On Thu, Jul 30, 2020 at 03:20:26PM -0400, Peilin Ye wrote:
> 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.

Of course, this is the difference between "{ 0 }" and "{}" initializations.

>
> Cc: stable@vger.kernel.org
> Fixes: f037590fff30 ("rds: fix a leak of kernel memory")
> Fixes: bdbe6fbc6a2f ("RDS: recv.c")
> Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
> ---
> Note: the "real" copy_to_user() happens in put_cmsg(), where
> `cmlen - sizeof(*cm)` equals to `sizeof(cmsg)`.
>
> Reference: https://lwn.net/Articles/417989/
>
> $ pahole -C "rds_rdma_notify" net/rds/recv.o
> struct rds_rdma_notify {
>       __u64                      user_token;           /*     0     8 */
>       __s32                      status;               /*     8     4 */
>
>       /* size: 16, cachelines: 1, members: 2 */
>       /* padding: 4 */
>       /* last cacheline: 16 bytes */
> };
>
>  net/rds/recv.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/rds/recv.c b/net/rds/recv.c
> index c8404971d5ab..aba4afe4dfed 100644
> --- a/net/rds/recv.c
> +++ b/net/rds/recv.c
> @@ -450,12 +450,13 @@ static int rds_still_queued(struct rds_sock *rs, struct rds_incoming *inc,
>  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 */

It works, but the right solution is to drop 0 from cmsg initialization
and write "struct rds_rdma_notify cmsg = {};" without any memset.

Thanks