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seccomp: Operation for checking if an action is available

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Userspace code that needs to check if the kernel supports a given action
may not be able to use the /proc/sys/kernel/seccomp/actions_avail
sysctl. The process may be running in a sandbox and, therefore,
sufficient filesystem access may not be available. This patch adds an
operation to the seccomp(2) syscall that allows userspace code to ask
the kernel if a given action is available.

If the action is supported by the kernel, 0 is returned. If the action
is not supported by the kernel, -1 is returned with errno set to
-EOPNOTSUPP. If this check is attempted on a kernel that doesn't support
this new operation, -1 is returned with errno set to -EINVAL meaning
that userspace code will have the ability to differentiate between the
two error cases.

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Kees Cook <keescook@chromium.org>
This commit is contained in:
Tyler Hicks 2017-08-11 04:33:53 +00:00 committed by Kees Cook
parent 8e5f1ad116
commit d612b1fd80
3 changed files with 65 additions and 2 deletions
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5
include/uapi/linux/seccomp.h
View File

@ -11,8 +11,9 @@
#define SECCOMP_MODE_FILTER 2 /* uses user-supplied filter. */
/* Valid operations for seccomp syscall. */
#define SECCOMP_SET_MODE_STRICT 0
#define SECCOMP_SET_MODE_FILTER 1
#define SECCOMP_SET_MODE_STRICT 0
#define SECCOMP_SET_MODE_FILTER 1
#define SECCOMP_GET_ACTION_AVAIL 2
/* Valid flags for SECCOMP_SET_MODE_FILTER */
#define SECCOMP_FILTER_FLAG_TSYNC 1

26
kernel/seccomp.c
View File

@ -808,6 +808,27 @@ static inline long seccomp_set_mode_filter(unsigned int flags,
}
#endif
static long seccomp_get_action_avail(const char __user *uaction)
{
u32 action;
if (copy_from_user(&action, uaction, sizeof(action)))
return -EFAULT;
switch (action) {
case SECCOMP_RET_KILL:
case SECCOMP_RET_TRAP:
case SECCOMP_RET_ERRNO:
case SECCOMP_RET_TRACE:
case SECCOMP_RET_ALLOW:
break;
default:
return -EOPNOTSUPP;
}
return 0;
}
/* Common entry point for both prctl and syscall. */
static long do_seccomp(unsigned int op, unsigned int flags,
const char __user *uargs)
@ -819,6 +840,11 @@ static long do_seccomp(unsigned int op, unsigned int flags,
return seccomp_set_mode_strict();
case SECCOMP_SET_MODE_FILTER:
return seccomp_set_mode_filter(flags, uargs);
case SECCOMP_GET_ACTION_AVAIL:
if (flags != 0)
return -EINVAL;
return seccomp_get_action_avail(uargs);
default:
return -EINVAL;
}

36
tools/testing/selftests/seccomp/seccomp_bpf.c
View File

@ -1731,6 +1731,10 @@ TEST_F_SIGNAL(TRACE_syscall, kill_after_ptrace, SIGSYS)
#define SECCOMP_SET_MODE_FILTER 1
#endif
#ifndef SECCOMP_GET_ACTION_AVAIL
#define SECCOMP_GET_ACTION_AVAIL 2
#endif
#ifndef SECCOMP_FILTER_FLAG_TSYNC
#define SECCOMP_FILTER_FLAG_TSYNC 1
#endif
@ -2469,6 +2473,38 @@ TEST(syscall_restart)
_metadata->passed = 0;
}
TEST(get_action_avail)
{
__u32 actions[] = { SECCOMP_RET_KILL, SECCOMP_RET_TRAP,
SECCOMP_RET_ERRNO, SECCOMP_RET_TRACE,
SECCOMP_RET_ALLOW };
__u32 unknown_action = 0x10000000U;
int i;
long ret;
ret = seccomp(SECCOMP_GET_ACTION_AVAIL, 0, &actions[0]);
ASSERT_NE(ENOSYS, errno) {
TH_LOG("Kernel does not support seccomp syscall!");
}
ASSERT_NE(EINVAL, errno) {
TH_LOG("Kernel does not support SECCOMP_GET_ACTION_AVAIL operation!");
}
EXPECT_EQ(ret, 0);
for (i = 0; i < ARRAY_SIZE(actions); i++) {
ret = seccomp(SECCOMP_GET_ACTION_AVAIL, 0, &actions[i]);
EXPECT_EQ(ret, 0) {
TH_LOG("Expected action (0x%X) not available!",
actions[i]);
}
}
/* Check that an unknown action is handled properly (EOPNOTSUPP) */
ret = seccomp(SECCOMP_GET_ACTION_AVAIL, 0, &unknown_action);
EXPECT_EQ(ret, -1);
EXPECT_EQ(errno, EOPNOTSUPP);
}
/*
* TODO:
* - add microbenchmarks