linux-kernel - [PATCH] lib/syscall: Clear return values when no stack

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Message-ID: <20170323224616.GA92694@beast>
Date:   Thu, 23 Mar 2017 15:46:16 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Robert Święcki <robert@...ecki.net>,
        Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org
Subject: [PATCH] lib/syscall: Clear return values when no stack

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@...ecki.net>
Fixes: aa1f1a639621 ("lib/syscall: Pin the task stack in collect_syscall()")
Cc: stable@...r.kernel.org # v4.9+
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 lib/syscall.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/syscall.c b/lib/syscall.c
index 17d5ff5fa6a3..2c6cd1b5c3ea 100644
--- a/lib/syscall.c
+++ b/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;
 	}
-- 
2.7.4


-- 
Kees Cook
Pixel Security