seccomp: Filter flag to log all actions except SECCOMP_RET_ALLOW

Add a new filter flag, SECCOMP_FILTER_FLAG_LOG, that enables logging for
all actions except for SECCOMP_RET_ALLOW for the given filter.

SECCOMP_RET_KILL actions are always logged, when "kill" is in the
actions_logged sysctl, and SECCOMP_RET_ALLOW actions are never logged,
regardless of this flag.

This flag can be used to create noisy filters that result in all
non-allowed actions to be logged. A process may have one noisy filter,
which is loaded with this flag, as well as a quiet filter that's not
loaded with this flag. This allows for the actions in a set of filters
to be selectively conveyed to the admin.

Since a system could have a large number of allocated seccomp_filter
structs, struct packing was taken in consideration. On 64 bit x86, the
new log member takes up one byte of an existing four byte hole in the
struct. On 32 bit x86, the new log member creates a new four byte hole
(unavoidable) and consumes one of those bytes.

Unfortunately, the tests added for SECCOMP_FILTER_FLAG_LOG are not
capable of inspecting the audit log to verify that the actions taken in
the filter were logged.

With this patch, the logic for deciding if an action will be logged is:

if action == RET_ALLOW:
  do not log
else if action == RET_KILL && RET_KILL in actions_logged:
  log
else if filter-requests-logging && action in actions_logged:
  log
else if audit_enabled && process-is-being-audited:
  log
else:
  do not log

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

1 files changed, 68 insertions, 1 deletions

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index abf708e09892..1c8c22ce7740 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -1739,6 +1739,10 @@ TEST_F_SIGNAL(TRACE_syscall, kill_after_ptrace, SIGSYS)
 #define SECCOMP_FILTER_FLAG_TSYNC 1
 #endif
 
+#ifndef SECCOMP_FILTER_FLAG_LOG
+#define SECCOMP_FILTER_FLAG_LOG 2
+#endif
+
 #ifndef seccomp
 int seccomp(unsigned int op, unsigned int flags, void *args)
 {
@@ -1844,7 +1848,8 @@ TEST(seccomp_syscall_mode_lock)
  */
 TEST(detect_seccomp_filter_flags)
 {
-        unsigned int flags[] = { SECCOMP_FILTER_FLAG_TSYNC };
+        unsigned int flags[] = { SECCOMP_FILTER_FLAG_TSYNC,
+                                 SECCOMP_FILTER_FLAG_LOG };
         unsigned int flag, all_flags;
         int i;
         long ret;
@@ -2533,6 +2538,67 @@ TEST(syscall_restart)
                 _metadata->passed = 0;
 }
 
+TEST_SIGNAL(filter_flag_log, SIGSYS)
+{
+        struct sock_filter allow_filter[] = {
+                BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
+        };
+        struct sock_filter kill_filter[] = {
+                BPF_STMT(BPF_LD|BPF_W|BPF_ABS,
+                        offsetof(struct seccomp_data, nr)),
+                BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_getpid, 0, 1),
+                BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_KILL),
+                BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
+        };
+        struct sock_fprog allow_prog = {
+                .len = (unsigned short)ARRAY_SIZE(allow_filter),
+                .filter = allow_filter,
+        };
+        struct sock_fprog kill_prog = {
+                .len = (unsigned short)ARRAY_SIZE(kill_filter),
+                .filter = kill_filter,
+        };
+        long ret;
+        pid_t parent = getppid();
+
+        ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+        ASSERT_EQ(0, ret);
+
+        /* Verify that the FILTER_FLAG_LOG flag isn't accepted in strict mode */
+        ret = seccomp(SECCOMP_SET_MODE_STRICT, SECCOMP_FILTER_FLAG_LOG,
+                      &allow_prog);
+        ASSERT_NE(ENOSYS, errno) {
+                TH_LOG("Kernel does not support seccomp syscall!");
+        }
+        EXPECT_NE(0, ret) {
+                TH_LOG("Kernel accepted FILTER_FLAG_LOG flag in strict mode!");
+        }
+        EXPECT_EQ(EINVAL, errno) {
+                TH_LOG("Kernel returned unexpected errno for FILTER_FLAG_LOG flag in strict mode!");
+        }
+
+        /* Verify that a simple, permissive filter can be added with no flags */
+        ret = seccomp(SECCOMP_SET_MODE_FILTER, 0, &allow_prog);
+        EXPECT_EQ(0, ret);
+
+        /* See if the same filter can be added with the FILTER_FLAG_LOG flag */
+        ret = seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_LOG,
+                      &allow_prog);
+        ASSERT_NE(EINVAL, errno) {
+                TH_LOG("Kernel does not support the FILTER_FLAG_LOG flag!");
+        }
+        EXPECT_EQ(0, ret);
+
+        /* Ensure that the kill filter works with the FILTER_FLAG_LOG flag */
+        ret = seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_LOG,
+                      &kill_prog);
+        EXPECT_EQ(0, ret);
+
+        EXPECT_EQ(parent, syscall(__NR_getppid));
+        /* getpid() should never return. */
+        EXPECT_EQ(0, syscall(__NR_getpid));
+}
+
 TEST(get_action_avail)
 {
         __u32 actions[] = { SECCOMP_RET_KILL,  SECCOMP_RET_TRAP,
@@ -2573,6 +2639,7 @@ TEST(get_action_avail)
  * - endianness checking when appropriate
  * - 64-bit arg prodding
  * - arch value testing (x86 modes especially)
+ * - verify that FILTER_FLAG_LOG filters generate log messages
  * - ...
  */