In the Linux kernel, the following vulnerability has been resolved: 6lowpan: fix NHC entry use-after-free on error path lowpan_nhc_do_unco

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.