seccomp: Provide matching filter for introspection

Both the upcoming logging improvements and changes to RET_KILL will need
to know which filter a given seccomp return value originated from. In
order to delay logic processing of result until after the seccomp loop,
this adds a single pointer assignment on matches. This will allow both
log and RET_KILL logic to work off the filter rather than doing more
expensive tests inside the time-critical run_filters loop.

Running tight cycles of getpid() with filters attached shows no measurable
difference in speed.

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

1 files changed, 15 insertions, 3 deletions

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 98b59b5db90b..1f3347fc2605 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -171,10 +171,14 @@ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
 /**
  * seccomp_run_filters - evaluates all seccomp filters against @sd
  * @sd: optional seccomp data to be passed to filters
+ * @match: stores struct seccomp_filter that resulted in the return value,
+ *         unless filter returned SECCOMP_RET_ALLOW, in which case it will
+ *         be unchanged.
  *
  * Returns valid seccomp BPF response codes.
  */
-static u32 seccomp_run_filters(const struct seccomp_data *sd)
+static u32 seccomp_run_filters(const struct seccomp_data *sd,
+                               struct seccomp_filter **match)
 {
         struct seccomp_data sd_local;
         u32 ret = SECCOMP_RET_ALLOW;
@@ -198,8 +202,10 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd)
         for (; f; f = f->prev) {
                 u32 cur_ret = BPF_PROG_RUN(f->prog, sd);
 
-                if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
+                if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION)) {
                         ret = cur_ret;
+                        *match = f;
+                }
         }
         return ret;
 }
@@ -566,6 +572,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
                             const bool recheck_after_trace)
 {
         u32 filter_ret, action;
+        struct seccomp_filter *match = NULL;
         int data;
 
         /*
@@ -574,7 +581,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
          */
         rmb();
 
-        filter_ret = seccomp_run_filters(sd);
+        filter_ret = seccomp_run_filters(sd, &match);
         data = filter_ret & SECCOMP_RET_DATA;
         action = filter_ret & SECCOMP_RET_ACTION;
 
@@ -638,6 +645,11 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
                 return 0;
 
         case SECCOMP_RET_ALLOW:
+                /*
+                 * Note that the "match" filter will always be NULL for
+                 * this action since SECCOMP_RET_ALLOW is the starting
+                 * state in seccomp_run_filters().
+                 */
                 return 0;
 
         case SECCOMP_RET_KILL: