[PATCH 4.9 020/212] rds: Prevent kernel-infoleak in rds_notify_queue_get()

[Greg Kroah-Hartman]

From: Peilin Ye <yepeilin.cs@xxxxxxxxx>

commit bbc8a99e952226c585ac17477a85ef1194501762 upstream.

rds_notify_queue_get() is potentially copying uninitialized kernel stack
memory to userspace since the compiler may leave a 4-byte hole at the end
of `cmsg`.

In 2016 we tried to fix this issue by doing `= { 0 };` on `cmsg`, which
unfortunately does not always initialize that 4-byte hole. Fix it by using
memset() instead.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: f037590fff30 ("rds: fix a leak of kernel memory")
Fixes: bdbe6fbc6a2f ("RDS: recv.c")
Suggested-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Peilin Ye <yepeilin.cs@xxxxxxxxx>
Acked-by: Santosh Shilimkar <santosh.shilimkar@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 net/rds/recv.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/net/rds/recv.c
+++ b/net/rds/recv.c
@@ -405,12 +405,13 @@ static int rds_still_queued(struct rds_s
 int rds_notify_queue_get(struct rds_sock *rs, struct msghdr *msghdr)
 {
 	struct rds_notifier *notifier;
-	struct rds_rdma_notify cmsg = { 0 }; /* fill holes with zero */
+	struct rds_rdma_notify cmsg;
 	unsigned int count = 0, max_messages = ~0U;
 	unsigned long flags;
 	LIST_HEAD(copy);
 	int err = 0;
 
+	memset(&cmsg, 0, sizeof(cmsg));	/* fill holes with zero */
 
 	/* put_cmsg copies to user space and thus may sleep. We can't do this
 	 * with rs_lock held, so first grab as many notifications as we can stuff