[PATCH net-next v2] sctp: copy zeroed stats to user in sctp_getsockopt_pr_streamstatus() when !streamoute

[Hui Peng <benquike@gmail.com>]

When an SCTP stream output extension structure (streamoute) has not yet
been allocated, sctp_getsockopt_pr_streamstatus() sets the local stack
variables params.sprstat_abandoned_unsent = 0 and
params.sprstat_abandoned_sent = 0, but then immediately jumps to 'out:',
bypassing copy_to_user(). As a result, when an uninitialized user buffer
is passed to getsockopt(SCTP_PR_STREAM_STATUS), the kernel returns
success (0) without copying the zeroed stats back to user space, leaving
uninitialized data in the user buffer.

Refactor the !streamoute check into an if-else chain so that when
streamoute is NULL, params is zeroed and falls through to copy_to_user(),
copying the zeroed stats to user space and setting retval = 0 once before
returning.

Tested in QEMU against Linux 7.3.0-rc3 using an uninitialized
sctp_prstatus user struct passed to getsockopt(SCTP_PR_STREAM_STATUS) on a
freshly created stream: on the unfixed kernel the user struct contains
uninitialized stack garbage; whereas with this fix applied, getsockopt()
copies zeroed stats into user space and returns 0.

Fixes: 28b7eab0f910 ("sctp: add SCTP_PR_STREAM_STATUS getsockopt")
Cc: stable@vger.kernel.org
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v2:
- Re-target subject tag to [PATCH net-next v2].
- Remove redundant retval = 0 assignment in !streamoute block and fall
  through to copy_to_user() as suggested by Paolo Abeni and Xin Long.

 net/sctp/socket.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index c7b9e325ec1c..1aa475d9dfdb 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -7569,11 +7569,7 @@ static int sctp_getsockopt_pr_streamstatus(struct sock *sk, int len,
 		/* Not allocated yet, means all stats are 0 */
 		params.sprstat_abandoned_unsent = 0;
 		params.sprstat_abandoned_sent = 0;
-		retval = 0;
-		goto out;
-	}
-
-	if (policy == SCTP_PR_SCTP_ALL) {
+	} else if (policy == SCTP_PR_SCTP_ALL) {
 		params.sprstat_abandoned_unsent = 0;
 		params.sprstat_abandoned_sent = 0;
 		for (policy = 0; policy <= SCTP_PR_INDEX(MAX); policy++) {