lib/syscall: Clear return values when no stack

commit 854fbd6e5f60fe99e8e3a569865409fca378f143 upstream.

Commit:

  aa1f1a639621 ("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 <robert@swiecki.net>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: aa1f1a639621 ("lib/syscall: Pin the task stack in collect_syscall()")
Link: http://lkml.kernel.org/r/20170323224616.GA92694@beast
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/lib/syscall.c b/lib/syscall.c
index 63239e0..a72cd09 100644 (file)
--- a/lib/syscall.c
+++ b/lib/syscall.c
@@ -11,6 +11,7 @@ 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;
        }