[Linux-kernel-mentees] [PATCH net] rds: Prevent kernel-infoleak in rds_notify_queue_get()

Peilin Ye
2020-07-30T19:22:01Z

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@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 */

/* 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
--
2.25.1