linux/kernel/seccomp.c
Will Drewry 8156b451f3 seccomp: fix build warnings when there is no CONFIG_SECCOMP_FILTER
If both audit and seccomp filter support are disabled, 'ret' is marked
as unused.

If just seccomp filter support is disabled, data and skip are considered
unused.

This change fixes those build warnings.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Will Drewry <wad@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>
2012-04-18 12:24:52 +10:00

504 lines
14 KiB
C

/*
* 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/ptrace.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_ALLOW;
/* Ensure unexpected behavior doesn't result in failing open. */
if (WARN_ON(current->seccomp.filter == NULL))
return SECCOMP_RET_KILL;
/*
* All filters in the list are evaluated and the lowest BPF return
* value always takes priority (ignoring the DATA).
*/
for (f = current->seccomp.filter; f; f = f->prev) {
u32 cur_ret = sk_run_filter(NULL, f->insns);
if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
ret = cur_ret;
}
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);
}
}
/**
* seccomp_send_sigsys - signals the task to allow in-process syscall emulation
* @syscall: syscall number to send to userland
* @reason: filter-supplied reason code to send to userland (via si_errno)
*
* Forces a SIGSYS with a code of SYS_SECCOMP and related sigsys info.
*/
static void seccomp_send_sigsys(int syscall, int reason)
{
struct siginfo info;
memset(&info, 0, sizeof(info));
info.si_signo = SIGSYS;
info.si_code = SYS_SECCOMP;
info.si_call_addr = (void __user *)KSTK_EIP(current);
info.si_errno = reason;
info.si_arch = syscall_get_arch(current, task_pt_regs(current));
info.si_syscall = syscall;
force_sig_info(SIGSYS, &info, current);
}
#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
int __secure_computing(int this_syscall)
{
int mode = current->seccomp.mode;
int exit_sig = 0;
int *syscall;
u32 ret;
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 0;
} while (*++syscall);
exit_sig = SIGKILL;
ret = SECCOMP_RET_KILL;
break;
#ifdef CONFIG_SECCOMP_FILTER
case SECCOMP_MODE_FILTER: {
int data;
ret = seccomp_run_filters(this_syscall);
data = ret & SECCOMP_RET_DATA;
ret &= SECCOMP_RET_ACTION;
switch (ret) {
case SECCOMP_RET_ERRNO:
/* Set the low-order 16-bits as a errno. */
syscall_set_return_value(current, task_pt_regs(current),
-data, 0);
goto skip;
case SECCOMP_RET_TRAP:
/* Show the handler the original registers. */
syscall_rollback(current, task_pt_regs(current));
/* Let the filter pass back 16 bits of data. */
seccomp_send_sigsys(this_syscall, data);
goto skip;
case SECCOMP_RET_TRACE:
/* Skip these calls if there is no tracer. */
if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP))
goto skip;
/* Allow the BPF to provide the event message */
ptrace_event(PTRACE_EVENT_SECCOMP, data);
/*
* The delivery of a fatal signal during event
* notification may silently skip tracer notification.
* Terminating the task now avoids executing a system
* call that may not be intended.
*/
if (fatal_signal_pending(current))
break;
return 0;
case SECCOMP_RET_ALLOW:
return 0;
case SECCOMP_RET_KILL:
default:
break;
}
exit_sig = SIGSYS;
break;
}
#endif
default:
BUG();
}
#ifdef SECCOMP_DEBUG
dump_stack();
#endif
audit_seccomp(this_syscall, exit_sig, ret);
do_exit(exit_sig);
#ifdef CONFIG_SECCOMP_FILTER
skip:
audit_seccomp(this_syscall, exit_sig, ret);
#endif
return -1;
}
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;
}