6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Howells <dhowells@redhat.com>

[ Upstream commit 333b6d5bb9f87827ac2639c737bf9613dbae7253 ]

Fix modification of the received skbuff in rxrpc_input_soft_acks() and a
potential incorrect access of the buffer in a fragmented UDP packet (the
packet would probably have to be deliberately pre-generated as fragmented)
when AF_RXRPC tries to extract the contents of the SACK table by copying
out the contents of the SACK table into a buffer before attempting to parse

AF_RXRPC assumes that it can just call skb_condense() and then validly
access the SACK table from skb->data and that it will be a flat buffer -
but skb_condense() can silently fail to do anything under some
circumstances.

Note that whilst rxrpc_input_soft_acks() should be able to parse extended
ACKs, the rest of AF_RXRPC doesn't currently support that.

Further, there's then no need to call skb_condense() in rxrpc_input_ack(),
so don't.

Fixes: d57a3a151660 ("rxrpc: Save last ACK's SACK table rather than marking txbufs")
Reported-by: Michael Bommarito <redacted>
Link: https://lore.kernel.org/r/20260513180907.2061972-1-michael.bommarito@gmail.com (local)
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <redacted>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
cc: stable@kernel.org
Link: https://patch.msgid.link/105362.1780573560@warthog.procyon.org.uk
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/rxrpc/input.c |   21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

--- a/net/rxrpc/input.c
+++ b/net/rxrpc/input.c

@@ -775,9 +775,23 @@ static void rxrpc_input_soft_acks(struct
 				  rxrpc_seq_t since)
 {
 	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
-	unsigned int i, old_nacks = 0;
+	unsigned int i, old_nacks = 0, nsack;
 	rxrpc_seq_t lowest_nak = seq + sp->ack.nr_acks;
-	u8 *acks = skb->data + sizeof(struct rxrpc_wire_header) + sizeof(struct rxrpc_ackpacket);
+	u8 sack[256] __aligned(sizeof(unsigned long));
+	u8 *acks = sack;
+
+	/* AF_RXRPC assumes that it can access the SACK table directly from
+	 * skb->data as a flat buffer, but the skb may be non-linear (e.g. a
+	 * fragmented UDP packet) and skb_condense() can silently fail to
+	 * linearise it.  Copy the SACK table out into a local buffer before
+	 * parsing it.
+	 */
+	memset(sack, 0, sizeof(sack));
+	nsack = umin(sp->ack.nr_acks, 256);
+	if (skb_copy_bits(skb,
+			  sizeof(struct rxrpc_wire_header) + sizeof(struct rxrpc_ackpacket),
+			  sack, nsack) < 0)
+		return;
 
 	for (i = 0; i < sp->ack.nr_acks; i++) {
 		if (acks[i] == RXRPC_ACK_TYPE_ACK) {

@@ -934,9 +948,6 @@ static void rxrpc_input_ack(struct rxrpc
 	    skb_copy_bits(skb, ioffset, &trailer, sizeof(trailer)) < 0)
 		return rxrpc_proto_abort(call, 0, rxrpc_badmsg_short_ack_trailer);
 
-	if (nr_acks > 0)
-		skb_condense(skb);
-
 	if (call->cong_last_nack) {
 		since = rxrpc_input_check_prev_ack(call, &summary, first_soft_ack);
 		rxrpc_free_skb(call->cong_last_nack, rxrpc_skb_put_last_nack);