[PATCH net] net: appletalk: fix NULL pointer dereference in aarp_send_ddp()

[Weiming Shi <bestswngs@gmail.com>]

aarp_send_ddp() calls atalk_find_dev_addr(dev) in the LocalTalk fast
path without checking for NULL. When the device has no AppleTalk
interface configured (dev->atalk_ptr == NULL), this leads to a NULL
pointer dereference at the at->s_net access.

 KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
 RIP: 0010:aarp_send_ddp (net/appletalk/aarp.c:552 (discriminator 2))
 Call Trace:
  <TASK>
  atalk_sendmsg (net/appletalk/ddp.c:1715)
  __sys_sendto (net/socket.c:2265 (discriminator 1))
  __x64_sys_sendto (net/socket.c:2272)
  do_syscall_64 (arch/x86/entry/syscall_64.c:94)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

Add a NULL check consistent with the other callers of
atalk_find_dev_addr().

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Assisted-by: Claude:claude-opus-4-7
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 net/appletalk/aarp.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/appletalk/aarp.c b/net/appletalk/aarp.c
index e7315c01a299..30493ea3c010 100644
--- a/net/appletalk/aarp.c
+++ b/net/appletalk/aarp.c
@@ -542,6 +542,11 @@ int aarp_send_ddp(struct net_device *dev, struct sk_buff *skb,
 		struct ddpehdr *ddp = (struct ddpehdr *)skb->data;
 		int ft = 2;
 
+		if (!at) {
+			kfree_skb(skb);
+			return NET_XMIT_DROP;
+		}
+
 		/*
 		 * Compressible ?
 		 *
-- 
2.43.0