CHROMIUM: signal: always clear sa_restorer on execve

When the new signal handlers are set up, 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().

Based on what should be considered "secret" about addresses, it only
matters across the exec not the fork (since the VMAs haven't changed until
the exec).  But since exec sets SIG_DFL and keeps sa_restorer, this is
where it should be fixed.

Given the few uses of sa_restorer, a "set" function was not written since
this would be the only use.  Instead, we use __ARCH_HAS_SA_RESTORER, as
already done in other places.

Example of the leak before applying this patch:

$ 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
...

BUG=chromium:177956
TEST=link build, PoC fails to show leaks

[sent upstream https://lkml.org/lkml/2013/3/11/498]
[changed #ifdef for pre-3.9 defines]
Change-Id: Icb92cc8a616f326f8df783b749ba1ffad24d98ce
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/45130
Reviewed-by: Julien Tinnes <jln@chromium.org>

diff --git a/kernel/signal.c b/kernel/signal.c
index 5b8bbc6..d620679 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -482,6 +482,9 @@ 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 SA_RESTORER
+               ka->sa.sa_restorer = NULL;
+#endif
                sigemptyset(&ka->sa.sa_mask);
                ka++;
        }