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From: Kees Cook <>
To: Ingo Molnar <>
Cc: "Robert Święcki" <>,
	"Andy Lutomirski" <>,
Subject: [PATCH] lib/syscall: Clear return values when no stack
Date: Thu, 23 Mar 2017 15:46:16 -0700
Message-ID: <20170323224616.GA92694@beast> (raw)

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 <>
Fixes: aa1f1a639621 ("lib/syscall: Pin the task stack in collect_syscall()")
Cc: # v4.9+
Signed-off-by: Kees Cook <>
 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;

Kees Cook
Pixel Security

             reply index

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-23 22:46 Kees Cook [this message]
2017-03-23 23:03 ` Andy Lutomirski
2017-03-24  6:45 ` [tip:core/urgent] " tip-bot for Kees Cook

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