[This patch depends on luto@xxxxxxx's no_new_privs patch: https://lkml.org/lkml/2012/1/30/264 ] This patch adds support for seccomp mode 2. Mode 2 introduces the ability for unprivileged processes to install system call filtering policy expressed in terms of a Berkeley Packet Filter (BPF) program. This program will be evaluated in the kernel for each system call the task makes and computes a result based on data in the format of struct seccomp_data. A filter program may be installed by calling: struct sock_fprog fprog = { ... }; ... prctl(PR_SET_SECCOMP, 2, &fprog); The return value of the filter program determines if the system call is allowed to proceed or denied. If the first filter program installed allows prctl(2) calls, then the above call may be made repeatedly by a task to further reduce its access to the kernel. All attached programs must be evaluated before a system call will be allowed to proceed. To avoid CONFIG_COMPAT related landmines, once a filter program is installed using specific is_compat_task() value, it is not allowed to make system calls using the alternate entry point. Filter programs will be inherited across fork/clone and execve. However, if the task attaching the filter is unprivileged (!CAP_SYS_ADMIN) the no_new_privs bit will be set on the task. This ensures that unprivileged tasks cannot attach filters that affect privileged tasks (e.g., setuid binary). There are a number of benefits to this approach. A few of which are as follows: - BPF has been exposed to userland for a long time - BPF optimization (and JIT'ing) are well understood - Userland already knows its ABI: system call numbers and desired arguments - No time-of-check-time-of-use vulnerable data accesses are possible. - system call arguments are loaded on access only to minimize copying required for system call policy decisions. Mode 2 support is restricted to architectures that enable HAVE_ARCH_SECCOMP_FILTER. In this patch, the primary dependency is on syscall_get_arguments(). The full desired scope of this feature will add a few minor additional requirements expressed later in this series. Based on discussion, SECCOMP_RET_ERRNO and SECCOMP_RET_TRACE seem to be the desired additional functionality. No architectures are enabled in this patch. v8: - use bpf_chk_filter, bpf_run_filter. update load_fns - Lots of fixes courtesy of indan@xxxxxx: -- fix up load behavior, compat fixups, and merge alloc code, -- renamed pc and dropped __packed, use bool compat. -- Added a hidden CONFIG_SECCOMP_FILTER to synthesize non-arch dependencies v7: (massive overhaul thanks to Indan, others) - added CONFIG_HAVE_ARCH_SECCOMP_FILTER - merged into seccomp.c - minimal seccomp_filter.h - no config option (part of seccomp) - no new prctl - doesn't break seccomp on systems without asm/syscall.h (works but arg access always fails) - dropped seccomp_init_task, extra free functions, ... - dropped the no-asm/syscall.h code paths - merges with network sk_run_filter and sk_chk_filter v6: - fix memory leak on attach compat check failure - require no_new_privs || CAP_SYS_ADMIN prior to filter installation. (luto@xxxxxxx) - s/seccomp_struct_/seccomp_/ for macros/functions (amwang@xxxxxxxxxx) - cleaned up Kconfig (amwang@xxxxxxxxxx) - on block, note if the call was compat (so the # means something) v5: - uses syscall_get_arguments (indan@xxxxxx,oleg@xxxxxxxxxx, mcgrathr@xxxxxxxxxxxx) - uses union-based arg storage with hi/lo struct to handle endianness. Compromises between the two alternate proposals to minimize extra arg shuffling and account for endianness assuming userspace uses offsetof(). (mcgrathr@xxxxxxxxxxxx, indan@xxxxxx) - update Kconfig description - add include/seccomp_filter.h and add its installation - (naive) on-demand syscall argument loading - drop seccomp_t (eparis@xxxxxxxxxx) v4: - adjusted prctl to make room for PR_[SG]ET_NO_NEW_PRIVS - now uses current->no_new_privs (luto@xxxxxxx,torvalds@xxxxxxxxxxxxxxxxxxxx) - assign names to seccomp modes (rdunlap@xxxxxxxxxxxx) - fix style issues (rdunlap@xxxxxxxxxxxx) - reworded Kconfig entry (rdunlap@xxxxxxxxxxxx) v3: - macros to inline (oleg@xxxxxxxxxx) - init_task behavior fixed (oleg@xxxxxxxxxx) - drop creator entry and extra NULL check (oleg@xxxxxxxxxx) - alloc returns -EINVAL on bad sizing (serge.hallyn@xxxxxxxxxxxxx) - adds tentative use of "always_unprivileged" as per torvalds@xxxxxxxxxxxxxxxxxxxx and luto@xxxxxxx v2: - (patch 2 only) Signed-off-by: Will Drewry <wad@xxxxxxxxxxxx> --- arch/Kconfig | 17 +++ include/linux/Kbuild | 1 + include/linux/seccomp.h | 69 ++++++++++- kernel/fork.c | 3 + kernel/seccomp.c | 327 ++++++++++++++++++++++++++++++++++++++++++++-- kernel/sys.c | 2 +- 6 files changed, 399 insertions(+), 20 deletions(-) diff --git a/arch/Kconfig b/arch/Kconfig index 4f55c73..c6ba1db 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -199,4 +199,21 @@ config HAVE_CMPXCHG_LOCAL config HAVE_CMPXCHG_DOUBLE bool +config HAVE_ARCH_SECCOMP_FILTER + bool + help + This symbol should be selected by an architecure if it provides + asm/syscall.h, specifically syscall_get_arguments(). + +config SECCOMP_FILTER + def_bool y + depends on HAVE_ARCH_SECCOMP_FILTER && SECCOMP && NET + help + Enable tasks to build secure computing environments defined + in terms of Berkeley Packet Filter programs which implement + task-defined system call filtering polices. + + See Documentation/prctl/seccomp_filter.txt for more + information on the topic of seccomp filtering. + source "kernel/gcov/Kconfig" diff --git a/include/linux/Kbuild b/include/linux/Kbuild index c94e717..d41ba12 100644 --- a/include/linux/Kbuild +++ b/include/linux/Kbuild @@ -330,6 +330,7 @@ header-y += scc.h header-y += sched.h header-y += screen_info.h header-y += sdla.h +header-y += seccomp.h header-y += securebits.h header-y += selinux_netlink.h header-y += sem.h diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h index d61f27f..2bee1f7 100644 --- a/include/linux/seccomp.h +++ b/include/linux/seccomp.h @@ -1,14 +1,60 @@ #ifndef _LINUX_SECCOMP_H #define _LINUX_SECCOMP_H +#include <linux/compiler.h> +#include <linux/types.h> + + +/* Valid values for seccomp.mode and prctl(PR_SET_SECCOMP, <mode>) */ +#define SECCOMP_MODE_DISABLED 0 /* seccomp is not in use. */ +#define SECCOMP_MODE_STRICT 1 /* uses hard-coded filter. */ +#define SECCOMP_MODE_FILTER 2 /* uses user-supplied filter. */ + +/* + * BPF programs may return a 32-bit value. + * The bottom 16-bits are reserved for future use. + * The upper 16-bits are ordered from least permissive values to most. + * + * The ordering ensures that a min_t() over composed return values always + * selects the least permissive choice. + */ +#define SECCOMP_RET_MASK 0xffff0000U +#define SECCOMP_RET_KILL 0x00000000U /* kill the task immediately */ +#define SECCOMP_RET_ALLOW 0x7fff0000U /* allow */ + +/* Format of the data the BPF program executes over. */ +struct seccomp_data { + int nr; + __u32 __reserved[3]; + struct { + __u32 lo; + __u32 hi; + } instruction_pointer; + __u32 lo32[6]; + __u32 hi32[6]; +}; +#ifdef __KERNEL__ #ifdef CONFIG_SECCOMP #include <linux/thread_info.h> #include <asm/seccomp.h> +struct seccomp_filter; +/** + * struct seccomp - the state of a seccomp'ed process + * + * @mode: indicates one of the valid values above for controlled + * system calls available to a process. + * @filter: The metadata and ruleset for determining what system calls + * are allowed for a task. + * + * @filter must only be accessed from the context of current as there + * is no locking. + */ struct seccomp { int mode; + struct seccomp_filter *filter; }; extern void __secure_computing(int); @@ -19,7 +65,7 @@ static inline void secure_computing(int this_syscall) } extern long prctl_get_seccomp(void); -extern long prctl_set_seccomp(unsigned long); +extern long prctl_set_seccomp(unsigned long, char __user *); static inline int seccomp_mode(struct seccomp *s) { @@ -31,15 +77,16 @@ static inline int seccomp_mode(struct seccomp *s) #include <linux/errno.h> struct seccomp { }; +struct seccomp_filter { }; -#define secure_computing(x) do { } while (0) +#define secure_computing(x) 0 static inline long prctl_get_seccomp(void) { return -EINVAL; } -static inline long prctl_set_seccomp(unsigned