CVE-2026-64452: 6lowpan: fix NHC entry use-after-free on error path

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-64452 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.1 with commit 92aa7c65d295f3cbb96904afe335f683e55584b8 and fixed in 5.10.261 with commit 9c2f5c0829a8c8b904dae36be6d8056b719ac605
	Issue introduced in 4.1 with commit 92aa7c65d295f3cbb96904afe335f683e55584b8 and fixed in 5.15.212 with commit 80b5c8779acee0550845394fb3e5176a398aa24c
	Issue introduced in 4.1 with commit 92aa7c65d295f3cbb96904afe335f683e55584b8 and fixed in 6.1.178 with commit cc27aea4d454abfb385ee2c9499c78b96db9b728
	Issue introduced in 4.1 with commit 92aa7c65d295f3cbb96904afe335f683e55584b8 and fixed in 6.6.145 with commit a8e3a94711134e898c6021a6b77374efa91b3639
	Issue introduced in 4.1 with commit 92aa7c65d295f3cbb96904afe335f683e55584b8 and fixed in 6.12.96 with commit 593b78bb3c7ef0c6e9ae6fdf5afa80a5f7573168
	Issue introduced in 4.1 with commit 92aa7c65d295f3cbb96904afe335f683e55584b8 and fixed in 6.18.39 with commit 0beccbcf50de125be5520d0ffc59af4bb8655482
	Issue introduced in 4.1 with commit 92aa7c65d295f3cbb96904afe335f683e55584b8 and fixed in 7.1.4 with commit b713aa0cc344f10f7a9928a230b5f5e780d04078
	Issue introduced in 4.1 with commit 92aa7c65d295f3cbb96904afe335f683e55584b8 and fixed in 7.2-rc1 with commit 1720db928e5a58ca7d75ac1d514c3b73fd7061a7

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-64452
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/6lowpan/nhc.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/9c2f5c0829a8c8b904dae36be6d8056b719ac605
	https://git.kernel.org/stable/c/80b5c8779acee0550845394fb3e5176a398aa24c
	https://git.kernel.org/stable/c/cc27aea4d454abfb385ee2c9499c78b96db9b728
	https://git.kernel.org/stable/c/a8e3a94711134e898c6021a6b77374efa91b3639
	https://git.kernel.org/stable/c/593b78bb3c7ef0c6e9ae6fdf5afa80a5f7573168
	https://git.kernel.org/stable/c/0beccbcf50de125be5520d0ffc59af4bb8655482
	https://git.kernel.org/stable/c/b713aa0cc344f10f7a9928a230b5f5e780d04078
	https://git.kernel.org/stable/c/1720db928e5a58ca7d75ac1d514c3b73fd7061a7