seccomp: recheck the syscall after RET_TRACE

When RET_TRACE triggers, a tracer may change a syscall into something that
should be filtered by seccomp. This re-runs seccomp after a trace event
to make sure things continue to pass.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@kernel.org>

1 files changed, 18 insertions, 3 deletions

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 14a37d71b612..54d15eb2b701 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -556,7 +556,8 @@ void secure_computing_strict(int this_syscall)
 #else
 
 #ifdef CONFIG_SECCOMP_FILTER
-static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd)
+static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
+                            const bool recheck_after_trace)
 {
         u32 filter_ret, action;
         int data;
@@ -588,6 +589,10 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd)
                 goto skip;
 
         case SECCOMP_RET_TRACE:
+                /* We've been put in this state by the ptracer already. */
+                if (recheck_after_trace)
+                        return 0;
+
                 /* ENOSYS these calls if there is no tracer attached. */
                 if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP)) {
                         syscall_set_return_value(current,
@@ -611,6 +616,15 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd)
                 if (this_syscall < 0)
                         goto skip;
 
+                /*
+                 * Recheck the syscall, since it may have changed. This
+                 * intentionally uses a NULL struct seccomp_data to force
+                 * a reload of all registers. This does not goto skip since
+                 * a skip would have already been reported.
+                 */
+                if (__seccomp_filter(this_syscall, NULL, true))
+                        return -1;
+
                 return 0;
 
         case SECCOMP_RET_ALLOW:
@@ -629,7 +643,8 @@ skip:
         return -1;
 }
 #else
-static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd)
+static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
+                            const bool recheck_after_trace)
 {
         BUG();
 }
@@ -652,7 +667,7 @@ int __secure_computing(const struct seccomp_data *sd)
                 __secure_computing_strict(this_syscall);  /* may call do_exit */
                 return 0;
         case SECCOMP_MODE_FILTER:
-                return __seccomp_filter(this_syscall, sd);
+                return __seccomp_filter(this_syscall, sd, false);
         default:
                 BUG();
         }