long arg2) +static inline long prctl_set_seccomp(unsigned long arg2, char __user *arg3) { return -EINVAL; } @@ -48,7 +95,21 @@ static inline int seccomp_mode(struct seccomp *s) { return 0; } - #endif /* CONFIG_SECCOMP */ +#ifdef CONFIG_SECCOMP_FILTER +extern void put_seccomp_filter(struct seccomp_filter *); +extern void copy_seccomp(struct seccomp *child, + const struct seccomp *parent); +#else /* CONFIG_SECCOMP_FILTER */ +/* The macro consumes the ->filter reference. */ +#define put_seccomp_filter(_s) do { } while (0) + +static inline void copy_seccomp(struct seccomp *child, + const struct seccomp *prev) +{ + return; +} +#endif /* CONFIG_SECCOMP_FILTER */ +#endif /* __KERNEL__ */ #endif /* _LINUX_SECCOMP_H */ diff --git a/kernel/fork.c b/kernel/fork.c index b77fd55..a5187b7 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -34,6 +34,7 @@ #include <linux/cgroup.h> #include <linux/security.h> #include <linux/hugetlb.h> +#include <linux/seccomp.h> #include <linux/swap.h> #include <linux/syscalls.h> #include <linux/jiffies.h> @@ -169,6 +170,7 @@ void free_task(struct task_struct *tsk) free_thread_info(tsk->stack); rt_mutex_debug_task_free(tsk); ftrace_graph_exit_task(tsk); + put_seccomp_filter(tsk->seccomp.filter); free_task_struct(tsk); } EXPORT_SYMBOL(free_task); @@ -1113,6 +1115,7 @@ static struct task_struct *copy_process(unsigned long clone_flags, goto fork_out; ftrace_graph_init_task(p); + copy_seccomp(&p->seccomp, ¤t->seccomp); rt_mutex_init_task(p); diff --git a/kernel/seccomp.c b/kernel/seccomp.c index e8d76c5..14d1869 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -3,16 +3,297 @@ * * Copyright 2004-2005 Andrea Arcangeli <andrea@xxxxxxxxxxxx> * - * This defines a simple but solid secure-computing mode. + * Copyright (C) 2012 Google, Inc. + * Will Drewry <wad@xxxxxxxxxxxx> + * + * This defines a simple but solid secure-computing facility. + * + * Mode 1 uses a fixed list of allowed system calls. + * Mode 2 allows user-defined system call filters in the form + * of Berkeley Packet Filters/Linux Socket Filters. */ #include <linux/audit.h> +#include <linux/filter.h> #include <linux/seccomp.h> #include <linux/sched.h> #include <linux/compat.h> +#include <linux/atomic.h> +#include <linux/security.h> + +#include <linux/slab.h> +#include <linux/uaccess.h> +#include <linux/user.h> + +#include <linux/tracehook.h> +#include <asm/syscall.h> + /* #define SECCOMP_DEBUG 1 */ -#define NR_SECCOMP_MODES 1 + +#ifdef CONFIG_SECCOMP_FILTER +/** + * struct seccomp_filter - container for seccomp BPF programs + * + * @usage: reference count to manage the object liftime. + * get/put helpers should be used when accessing an instance + * outside of a lifetime-guarded section. In general, this + * is only needed for handling filters shared across tasks. + * @prev: points to a previously installed, or inherited, filter + * @compat: indicates the value of is_compat_task() at creation time + * @insns: the BPF program instructions to evaluate + * @count: the number of instructions in the program + * + * seccomp_filter objects are organized in a tree linked via the @prev + * pointer. For any task, it appears to be a singly-linked list starting + * with current->seccomp.filter, the most recently attached or inherited filter. + * However, multiple filters may share a @prev node, by way of fork(), which + * results in a unidirectional tree existing in memory. This is similar to + * how namespaces work. + * + * seccomp_filter objects should never be modified after being attached + * to a task_struct (other than @usage). + */ +struct seccomp_filter { + atomic_t usage; + struct seccomp_filter *prev; + bool compat; + unsigned short count; /* Instruction count */ + struct sock_filter insns[]; +}; + +static void seccomp_filter_log_failure(int syscall) +{ + int compat = 0; +#ifdef CONFIG_COMPAT + compat = is_compat_task(); +#endif + pr_info("%s[%d]: %ssystem call %d blocked at 0x%lx\n", + current->comm, task_pid_nr(current), + (compat ? "compat " : ""), + syscall, KSTK_EIP(current)); +} + +static inline u32 get_high_bits(unsigned long value) +{ + int bits = 