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>

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 14a37d7..54d15eb 100644 (file)
--- 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();
        }