[PATCH] signal: always clear sa_restorer on execve

[Kees Cook <keescook@chromium.org>]

When the new signal handlers are set up for a fork, the location of
sa_restorer is not cleared, leaking a parent process's address space
location to children. This allows for a potential bypass of the parent's
ASLR by examining the sa_restorer value returned when calling sigaction().

$ cat /proc/$$/maps
...
7fb9f3083000-7fb9f3238000 r-xp 00000000 fd:01 404469 .../libc-2.15.so
...
$ ./leak
...
7f278bc74000-7f278be29000 r-xp 00000000 fd:01 404469 .../libc-2.15.so
...
1 0 (nil) 0x7fb9f30b94a0
2 4000000 (nil) 0x7f278bcaa4a0
3 4000000 (nil) 0x7f278bcaa4a0
4 0 (nil) 0x7fb9f30b94a0
...

Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Emese Revfy <re.emese@gmail.com>
Cc: Emese Revfy <re.emese@gmail.com>
Cc: PaX Team <pageexec@freemail.hu>
Cc: stable@vger.kernel.org
---
 kernel/signal.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/signal.c b/kernel/signal.c
index 2ec870a..8c8e3ca 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -485,6 +485,9 @@ flush_signal_handlers(struct task_struct *t, int force_default)
 		if (force_default || ka->sa.sa_handler != SIG_IGN)
 			ka->sa.sa_handler = SIG_DFL;
 		ka->sa.sa_flags = 0;
+#ifdef __ARCH_HAS_SA_RESTORER
+		ka->sa.sa_restorer = NULL;
+#endif
 		sigemptyset(&ka->sa.sa_mask);
 		ka++;
 	}
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security