[PATCH] compat: Fix RT signal mask corruption via sigprocmask

[Jan Kiszka <jan.kiszka@siemens.com>]

compat_sys_sigprocmask reads a smaller signal mask from userspace than
sigprogmask accepts for setting. So the high word of blocked.sig[0] will
be cleared, releasing any potentially blocked RT signal.

This was discovered via userspace code that relies on get/setcontext.
glibc's i386 versions of those functions use sigprogmask instead of
rt_sigprogmask to save/restore the signal mask and caused RT signal
unblocking this way.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---
 kernel/compat.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/kernel/compat.c b/kernel/compat.c
index 74ff849..03e491d 100644
--- a/kernel/compat.c
+++ b/kernel/compat.c
@@ -381,6 +381,8 @@ asmlinkage long compat_sys_sigprocmask(int how, compat_old_sigset_t __user *set,
 
 	if (set && get_user(s, set))
 		return -EFAULT;
+	s |= current->blocked.sig[0] &
+		~((old_sigset_t)(compat_old_sigset_t)-1);
 	old_fs = get_fs();
 	set_fs(KERNEL_DS);
 	ret = sys_sigprocmask(how,
-- 
1.7.3.4