Re: [PATCH v5 5/6] seccomp: Action to log before allowing

From: Kees Cook
Date: Thu Aug 03 2017 - 13:00:02 EST

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On Fri, Jul 28, 2017 at 1:55 PM, Tyler Hicks <tyhicks@xxxxxxxxxxxxx> wrote:
> Add a new action, SECCOMP_RET_LOG, that logs a syscall before allowing
> the syscall. At the implementation level, this action is identical to
> the existing SECCOMP_RET_ALLOW action. However, it can be very useful when
> initially developing a seccomp filter for an application. The developer
> can set the default action to be SECCOMP_RET_LOG, maybe mark any
> obviously needed syscalls with SECCOMP_RET_ALLOW, and then put the
> application through its paces. A list of syscalls that triggered the
> default action (SECCOMP_RET_LOG) can be easily gleaned from the logs and
> that list can be used to build the syscall whitelist. Finally, the
> developer can change the default action to the desired value.
>
> This provides a more friendly experience than seeing the application get
> killed, then updating the filter and rebuilding the app, seeing the
> application get killed due to a different syscall, then updating the
> filter and rebuilding the app, etc.
>
> The functionality is similar to what's supported by the various LSMs.
> SELinux has permissive mode, AppArmor has complain mode, SMACK has
> bring-up mode, etc.
>
> SECCOMP_RET_LOG is given a lower value than SECCOMP_RET_ALLOW as allow
> while logging is slightly more restrictive than quietly allowing.
>
> Unfortunately, the tests added for SECCOMP_RET_LOG are not capable of
> inspecting the audit log to verify that the syscall was logged.
>
> Signed-off-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
> ---
>
> * Change since v4:
> - folded the previously separate selftest patch into this patch
> - add TODO to verify that RET_LOG generates log messages in selftests
> - adjust for new reStructuredText format
>
> Documentation/userspace-api/seccomp_filter.rst | 6 ++
> include/uapi/linux/seccomp.h | 1 +
> kernel/seccomp.c | 17 ++++-
> tools/testing/selftests/seccomp/seccomp_bpf.c | 97 +++++++++++++++++++++++++-
> 4 files changed, 118 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
> index 2d1d8ab..2ece856 100644
> --- a/Documentation/userspace-api/seccomp_filter.rst
> +++ b/Documentation/userspace-api/seccomp_filter.rst
> @@ -141,6 +141,12 @@ In precedence order, they are:
> allow use of ptrace, even of other sandboxed processes, without
> extreme care; ptracers can use this mechanism to escape.)
>
> +``SECCOMP_RET_LOG``:
> + Results in the system call being executed after it is logged. This
> + should be used by application developers to learn which syscalls their
> + application needs without having to iterate through multiple test and
> + development cycles to build the list.

Perhaps this should note that it'll only be logged if the admin has
left the default sysctl value alone.

