/*
* linux/kernel/seccomp.c
*
* Copyright 2004-2005 Andrea Arcangeli <andrea@cpushare.com>
*
* Copyright (C) 2012 Google, Inc.
* Will Drewry <wad@chromium.org>
*
* 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/atomic.h>
#include <linux/audit.h>
#include <linux/compat.h>
#include <linux/sched.h>
#include <linux/seccomp.h>
/* #define SECCOMP_DEBUG 1 */
#ifdef CONFIG_SECCOMP_FILTER
#include <asm/syscall.h>
#include <linux/filter.h>
#include <linux/security.h>
#include <linux/slab.h>
#include <linux/tracehook.h>
#include <linux/uaccess.h>
/**
* struct seccomp_filter - container for seccomp BPF programs
*
* @usage: reference count to manage the object lifetime.
* 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
* @len: the number of instructions in the program
* @insns: the BPF program instructions to evaluate
*
* 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;
unsigned short len; /* Instruction count */
struct sock_filter insns[];
};
/* Limit any path through the tree to 256KB worth of instructions. */
#define MAX_INSNS_PER_PATH ((1 << 18) / sizeof(struct sock_filter))
/**
* get_u32 - returns a u32 offset into data
* @data: a unsigned 64 bit value
* @index: 0 or 1 to return the first or second 32-bits
*
* This inline exists to hide the length of unsigned long. If a 32-bit
* unsigned long is passed in, it will be extended and the top 32-bits will be
* 0. If it is a 64-bit unsigned long, then whatever data is resident will be
* properly returned.
*
* Endianness is explicitly ignored and left for BPF program authors to manage
* as per the specific architecture.
*/
static inline u32 get_u32(u64 data, int index)
{
return ((u32 *)&data)[index];
}
/* Helper for bpf_load below. */
#define BPF_DATA(_name) offsetof(struct seccomp_data, _name)
/**
* bpf_load: checks and returns a pointer to the requested offset
* @off: offset into struct seccomp_data to load from
*
* Returns the requested 32-bits of data.
* seccomp_check_filter() should assure that @off is 32-bit aligned
* and not out of bounds. Failure to do so is a BUG.
*/
u32 seccomp_bpf_load(int off)
{
struct pt_regs *regs = task_pt_regs(current);
if (off == BPF_DATA(nr))
return syscall_get_nr(current, regs);
if (off == BPF_DATA(arch))
return syscall_get_arch(current, regs);
if (off >= BPF_DATA(args[0]) && off < BPF_DATA(args[6])) {
unsigned long value;
int arg = (off - BPF_DATA(args[0])) / sizeof(u64);
int index = !!(off % sizeof(u64));
syscall_get_arguments(current, regs, arg, 1, &value);
return get_u32(value, index);
}
if (off == BPF_DATA(instruction_pointer))
return get_u32(KSTK_EIP(current), 0);
if (off == BPF_DATA(instruction_pointer) + sizeof(u32))
return get_u32(KSTK_EIP(current), 1);
/* seccomp_check_filter should make this impossible. */
BUG();
}
/**
* seccomp_check_filter - verify seccomp filter code
* @filter: filter to verify
* @flen: length of filter
*
* Takes a previously checked filter (by sk_chk_filter) and
* redirects all filter code that loads struct sk_buff data
* and related data through seccomp_bpf_load. It also
* enforces length and alignment checking of those loads.
*
* Returns 0 if the rule set is legal or -EINVAL if not.
