx) \ >> + BPF_STMT(BPF_LD+BPF_W+BPF_ABS, LO_ARG(idx)), \ >> + BPF_STMT(BPF_ST, 0), /* lo -> M[0] */ \ >> + BPF_STMT(BPF_LD+BPF_W+BPF_ABS, HI_ARG(idx)), \ >> + BPF_STMT(BPF_ST, 1) /* hi -> M[1] */ >> + >> +#define JEQ32(value, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (value), 0, 1), \ >> + jt >> + >> +#define JNE32(value, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (value), 1, 0), \ >> + jt >> + >> +/* Checks the lo, then swaps to check the hi. A=lo,X=hi */ >> +#define JEQ64(lo, hi, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (lo), 0, 2), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \ >> + jt, \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */ >> + >> +#define JNE64(lo, hi, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 5, 0), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (lo), 2, 0), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \ >> + jt, \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */ >> + >> +#define JA32(value, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (value), 0, 1), \ >> + jt >> + >> +#define JA64(lo, hi, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (hi), 3, 0), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \ >> + BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (lo), 0, 2), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \ >> + jt, \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */ >> + >> +#define JGE32(value, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (value), 0, 1), \ >> + jt >> + >> +#define JLT32(value, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (value), 1, 0), \ >> + jt >> + >> +/* Shortcut checking if hi > arg.hi. */ >> +#define JGE64(lo, hi, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 4, 0), \ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \ >> + BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (lo), 0, 2), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \ >> + jt, \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */ >> + >> +#define JLT64(lo, hi, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (hi), 0, 4), \ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \ >> + BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 2, 0), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \ >> + jt, \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */ >> + >> +#define JGT32(value, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (value), 0, 1), \ >> + jt >> + >> +#define JLE32(value, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (value), 1, 0), \ >> + jt >> + >> +/* Check hi > args.hi first, then do the GE checking */ >> +#define JGT64(lo, hi, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 4, 0), \ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \ >> + BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 0, 2), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \ >> + jt, \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */ >> + >> +#define JLE64(lo, hi, jt) \ >> + BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 6, 0), \ >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 3), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \ >> + BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 2, 0), \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1), /* passed: swap hi back in */ \ >> + jt, \ >> + BPF_STMT(BPF_LD+BPF_MEM, 1) /* failed: swap hi back in */ >> + >> +#define LOAD_SYSCALL_NR \ >> + BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \ >> + offsetof(struct seccomp_data, nr)) >> + >> +#endif /* __BPF_HELPER_H__ */ >> diff --git a/samples/seccomp/dropper.c b/samples/seccomp/dropper.c >> new file mode 100644 >> index 0000000..c69c347 >> --- /dev/null >> +++ b/samples/seccomp/dropper.c >> @@ -0,0 +1,68 @@ >> +/* >> + * Naive system call dropper built on seccomp_filter. >> + * >> + * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@xxxxxxxxxxxx> >> + * Author: Will Drewry <wad@xxxxxxxxxxxx> >> + * >> + * The code may be used by anyone for any purpose, >> + * and can serve as a starting point for developing >> + * applications using prctl(PR_SET_SECCOMP, 2, ...). >> + * >> + * When run, returns the specified errno for the specified >> + * system call number against the given architecture. >> + * >> + * Run this one as root as PR_SET_NO_NEW_PRIVS is not called. >> + */ >> + >> +#include <errno.h> >> +#include <linux/audit.h> >> +#include <linux/filter.h> >> +#include <linux/seccomp.h> >> +#include <linux/unistd.h> >> +#include <stdio.h> >> +#include <stddef.h> >> +#include <stdlib.h> >> +#include <sys/prctl.h> >> +#include <unistd.h> >> + >> +static int install_filter(int nr, int arch, int error) >> +{ >> + struct sock_filter filter[] = { >> + BPF_STMT(BPF_LD+BPF_W+BPF_ABS, >> + (offsetof(struct seccomp_data, arch))), >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, arch, 0, 3), >> + BPF_STMT(BPF_LD+BPF_W+BPF_ABS, >> + (offsetof(struct seccomp_data, nr))), >> + BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1), >> + BPF_STMT(BPF_RET+BPF_K, >> + SECCOMP_RET_ERRNO|(error & SECCOMP_RET_DATA)), >> + BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), >> + }; >> + struct sock_fprog prog = { >> + .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])), >> + .filter = filter, >> + }; >> + if (prctl(PR_SET_SECCOMP, 2, &prog)) { >> + perror("prctl"); >> + return 1; >> + } >> + return 0; >> +} >> + >> +int main(int argc, char **argv) >> +{ >> + if (argc < 5) { >> + fprintf(stderr, "Usage:\n" >> + "dropper <syscall_nr> <arch> <errno> <prog> [<args>]\n" >> + "Hint: AUDIT_ARCH_I386: 0x%X\n" >> + " AUDIT_ARCH_X86_64: 0x%X\n" >> + "\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64); >> + return 1; >> + } >> + if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0), >> + strtol(argv[3], NULL, 0))) >> + return 1; >> + execv(argv[4], &argv[4]); >> + printf("Failed to execv\n"); >> + return 255; >> +} >> -- >> 1.7.5.4 >> >> -- >> To unsubscribe from this list: send the line "unsubscribe netdev" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