> +
> ``SECCOMP_RET_ALLOW``:
> Results in the system call being executed.
>
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 19a611d..f944332 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -31,6 +31,7 @@
> #define SECCOMP_RET_TRAP 0x00030000U /* disallow and force a SIGSYS */
> #define SECCOMP_RET_ERRNO 0x00050000U /* returns an errno */
> #define SECCOMP_RET_TRACE 0x7ff00000U /* pass to a tracer or disallow */
> +#define SECCOMP_RET_LOG 0x7ffc0000U /* allow after logging */
> #define SECCOMP_RET_ALLOW 0x7fff0000U /* allow */
>
> /* Masks for the return value sections. */
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 03ad3ba..c12d3dd 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -533,10 +533,12 @@ static void seccomp_send_sigsys(int syscall, int reason)
> #define SECCOMP_LOG_TRAP (1 << 2)
> #define SECCOMP_LOG_ERRNO (1 << 3)
> #define SECCOMP_LOG_TRACE (1 << 4)
> -#define SECCOMP_LOG_ALLOW (1 << 5)
> +#define SECCOMP_LOG_LOG (1 << 5)
> +#define SECCOMP_LOG_ALLOW (1 << 6)
>
> static u32 seccomp_actions_logged = SECCOMP_LOG_KILL | SECCOMP_LOG_TRAP |
> - SECCOMP_LOG_ERRNO | SECCOMP_LOG_TRACE;
> + SECCOMP_LOG_ERRNO | SECCOMP_LOG_TRACE |
> + SECCOMP_LOG_LOG;
>
> static inline void seccomp_log(unsigned long syscall, long signr, u32 action,
> bool requested)
> @@ -554,6 +556,9 @@ static inline void seccomp_log(unsigned long syscall, long signr, u32 action,
> case SECCOMP_RET_TRACE:
> log = seccomp_actions_logged & SECCOMP_LOG_TRACE;
> break;
> + case SECCOMP_RET_LOG:
> + log = seccomp_actions_logged & SECCOMP_LOG_LOG;
> + break;
> case SECCOMP_RET_ALLOW:
> log = false;
> break;
> @@ -701,6 +706,10 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>
> return 0;
>
> + case SECCOMP_RET_LOG:
> + seccomp_log(this_syscall, 0, action, true);
> + return 0;
> +
> case SECCOMP_RET_ALLOW:
> return 0;
>
> @@ -870,6 +879,7 @@ static long seccomp_get_action_avail(const char __user *uaction)
> case SECCOMP_RET_TRAP:
> case SECCOMP_RET_ERRNO:
> case SECCOMP_RET_TRACE:
> + case SECCOMP_RET_LOG:
> case SECCOMP_RET_ALLOW:
> break;
> default:
> @@ -1020,12 +1030,14 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
> #define SECCOMP_RET_TRAP_NAME "trap"
> #define SECCOMP_RET_ERRNO_NAME "errno"
> #define SECCOMP_RET_TRACE_NAME "trace"
> +#define SECCOMP_RET_LOG_NAME "log"
> #define SECCOMP_RET_ALLOW_NAME "allow"
>
> static const char seccomp_actions_avail[] = SECCOMP_RET_KILL_NAME " "
> SECCOMP_RET_TRAP_NAME " "
> SECCOMP_RET_ERRNO_NAME " "
> SECCOMP_RET_TRACE_NAME " "
> + SECCOMP_RET_LOG_NAME " "
> SECCOMP_RET_ALLOW_NAME;
>
> #define SECCOMP_ACTIONS_AVAIL_LEN strlen(seccomp_actions_avail)
> @@ -1040,6 +1052,7 @@ static const struct seccomp_log_name seccomp_log_names[] = {
> { SECCOMP_LOG_TRAP, SECCOMP_RET_TRAP_NAME },
> { SECCOMP_LOG_ERRNO, SECCOMP_RET_ERRNO_NAME },
> { SECCOMP_LOG_TRACE, SECCOMP_RET_TRACE_NAME },
> + { SECCOMP_LOG_LOG, SECCOMP_RET_LOG_NAME },
> { SECCOMP_LOG_ALLOW, SECCOMP_RET_ALLOW_NAME },
> { }
> };
> diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> index 8f0872b..040e875 100644
> --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> @@ -87,6 +87,10 @@ struct seccomp_data {
> };
> #endif
>
> +#ifndef SECCOMP_RET_LOG
> +#define SECCOMP_RET_LOG 0x7ffc0000U /* allow after logging */
> +#endif
> +
> #if __BYTE_ORDER == __LITTLE_ENDIAN
> #define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
> #elif __BYTE_ORDER == __BIG_ENDIAN
> @@ -342,6 +346,28 @@ TEST(empty_prog)
> EXPECT_EQ(EINVAL, errno);
> }
>
> +TEST(log_all)
> +{
> + struct sock_filter filter[] = {
> + BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_LOG),
> + };
> + struct sock_fprog prog = {
> + .len = (unsigned short)ARRAY_SIZE(filter),
> + .filter = filter,
> + };
> + long ret;
> + pid_t parent = getppid();
> +
> + ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
> + ASSERT_EQ(0, ret);
> +
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog);
> + ASSERT_EQ(0, ret);
> +
> + /* getppid() should succeed and be logged (no check for logging) */
> + EXPECT_EQ(parent, syscall(__NR_getppid));
> +}
> +
> TEST_SIGNAL(unknown_ret_is_kill_inside, SIGSYS)
> {
> struct sock_filter filter[] = {
> @@ -735,6 +761,7 @@ TEST_F(TRAP, handler)
>
> FIXTURE_DATA(precedence) {
> struct sock_fprog allow;
> + struct sock_fprog log;
> struct sock_fprog trace;
> struct sock_fprog error;
> struct sock_fprog trap;
> @@ -746,6 +773,13 @@ FIXTURE_SETUP(precedence)
> struct sock_filter allow_insns[] = {
> BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
> };
> + struct sock_filter log_insns[] = {
> + BPF_STMT(BPF_LD|BPF_W|BPF_ABS,
> + offsetof(struct seccomp_data, nr)),
> + BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_getpid, 1, 0),
> + BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
> + BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_LOG),
> + };
> struct sock_filter trace_insns[] = {
> BPF_STMT(BPF_LD|BPF_W|BPF_ABS,
> offsetof(struct seccomp_data, nr)),
> @@ -782,6 +816,7 @@ FIXTURE_SETUP(precedence)
> memcpy(self->_x.filter, &_x##_insns, sizeof(_x##_insns)); \
> self->_x.len = (unsigned short)ARRAY_SIZE(_x##_insns)