*/
static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
{
int pc;
for (pc = 0; pc < flen; pc++) {
struct sock_filter *ftest = &filter[pc];
u16 code = ftest->code;
u32 k = ftest->k;
switch (code) {
case BPF_S_LD_W_ABS:
ftest->code = BPF_S_ANC_SECCOMP_LD_W;
/* 32-bit aligned and not out of bounds. */
if (k >= sizeof(struct seccomp_data) || k & 3)
return -EINVAL;
continue;
case BPF_S_LD_W_LEN:
ftest->code = BPF_S_LD_IMM;
ftest->k = sizeof(struct seccomp_data);
continue;
case BPF_S_LDX_W_LEN:
ftest->code = BPF_S_LDX_IMM;
ftest->k = sizeof(struct seccomp_data);
continue;
/* Explicitly include allowed calls. */
case BPF_S_RET_K:
case BPF_S_RET_A:
case BPF_S_ALU_ADD_K:
case BPF_S_ALU_ADD_X:
case BPF_S_ALU_SUB_K:
case BPF_S_ALU_SUB_X:
case BPF_S_ALU_MUL_K:
case BPF_S_ALU_MUL_X:
case BPF_S_ALU_DIV_X:
case BPF_S_ALU_AND_K:
case BPF_S_ALU_AND_X:
case BPF_S_ALU_OR_K:
case BPF_S_ALU_OR_X:
case BPF_S_ALU_LSH_K:
case BPF_S_ALU_LSH_X:
case BPF_S_ALU_RSH_K:
case BPF_S_ALU_RSH_X:
case BPF_S_ALU_NEG:
case BPF_S_LD_IMM:
case BPF_S_LDX_IMM:
case BPF_S_MISC_TAX:
case BPF_S_MISC_TXA:
case BPF_S_ALU_DIV_K:
case BPF_S_LD_MEM:
case BPF_S_LDX_MEM:
case BPF_S_ST:
case BPF_S_STX:
case BPF_S_JMP_JA:
case BPF_S_JMP_JEQ_K:
case BPF_S_JMP_JEQ_X:
case BPF_S_JMP_JGE_K:
case BPF_S_JMP_JGE_X:
case BPF_S_JMP_JGT_K:
case BPF_S_JMP_JGT_X:
case BPF_S_JMP_JSET_K:
case BPF_S_JMP_JSET_X:
continue;
default:
return -EINVAL;
}
}
return 0;
}
/**
* seccomp_run_filters - evaluates all seccomp filters against @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;
u32 ret = SECCOMP_RET_KILL;
/*
* All filters in the list are evaluated and the lowest BPF return
* value always takes priority.
*/
for (f = current->seccomp.filter; f; f = f->prev) {
ret = sk_run_filter(NULL, f->insns);
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;
unsigned long fp_size = fprog->len * sizeof(struct sock_filter);
unsigned long total_insns = fprog->len;
long ret;
if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
return -EINVAL;
for (filter = current->seccomp.filter; filter; filter = filter->prev)
total_insns += filter->len + 4; /* include a 4 instr penalty */
if (total_insns > MAX_INSNS_PER_PATH)
return -ENOMEM;
/*
* 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.
*/
if (!current->no_new_privs &&
security_capable_noaudit(current_cred(), current_user_ns(),
CAP_SYS_ADMIN) != 0)
return -EACCES;
/* Allocate a new seccomp_filter */
filter = kzalloc(sizeof(struct seccomp_filter) + fp_size,
GFP_KERNEL|__GFP_NOWARN);
if (!filter)
return -ENOMEM;
atomic_set(&filter->usage, 1);
filter->len = fprog->len;
/* Copy the instructions from fprog. */
ret = -EFAULT;
if (copy_from_user(filter->insns, fprog->filter, fp_size))
goto fail;
/* Check and rewrite the fprog via the skb checker */
ret = sk_chk_filter(filter->insns, filter->len);
if (ret)
goto fail;
/* Check and rewrite the fprog for seccomp use */
ret = seccomp_check_filter(filter->insns, filter->len);
if (ret)
goto fail;
/*
* 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;
fail:
kfree(filter);
return ret;
}
/**
* seccomp_attach_user_filter - attaches a user-supplied sock_fprog
* @user_filter: pointer to the user data containing a sock_fprog.
*
* Returns 0 on success and non-zero otherwise.
