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" Thanks, Paul. -- > > 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