> FILTER_ALLOC(allow);
> + FILTER_ALLOC(log);
> FILTER_ALLOC(trace);
> FILTER_ALLOC(error);
> FILTER_ALLOC(trap);
> @@ -792,6 +827,7 @@ FIXTURE_TEARDOWN(precedence)
> {
> #define FILTER_FREE(_x) if (self->_x.filter) free(self->_x.filter)
> FILTER_FREE(allow);
> + FILTER_FREE(log);
> FILTER_FREE(trace);
> FILTER_FREE(error);
> FILTER_FREE(trap);
> @@ -809,6 +845,8 @@ TEST_F(precedence, allow_ok)
>
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
> ASSERT_EQ(0, ret);
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
> + ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
> ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
> @@ -833,6 +871,8 @@ TEST_F_SIGNAL(precedence, kill_is_highest, SIGSYS)
>
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
> ASSERT_EQ(0, ret);
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
> + ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
> ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
> @@ -864,6 +904,8 @@ TEST_F_SIGNAL(precedence, kill_is_highest_in_any_order, SIGSYS)
> ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
> ASSERT_EQ(0, ret);
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
> + ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
> ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trap);
> @@ -885,6 +927,8 @@ TEST_F_SIGNAL(precedence, trap_is_second, SIGSYS)
>
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
> ASSERT_EQ(0, ret);
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
> + ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
> ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
> @@ -910,6 +954,8 @@ TEST_F_SIGNAL(precedence, trap_is_second_in_any_order, SIGSYS)
> ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trap);
> ASSERT_EQ(0, ret);
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
> + ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
> ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
> @@ -931,6 +977,8 @@ TEST_F(precedence, errno_is_third)
>
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
> ASSERT_EQ(0, ret);
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
> + ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
> ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
> @@ -949,6 +997,8 @@ TEST_F(precedence, errno_is_third_in_any_order)
> ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
> ASSERT_EQ(0, ret);
>
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
> + ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
> ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
> @@ -971,6 +1021,8 @@ TEST_F(precedence, trace_is_fourth)
>
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
> ASSERT_EQ(0, ret);
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
> + ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
> ASSERT_EQ(0, ret);
> /* Should work just fine. */
> @@ -992,12 +1044,54 @@ TEST_F(precedence, trace_is_fourth_in_any_order)
> ASSERT_EQ(0, ret);
> ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
> ASSERT_EQ(0, ret);
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
> + ASSERT_EQ(0, ret);
> /* Should work just fine. */
> EXPECT_EQ(parent, syscall(__NR_getppid));
> /* No ptracer */
> EXPECT_EQ(-1, syscall(__NR_getpid));
> }
>
> +TEST_F(precedence, log_is_fifth)
> +{
> + pid_t mypid, parent;
> + long ret;
> +
> + mypid = getpid();
> + parent = getppid();
> + ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
> + ASSERT_EQ(0, ret);
> +
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
> + ASSERT_EQ(0, ret);
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
> + ASSERT_EQ(0, ret);
> + /* Should work just fine. */
> + EXPECT_EQ(parent, syscall(__NR_getppid));
> + /* Should also work just fine */
> + EXPECT_EQ(mypid, syscall(__NR_getpid));
> +}
> +
> +TEST_F(precedence, log_is_fifth_in_any_order)
> +{
> + pid_t mypid, parent;
> + long ret;
> +
> + mypid = getpid();
> + parent = getppid();
> + ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
> + ASSERT_EQ(0, ret);
> +
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
> + ASSERT_EQ(0, ret);
> + ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
> + ASSERT_EQ(0, ret);
> + /* Should work just fine. */
> + EXPECT_EQ(parent, syscall(__NR_getppid));
> + /* Should also work just fine */
> + EXPECT_EQ(mypid, syscall(__NR_getpid));
> +}
> +
> #ifndef PTRACE_O_TRACESECCOMP
> #define PTRACE_O_TRACESECCOMP 0x00000080
> #endif
> @@ -2494,7 +2588,7 @@ TEST(get_action_avail)
> {
> __u32 actions[] = { SECCOMP_RET_KILL, SECCOMP_RET_TRAP,
> SECCOMP_RET_ERRNO, SECCOMP_RET_TRACE,
> - SECCOMP_RET_ALLOW };
> + SECCOMP_RET_LOG, SECCOMP_RET_ALLOW };
> __u32 unknown_action = 0x10000000U;
> int i;
> long ret;
> @@ -2531,6 +2625,7 @@ TEST(get_action_avail)
> * - 64-bit arg prodding
> * - arch value testing (x86 modes especially)
> * - verify that FILTER_FLAG_LOG filters generate log messages
> + * - verify that RET_LOG generates log messages
> * - ...
> */
>
> --
> 2.7.4
>

Test updates look great, thanks!

-Kees

--
Kees Cook
Pixel Security

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