6lowpan: fix NHC entry use-after-free on error path

commit 1720db9 upstream.

lowpan_nhc_do_uncompression() looks up an NHC descriptor while holding
lowpan_nhc_lock.  If the descriptor has no uncompress callback, the error
path drops the lock before printing nhc->name.

lowpan_nhc_del() removes descriptors under the same lock and then relies
on synchronize_net() before the owning module can be unloaded.  That only
waits for net RX RCU readers.  lowpan_header_decompress() is also exported
and can be reached from callers that are not necessarily covered by the net
core RX critical section, for example the Bluetooth 6LoWPAN L2CAP receive
path.

This leaves a race where one task drops lowpan_nhc_lock in the error path,
another task unregisters and frees the matching descriptor after
synchronize_net() returns, and the first task then dereferences nhc->name
for the warning.

With the post-unlock window widened, KASAN reports:

  BUG: KASAN: slab-use-after-free in lowpan_nhc_do_uncompression+0x1f4/0x220
  Read of size 8
  lowpan_nhc_do_uncompression
  lowpan_header_decompress

Fix this by printing the warning before dropping lowpan_nhc_lock, so the
descriptor name is read while unregister is still excluded.  The malformed
packet is still rejected with -ENOTSUPP.

Fixes: 92aa7c6 ("6lowpan: add generic nhc layer interface")
Cc: stable@vger.kernel.org
Reported-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Reported-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
Reported-by: Ao Wang <wangao@seu.edu.cn>
Reported-by: Xuewei Feng <fengxw06@126.com>
Reported-by: Qi Li <qli01@tsinghua.edu.cn>
Reported-by: Ke Xu <xuke@tsinghua.edu.cn>
Signed-off-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Acked-by: Alexander Aring <aahringo@redhat.com>
Link: https://patch.msgid.link/20260609080054.4541-1-zhaoyz24@mails.tsinghua.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: ArtisticFantasy

net/6lowpan/nhc.c

@@ -117,9 +117,9 @@ int lowpan_nhc_do_uncompression(struct sk_buff *skb,
 				return ret;
 			}
 		} else {
-			spin_unlock_bh(&lowpan_nhc_lock);
 			netdev_warn(dev, "received nhc id for %s which is not implemented.\n",
 				    nhc->name);
+			spin_unlock_bh(&lowpan_nhc_lock);
 			return -ENOTSUPP;
 		}
 	} else {