[PATCH net 2/4] sctp: abort unordered partial delivery when its message is skipped

[Tim Budding <tim.budding@gmail.com>]

When an I-FORWARD-TSN skips unordered messages on a stream,
sctp_intl_skip() aborts the stream's unordered partial delivery only if
the MID being delivered is lower than the skipped MID. If the skipped
message is the one being partially delivered, partial delivery is left
in progress: the application never gets SCTP_PARTIAL_DELIVERY_ABORTED,
and the next unordered message on that stream is delivered with MSG_EOR
as if it were the rest of the abandoned message, so the application
sees the two messages merged into one. Until a later unordered message
on that stream is abandoned, no other unordered partial delivery can
start there either.

Abort the partial delivery when the MID being delivered is at or below
the skipped MID, as the ordered case already does.

Found by an AI assistant while working on the previous patch. Reproduced
on an unmodified kernel with the uo_pd test of the sctp_ifwdtsn.sh
selftest added later in this series, which passes with this fix.

Fixes: de60fe910543 ("sctp: implement handle_ftsn for sctp_stream_interleave")
Cc: stable@vger.kernel.org
Signed-off-by: Tim Budding <tim.budding@gmail.com>
---
 net/sctp/stream_interleave.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sctp/stream_interleave.c b/net/sctp/stream_interleave.c
index f8c818a5533c..77fe0db5ff77 100644
--- a/net/sctp/stream_interleave.c
+++ b/net/sctp/stream_interleave.c
@@ -1246,7 +1246,7 @@ static void sctp_intl_skip(struct sctp_ulpq *ulpq, __u16 sid, __u32 mid,
 	struct sctp_stream *stream  = &ulpq->asoc->stream;
 
 	if (flags & SCTP_FTSN_U_BIT) {
-		if (sin->pd_mode_uo && MID_lt(sin->mid_uo, mid)) {
+		if (sin->pd_mode_uo && !MID_lt(mid, sin->mid_uo)) {
 			sin->pd_mode_uo = 0;
 			sctp_intl_stream_abort_pd(ulpq, sid, mid, 0x1,
 						  GFP_ATOMIC);
-- 
2.34.1