*/
long seccomp_attach_user_filter(char __user *user_filter)
{
struct sock_fprog fprog;
long ret = -EFAULT;
#ifdef CONFIG_COMPAT
if (is_compat_task()) {
struct compat_sock_fprog fprog32;
if (copy_from_user(&fprog32, user_filter, sizeof(fprog32)))
goto out;
fprog.len = fprog32.len;
fprog.filter = compat_ptr(fprog32.filter);
} else /* falls through to the if below. */
#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 the filter on @tsk */
void get_seccomp_filter(struct task_struct *tsk)
{
struct seccomp_filter *orig = tsk->seccomp.filter;
if (!orig)
return;
/* Reference count is bounded by the number of total processes. */
atomic_inc(&orig->usage);
}
/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
void put_seccomp_filter(struct task_struct *tsk)
{
struct seccomp_filter *orig = tsk->seccomp.filter;
/* Clean up single-reference branches iteratively. */
while (orig && atomic_dec_and_test(&orig->usage)) {
struct seccomp_filter *freeme = orig;
orig = orig->prev;
kfree(freeme);
}
}
#endif /* CONFIG_SECCOMP_FILTER */
/*
* Secure computing mode 1 allows only read/write/exit/sigreturn.
* To be fully secure this must be combined with rlimit
* to limit the stack allocations too.
*/
static int mode1_syscalls[] = {
__NR_seccomp_read, __NR_seccomp_write, __NR_seccomp_exit, __NR_seccomp_sigreturn,
0, /* null terminated */
};
#ifdef CONFIG_COMPAT
static int mode1_syscalls_32[] = {
__NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, __NR_seccomp_sigreturn_32,
0, /* null terminated */
};
#endif
void __secure_computing(int this_syscall)
{
int mode = current->seccomp.mode;
int exit_sig = 0;
int *syscall;
switch (mode) {
case SECCOMP_MODE_STRICT:
syscall = mode1_syscalls;
#ifdef CONFIG_COMPAT
if (is_compat_task())
syscall = mode1_syscalls_32;
#endif
do {
if (*syscall == this_syscall)
return;
} while (*++syscall);
exit_sig = SIGKILL;
break;
#ifdef CONFIG_SECCOMP_FILTER
case SECCOMP_MODE_FILTER:
if (seccomp_run_filters(this_syscall) == SECCOMP_RET_ALLOW)
return;
exit_sig = SIGSYS;
break;
#endif
default:
BUG();
}
#ifdef SECCOMP_DEBUG
dump_stack();
#endif
audit_seccomp(this_syscall, exit_code, SECCOMP_RET_KILL);
do_exit(exit_sig);
}
long prctl_get_seccomp(void)
{
return current->seccomp.mode;
}
/**
* prctl_set_seccomp: configures current->seccomp.mode
* @seccomp_mode: requested mode to use
* @filter: optional struct sock_fprog for use with SECCOMP_MODE_FILTER
*
* This function may be called repeatedly with a @seccomp_mode of
* SECCOMP_MODE_FILTER to install additional filters. Every filter
* successfully installed will be evaluated (in reverse order) for each system
* call the task makes.
*
* Once current->seccomp.mode is non-zero, it may not be changed.
*
* Returns 0 on success or -EINVAL on failure.
*/
long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
{
long ret = -EINVAL;
if (current->seccomp.mode &&
current->seccomp.mode != seccomp_mode)
goto out;
switch (seccomp_mode) {
case SECCOMP_MODE_STRICT:
ret = 0;
#ifdef TIF_NOTSC
disable_TSC();
#endif
break;
#ifdef CONFIG_SECCOMP_FILTER
case SECCOMP_MODE_FILTER:
ret = seccomp_attach_user_filter(filter);
if (ret)
goto out;
break;
#endif
default:
goto out;
}
current->seccomp.mode = seccomp_mode;
set_thread_flag(TIF_SECCOMP);
out:
return ret;
}
