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

Overview

In the Linux kernel, the following vulnerability has been resolved:

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

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.

Affected

• Linux >= 92aa7c65d295f3cbb96904afe335f683e55584b8 < 9c2f5c0829a8c8b904dae36be6d8056b719ac605
• Linux >= 92aa7c65d295f3cbb96904afe335f683e55584b8 < 80b5c8779acee0550845394fb3e5176a398aa24c
• Linux >= 92aa7c65d295f3cbb96904afe335f683e55584b8 < cc27aea4d454abfb385ee2c9499c78b96db9b728
• Linux >= 92aa7c65d295f3cbb96904afe335f683e55584b8 < a8e3a94711134e898c6021a6b77374efa91b3639
• Linux >= 92aa7c65d295f3cbb96904afe335f683e55584b8 < 593b78bb3c7ef0c6e9ae6fdf5afa80a5f7573168
• Linux >= 92aa7c65d295f3cbb96904afe335f683e55584b8 < 0beccbcf50de125be5520d0ffc59af4bb8655482
• Linux >= 92aa7c65d295f3cbb96904afe335f683e55584b8 < b713aa0cc344f10f7a9928a230b5f5e780d04078
• Linux >= 92aa7c65d295f3cbb96904afe335f683e55584b8 < 1720db928e5a58ca7d75ac1d514c3b73fd7061a7
• Linux 4.1

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.