[SRU][N][PATCH 2/3] net: tls: fix off-by-one in sg_chain entry count for wrapped sk_msg ring

[Cengiz Can]

From: Jakub Kicinski <kuba at kernel.org>

BugLink: https://bugs.launchpad.net/bugs/2155609

When an sk_msg scatterlist ring wraps (sg.end < sg.start),
tls_push_record() chains the tail portion of the ring to the head
using sg_chain(). An extra entry in the sg array is reserved for
this:

  struct sk_msg_sg {
        [...]
        /* The extra two elements:
         * 1) used for chaining the front and sections when the list becomes
         *    partitioned (e.g. end < start). The crypto APIs require the
         *    chaining;
         * 2) to chain tailer SG entries after the message.
         */
        struct scatterlist              data[MAX_MSG_FRAGS + 2];

The current code uses MAX_SKB_FRAGS + 1 as the ring size:

    sg_chain(&msg_pl->sg.data[msg_pl->sg.start],
             MAX_SKB_FRAGS - msg_pl->sg.start + 1,
             msg_pl->sg.data);

This places the chain pointer at

  sg_chain(data[start], (MAX_SKB_FRAGS - msg_start + 1) .. =
  &data[start] + (MAX_SKB_FRAGS - msg_start + 1) - 1 =
  data[start + (MAX_SKB_FRAGS - start + 1) - 1] =
  data[MAX_SKB_FRAGS]

instead of the true last entry. This is likely due to a "race" of
the commit under Fixes landing close to
commit 031097d9e079 ("bpf: sk_msg, zap ingress queue on psock down")

Convert to ARRAY_SIZE and drop the data[start] / - start (as suggested
by Sabrina).

Reported-by: 钱一铭 <yimingqian591 at gmail.com>
Fixes: 9aaaa56845a0 ("bpf: Sockmap/tls, skmsg can have wrapped skmsg that needs extra chaining")
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
Reviewed-by: Sabrina Dubroca <sd at queasysnail.net>
Link: https://patch.msgid.link/20260511174920.433155-2-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni at redhat.com>
(cherry picked from commit 285943c6e7ca309bbea84b253745154241d9788a)
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
 net/tls/tls_sw.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
index 71b3100ccb43..a9b447159d86 100644
--- a/net/tls/tls_sw.c
+++ b/net/tls/tls_sw.c
@@ -800,11 +800,9 @@ static int tls_push_record(struct sock *sk, int flags,
 		sg_mark_end(sk_msg_elem(msg_pl, i));
 	}
 
-	if (msg_pl->sg.end < msg_pl->sg.start) {
-		sg_chain(&msg_pl->sg.data[msg_pl->sg.start],
-			 MAX_SKB_FRAGS - msg_pl->sg.start + 1,
+	if (msg_pl->sg.end < msg_pl->sg.start)
+		sg_chain(msg_pl->sg.data, ARRAY_SIZE(msg_pl->sg.data),
 			 msg_pl->sg.data);
-	}
 
 	i = msg_pl->sg.start;
 	sg_chain(rec->sg_aead_in, 2, &msg_pl->sg.data[i]);
-- 
2.43.0