32; + return value >> bits; +} + +static inline u32 bpf_length(const void *data) +{ + return sizeof(struct seccomp_data); +} + +/** + * bpf_pointer: checks and returns a pointer to the requested offset + * @nr: int syscall passed as a void * to bpf_run_filter + * @off: index to load a from in @data + * @size: load width requested + * @buffer: temporary storage supplied by bpf_run_filter + * + * Returns a pointer to @buffer where the value was stored. + * On failure, returns NULL. + */ +static void *bpf_pointer(const void *nr, int off, unsigned int size, void *buf) +{ + unsigned long value; + u32 *A = (u32 *)buf; + + if (size != sizeof(u32)) + return NULL; + +#define BPF_DATA(_name) offsetof(struct seccomp_data, _name) + /* Index by entry instead of by byte. */ + if (off == BPF_DATA(nr)) { + *A = (u32)(uintptr_t)nr; + } else if (off == BPF_DATA(instruction_pointer.lo)) { + *A = KSTK_EIP(current); + } else if (off == BPF_DATA(instruction_pointer.hi)) { + *A = get_high_bits(KSTK_EIP(current)); + } else if (off >= BPF_DATA(lo32[0]) && off <= BPF_DATA(lo32[5])) { + struct pt_regs *regs = task_pt_regs(current); + int arg = (off - BPF_DATA(lo32[0])) >> 2; + syscall_get_arguments(current, regs, arg, 1, &value); + *A = value; + } else if (off >= BPF_DATA(hi32[0]) && off <= BPF_DATA(hi32[5])) { + struct pt_regs *regs = task_pt_regs(current); + int arg = (off - BPF_DATA(hi32[0])) >> 2; + syscall_get_arguments(current, regs, arg, 1, &value); + *A = get_high_bits(value); + } else { + return NULL; + } +#undef BPF_DATA + return buf; +} + +/** + * seccomp_run_filters - run 'current' against the given syscall + * @syscall: number of the current system call + * + * Returns valid seccomp BPF response codes. + */ +static u32 seccomp_run_filters(int syscall) +{ + struct seccomp_filter *f; + const struct bpf_load_fns loaders = { bpf_pointer, bpf_length }; + u32 ret = SECCOMP_RET_KILL; + const void *sc_ptr = (const void *)(uintptr_t)syscall; + + /* It's not possible for the filter to be NULL here. */ +#ifdef CONFIG_COMPAT + if (current->seccomp.filter->compat != !!(is_compat_task())) + return ret; +#endif + + /* + * All filters are evaluated in order of youngest to oldest. The lowest + * BPF return value always takes priority. + */ + for (f = current->seccomp.filter; f; f = f->prev) { + ret = bpf_run_filter(sc_ptr, f->insns, &loaders); + if (ret != SECCOMP_RET_ALLOW) + break; + } + return ret; +} + +/** + * seccomp_attach_filter: Attaches a seccomp filter to current. + * @fprog: BPF program to install + * + * Returns 0 on success or an errno on failure. + */ +static long seccomp_attach_filter(struct sock_fprog *fprog) +{ + struct seccomp_filter *filter = NULL; + unsigned long fp_size = fprog->len * sizeof(struct sock_filter); + long ret = -EINVAL; + + if (fprog->len == 0 || fprog->len > BPF_MAXINSNS) + goto out; + + /* Allocate a new seccomp_filter */ + ret = -ENOMEM; + filter = kzalloc(sizeof(struct seccomp_filter) + fp_size, GFP_KERNEL); + if (!filter) + goto out; + atomic_set(&filter->usage, 1); + filter->count = fprog->len; + + /* Copy the instructions from fprog. */ + ret = -EFAULT; + if (copy_from_user(filter->insns, fprog->filter, fp_size)) + goto out; + + /* Check the fprog */ + ret = bpf_chk_filter(filter->insns, filter->count, BPF_CHK_FLAGS_NO_SKB); + if (ret) + goto out; + + /* + * Installing a seccomp filter requires that the task + * have CAP_SYS_ADMIN in its namespace or be running with + * no_new_privs. This avoids scenarios where unprivileged + * tasks can affect the behavior of privileged children. + */ + ret = -EACCES; + if (!current->no_new_privs && + security_capable_noaudit(current_cred(), current_user_ns(), + CAP_SYS_ADMIN) != 0) + goto out; + + /* Lock the filter to the current calling convention. */ +#ifdef CONFIG_COMPAT + filter->compat = !!