seccomp: Make syscall skipping and nr changes more consistent

This fixes two issues that could cause incompatibility between
kernel versions:

 - If a tracer uses SECCOMP_RET_TRACE to select a syscall number
   higher than the largest known syscall, emulate the unknown
   vsyscall by returning -ENOSYS.  (This is unlikely to make a
   noticeable difference on x86-64 due to the way the system call
   entry works.)

 - On x86-64 with vsyscall=emulate, skipped vsyscalls were buggy.

This updates the documentation accordingly.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Will Drewry <wad@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>

1 files changed, 10 insertions, 3 deletions

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index ee376beedaf9..5af44b593770 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -396,25 +396,29 @@ int __secure_computing(int this_syscall)
 #ifdef CONFIG_SECCOMP_FILTER
 	case SECCOMP_MODE_FILTER: {
 		int data;
+		struct pt_regs *regs = task_pt_regs(current);
 		ret = seccomp_run_filters(this_syscall);
 		data = ret & SECCOMP_RET_DATA;
 		ret &= SECCOMP_RET_ACTION;
 		switch (ret) {
 		case SECCOMP_RET_ERRNO:
 			/* Set the low-order 16-bits as a errno. */
-			syscall_set_return_value(current, task_pt_regs(current),
+			syscall_set_return_value(current, regs,
 						 -data, 0);
 			goto skip;
 		case SECCOMP_RET_TRAP:
 			/* Show the handler the original registers. */
-			syscall_rollback(current, task_pt_regs(current));
+			syscall_rollback(current, regs);
 			/* Let the filter pass back 16 bits of data. */
 			seccomp_send_sigsys(this_syscall, data);
 			goto skip;
 		case SECCOMP_RET_TRACE:
 			/* Skip these calls if there is no tracer. */
-			if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP))
+			if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP)) {
+				syscall_set_return_value(current, regs,
+							 -ENOSYS, 0);
 				goto skip;
+			}
 			/* Allow the BPF to provide the event message */
 			ptrace_event(PTRACE_EVENT_SECCOMP, data);
 			/*
@@ -425,6 +429,9 @@ int __secure_computing(int this_syscall)
 			 */
 			if (fatal_signal_pending(current))
 				break;
+			if (syscall_get_nr(current, regs) < 0)
+				goto skip;  /* Explicit request to skip. */
+
 			return 0;
 		case SECCOMP_RET_ALLOW:
 			return 0;