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Commit d612b1fd authored by Tyler Hicks's avatar Tyler Hicks Committed by Kees Cook
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seccomp: Operation for checking if an action is available 



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: default avatarTyler Hicks <tyhicks@canonical.com>
Suggested-by: default avatarAndy Lutomirski <luto@amacapital.net>
Signed-off-by: default avatarKees Cook <keescook@chromium.org>
parent 8e5f1ad1

include/uapi/linux/seccomp.h

+3 −2
Original line number Diff line number Diff line
@@ -13,6 +13,7 @@ 
/* Valid operations for seccomp syscall. */

#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

kernel/seccomp.c

+26 −0
Original line number Diff line number Diff line
@@ -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;

	}

tools/testing/selftests/seccomp/seccomp_bpf.c

+36 −0
Original line number Diff line number Diff line
@@ -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