ipc: shm: fix information leak to userland

The shmid_ds structure is copied to userland with shm_unused{,2,3}
fields unitialized.  It leads to leaking of contents of kernel stack
memory.
Signed-off-by: NVasiliy Kulikov <segooon@gmail.com>
Acked-by: NAl Viro <viro@ZenIV.linux.org.uk>
Cc: stable@kernel.org
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

ipc/shm.c

@@ -479,6 +479,7 @@ static inline unsigned long copy_shmid_to_user(void __user *buf, struct shmid64_
 	    {
 		struct shmid_ds out;
 
+		memset(&out, 0, sizeof(out));
 		ipc64_perm_to_ipc_perm(&in->shm_perm, &out.shm_perm);
 		out.shm_segsz	= in->shm_segsz;
 		out.shm_atime	= in->shm_atime;