[PATCH net v2] atm: lec: fix null-ptr-deref in lec_arp_clear_vccs

[Jiayuan Chen <jiayuan.chen@linux.dev>]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

syzkaller reported a null-ptr-deref in lec_arp_clear_vccs().
This issue can be easily reproduced using the syzkaller reproducer.

In the ATM LANE (LAN Emulation) module, the same atm_vcc can be shared by
multiple lec_arp_table entries (e.g., via entry->vcc or entry->recv_vcc).
When the underlying VCC is closed, lec_vcc_close() iterates over all
ARP entries and calls lec_arp_clear_vccs() for each matched entry.

For example, when lec_vcc_close() iterates through the hlists in
priv->lec_arp_empty_ones or other ARP tables:

1. In the first iteration, for the first matched ARP entry sharing the VCC,
lec_arp_clear_vccs() frees the associated vpriv (which is vcc->user_back)
and sets vcc->user_back to NULL.
2. In the second iteration, for the next matched ARP entry sharing the same
VCC, lec_arp_clear_vccs() is called again. It obtains a NULL vpriv from
vcc->user_back (via LEC_VCC_PRIV(vcc)) and then attempts to dereference it
via `vcc->pop = vpriv->old_pop`, leading to a null-ptr-deref crash.

Fix this by adding a null check for vpriv before dereferencing
it. If vpriv is already NULL, it means the VCC has been cleared
by a previous call, so we can safely skip the cleanup and just
clear the entry's vcc/recv_vcc pointers.

The entire cleanup block (including vcc_release_async()) is placed inside
the vpriv guard because a NULL vpriv indicates the VCC has already been
fully released by a prior iteration — repeating the teardown would
redundantly set flags and trigger callbacks on an already-closing socket.

The Fixes tag points to the initial commit because the entry->vcc path has
been vulnerable since the original code. The entry->recv_vcc path was later
added by commit 8d9f73c0ad2f ("atm: fix a memory leak of vcc->user_back")
with the same pattern, and both paths are fixed here.

Reported-by: syzbot+72e3ea390c305de0e259@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/68c95a83.050a0220.3c6139.0e5c.GAE@google.com/T/
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
- Add Reviewed-by from Simon Horman
- Drop the entry->recv_vcc->push to vcc->push cleanup to keep
  the patch minimal (Dan Carpenter)
- Clarify why vcc_release_async() is inside the vpriv guard
- Clarify why the Fixes tag points to the initial commit

v1: https://lore.kernel.org/netdev/6bc6f7fe686e53fe2aaca467920ccf300546d5bf@linux.dev/T/#t
---
 net/atm/lec.c | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/net/atm/lec.c b/net/atm/lec.c
index afb8d3eb2185..c39dc5d36797 100644
--- a/net/atm/lec.c
+++ b/net/atm/lec.c
@@ -1260,24 +1260,28 @@ static void lec_arp_clear_vccs(struct lec_arp_table *entry)
 		struct lec_vcc_priv *vpriv = LEC_VCC_PRIV(vcc);
 		struct net_device *dev = (struct net_device *)vcc->proto_data;
 
-		vcc->pop = vpriv->old_pop;
-		if (vpriv->xoff)
-			netif_wake_queue(dev);
-		kfree(vpriv);
-		vcc->user_back = NULL;
-		vcc->push = entry->old_push;
-		vcc_release_async(vcc, -EPIPE);
+		if (vpriv) {
+			vcc->pop = vpriv->old_pop;
+			if (vpriv->xoff)
+				netif_wake_queue(dev);
+			kfree(vpriv);
+			vcc->user_back = NULL;
+			vcc->push = entry->old_push;
+			vcc_release_async(vcc, -EPIPE);
+		}
 		entry->vcc = NULL;
 	}
 	if (entry->recv_vcc) {
 		struct atm_vcc *vcc = entry->recv_vcc;
 		struct lec_vcc_priv *vpriv = LEC_VCC_PRIV(vcc);
 
-		kfree(vpriv);
-		vcc->user_back = NULL;
+		if (vpriv) {
+			kfree(vpriv);
+			vcc->user_back = NULL;
 
-		entry->recv_vcc->push = entry->old_recv_push;
-		vcc_release_async(entry->recv_vcc, -EPIPE);
+			entry->recv_vcc->push = entry->old_recv_push;
+			vcc_release_async(entry->recv_vcc, -EPIPE);
+		}
 		entry->recv_vcc = NULL;
 	}
 }
-- 
2.43.0