lib/syscall: Clear return values when no stack

Commit:

  aa1f1a6396 ("lib/syscall: Pin the task stack in collect_syscall()")

... added logic to handle a process stack not existing, but left sp and pc
uninitialized, which can be later reported via /proc/$pid/syscall for zombie
processes, potentially exposing kernel memory to userspace.

  Zombie /proc/$pid/syscall before:
  -1 0xffffffff9a060100 0xffff92f42d6ad900

  Zombie /proc/$pid/syscall after:
  -1 0x0 0x0

Reported-by: Robert Święcki <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected] # v4.9+
Fixes: aa1f1a6396 ("lib/syscall: Pin the task stack in collect_syscall()")
Link: http://lkml.kernel.org/r/20170323224616.GA92694@beast
Signed-off-by: Ingo Molnar <[email protected]>

lib/syscall.c

@@ -12,6 +12,7 @@ static int collect_syscall(struct task_struct *target, long *callno,
 
 	if (!try_get_task_stack(target)) {
 		/* Task has no stack, so the task isn't in a syscall. */
+		*sp = *pc = 0;
 		*callno = -1;
 		return 0;
 	}