(is_compat_task()); +#endif + + /* + * If there is an existing filter, make it the prev + * and don't drop its task reference. + */ + filter->prev = current->seccomp.filter; + current->seccomp.filter = filter; + return 0; +out: + put_seccomp_filter(filter); /* for get or task, on err */ + return ret; +} + +/** + * seccomp_attach_user_filter - attaches a user-supplied sock_fprog + * @user_filter: pointer to the user data containing a sock_fprog. + * + * This function may be called repeatedly to install additional filters. + * Every filter successfully installed will be evaluated (in reverse order) + * for each system call the task makes. + * + * Returns 0 on success and non-zero otherwise. + */ +long seccomp_attach_user_filter(char __user *user_filter) +{ + struct sock_fprog fprog; + long ret = -EFAULT; + + if (!user_filter) + goto out; +#ifdef CONFIG_COMPAT + if (is_compat_task()) { + /* XXX: Share with net/compat.c */ + struct { + u16 len; + compat_uptr_t filter; /* struct sock_filter */ + } fprog32; + if (copy_from_user(&fprog32, user_filter, sizeof(fprog32))) + goto out; + fprog.len = fprog32.len; + fprog.filter = compat_ptr(fprog32.filter); + } else +#endif + if (copy_from_user(&fprog, user_filter, sizeof(fprog))) + goto out; + ret = seccomp_attach_filter(&fprog); +out: + return ret; +} + +/* get_seccomp_filter - increments the reference count of @orig. */ +static struct seccomp_filter *get_seccomp_filter(struct seccomp_filter *orig) +{ + if (!orig) + return NULL; + /* Reference count is bounded by the number of total processes. */ + atomic_inc(&orig->usage); + return orig; +} + +/* put_seccomp_filter - decrements the ref count of @orig and may free. */ +void put_seccomp_filter(struct seccomp_filter *orig) +{ + /* Clean up single-reference branches iteratively. */ + while (orig && atomic_dec_and_test(&orig->usage)) { + struct seccomp_filter *freeme = orig; + orig = orig->prev; + kfree(freeme); + } +} + +/** + * copy_seccomp: manages inheritance on fork + * @child: forkee's seccomp + * @prev: forker's seccomp + * + * Ensures that @child inherits seccomp mode and state if + * seccomp filtering is in use. + */ +void copy_seccomp(struct seccomp *child, + const struct seccomp *prev) +{ + child->mode = prev->mode; + child->filter = get_seccomp_filter(prev->filter); +} +#endif /* CONFIG_SECCOMP_FILTER */ /* * Secure computing mode 1 allows only read/write/exit/sigreturn. @@ -34,10 +315,10 @@ static int mode1_syscalls_32[] = { void __secure_computing(int this_syscall) { int mode = current->seccomp.mode; - int * syscall; + int *syscall; switch (mode) { - case 1: + case SECCOMP_MODE_STRICT: syscall = mode1_syscalls; #ifdef CONFIG_COMPAT if (is_compat_task()) @@ -48,6 +329,13 @@ void __secure_computing(int this_syscall) return; } while (*++syscall); break; +#ifdef CONFIG_SECCOMP_FILTER + case SECCOMP_MODE_FILTER: + if (seccomp_run_filters(this_syscall) == SECCOMP_RET_ALLOW) + return; + seccomp_filter_log_failure(this_syscall); + break; +#endif default: BUG(); } @@ -64,25 +352,34 @@ long prctl_get_seccomp(void) return current->seccomp.mode; } -long prctl_set_seccomp(unsigned long seccomp_mode) +long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter) { - long ret; + long ret = -EINVAL; - /* can set it only once to be even more secure */ - ret = -EPERM; - if (unlikely(current->seccomp.mode)) + if (current->seccomp.mode && + current->seccomp.mode != seccomp_mode) goto out; - ret = -EINVAL; - if (seccomp_mode && seccomp_mode <= NR_SECCOMP_MODES) { - current->seccomp.mode = seccomp_mode; - set_thread_flag(TIF_SECCOMP); + switch (seccomp_mode) { + case SECCOMP_MODE_STRICT: + ret = 0; #ifdef TIF_NOTSC disable_TSC(); #endif - ret = 0; + break; +#ifdef CONFIG_SECCOMP_FILTER + case SECCOMP_MODE_FILTER: + ret = seccomp_attach_user_filter(filter); + if (ret) + goto out; + break; +#endif + default: + goto out; } - out: + current->seccomp.mode = seccomp_mode; + set_thread_flag(TIF_SECCOMP); +out: return ret; } diff --git a/kernel/sys.c b/kernel/sys.c index 4070153..905031e 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -1899,7 +1899,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, error = prctl_get_seccomp(); break; case PR_SET_SECCOMP: - error = prctl_set_seccomp(arg2); + error = prctl_set_seccomp(arg2, (char __user *)arg3); break; case PR_GET_TSC: error = GET_TSC_CTL(arg2); -- 1.7.5.4 -- 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
