[PATCH AUTOSEL 5.4] rxrpc: Fix oops due to non-existence of prealloc backlog struct

[Sasha Levin <sashal@kernel.org>]

From: David Howells <dhowells@redhat.com>

[ Upstream commit 880a88f318cf1d2a0f4c0a7ff7b07e2062b434a4 ]

If an AF_RXRPC service socket is opened and bound, but calls are
preallocated, then rxrpc_alloc_incoming_call() will oops because the
rxrpc_backlog struct doesn't get allocated until the first preallocation is
made.

Fix this by returning NULL from rxrpc_alloc_incoming_call() if there is no
backlog struct.  This will cause the incoming call to be aborted.

Reported-by: Junvyyang, Tencent Zhuque Lab <zhuque@tencent.com>
Suggested-by: Junvyyang, Tencent Zhuque Lab <zhuque@tencent.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: LePremierHomme <kwqcheii@proton.me>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Willy Tarreau <w@1wt.eu>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250708211506.2699012-3-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

**YES**

This commit should be backported to stable kernel trees for the
following reasons:

1. **Fixes a Critical Kernel Oops**: The commit addresses a NULL pointer
   dereference that causes a kernel crash when `rx->backlog` is NULL. At
   line 257 of the original code,
   `smp_load_acquire(&b->call_backlog_head)` would dereference a NULL
   pointer if no preallocation was done.

2. **Minimal and Safe Fix**: The fix is a simple defensive check:
  ```c
  +       if (!b)
  +               return NULL;
  ```
  This is placed immediately after obtaining the backlog pointer and
  before any usage. The fix has zero risk of regression - if `b` is
  NULL, the code would have crashed anyway.

3. **Clear Reproducible Scenario**: The bug occurs in a specific but
   realistic scenario - when an AF_RXRPC service socket is opened and
   bound but no calls are preallocated (meaning
   `rxrpc_service_prealloc()` was never called to allocate the backlog
   structure).

4. **Follows Stable Kernel Rules**: This fix meets all criteria for
   stable backporting:
   - Fixes a real bug that users can hit
   - Small and contained change (2 lines)
   - Obviously correct with no side effects
   - Already tested and merged upstream

5. **Similar to Previously Backported Fixes**: Looking at Similar Commit
   #2 which was marked YES, it also fixed an oops in the rxrpc
   preallocation/backlog system with minimal changes.

The commit prevents a kernel crash with a trivial NULL check, making it
an ideal candidate for stable backporting.

 net/rxrpc/call_accept.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/rxrpc/call_accept.c b/net/rxrpc/call_accept.c
index 55fb3744552de..99f05057e4c90 100644
--- a/net/rxrpc/call_accept.c
+++ b/net/rxrpc/call_accept.c
@@ -281,6 +281,9 @@ static struct rxrpc_call *rxrpc_alloc_incoming_call(struct rxrpc_sock *rx,
 	unsigned short call_tail, conn_tail, peer_tail;
 	unsigned short call_count, conn_count;
 
+	if (!b)
+		return NULL;
+
 	/* #calls >= #conns >= #peers must hold true. */
 	call_head = smp_load_acquire(&b->call_backlog_head);
 	call_tail = b->call_backlog_tail;
-- 
2.39.5