CVE-2026-52947: net: qrtr: fix refcount saturation and potential UAF in qrtr_port_remove

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

net: qrtr: fix refcount saturation and potential UAF in qrtr_port_remove

In qrtr_port_remove(), the socket reference count is decremented via
__sock_put() before the port is removed from the qrtr_ports XArray and
before the RCU grace period elapses.

This breaks the fundamental RCU update paradigm. It exposes a race
window where a concurrent RCU reader (such as qrtr_reset_ports() or
qrtr_port_lookup()) can obtain a pointer to the socket from the XArray,
and attempt to call sock_hold() on a socket whose reference count has
already dropped to zero.

This exact race condition was hit during syzkaller fuzzing, leading to
the following refcount saturation warning and a potential Use-After-Free:

  refcount_t: saturated; leaking memory.
  WARNING: CPU: 3 PID: 1273 at lib/refcount.c:22 refcount_warn_saturate+0xae/0x1d0
  Modules linked in: qrtr(+) bochs drm_shmem_helper ...
  Call Trace:
   <TASK>
   qrtr_reset_ports net/qrtr/af_qrtr.c:768 [inline] [qrtr]
   __qrtr_bind.isra.0+0x48b/0x570 net/qrtr/af_qrtr.c:805 [qrtr]
   qrtr_bind+0x17d/0x210 net/qrtr/af_qrtr.c:901 [qrtr]
   kernel_bind+0xe4/0x120 net/socket.c:3592
   qrtr_ns_init+0x1a6/0x380 net/qrtr/ns.c:715 [qrtr]
   qrtr_proto_init+0x3b/0xff0 net/qrtr/af_qrtr.c:169 [qrtr]
   do_one_initcall+0xf5/0x5e0 init/main.c:1283
   ...
   </TASK>

Fix this by deferring the reference count decrement until after the
xa_erase() and the synchronize_rcu() complete.

(Note: The v1 of this patch incorrectly replaced __sock_put() with
sock_put(). As Simon Horman pointed out, the callers of qrtr_port_remove()
still hold a reference to the socket, so freeing the socket memory here
would lead to a subsequent UAF in the caller. Thus, the __sock_put() is
kept, but only repositioned to close the RCU race.)

The Linux kernel CVE team has assigned CVE-2026-52947 to this issue.


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

	Issue introduced in 4.7 with commit bdabad3e363d825ddf9679dd431cca0b2c30f881 and fixed in 5.10.259 with commit 2aa4c12723fe432e623462a3be42a197a128722b
	Issue introduced in 4.7 with commit bdabad3e363d825ddf9679dd431cca0b2c30f881 and fixed in 5.15.210 with commit 03bfa95e452e2b6ccd76a332060ae4feaf5ad84d
	Issue introduced in 4.7 with commit bdabad3e363d825ddf9679dd431cca0b2c30f881 and fixed in 6.1.176 with commit 474293d90880622fde9d2430fb0165767090f7b3
	Issue introduced in 4.7 with commit bdabad3e363d825ddf9679dd431cca0b2c30f881 and fixed in 6.6.143 with commit 2047c2aa0963bb2872fd722300a15bcb441a4c00
	Issue introduced in 4.7 with commit bdabad3e363d825ddf9679dd431cca0b2c30f881 and fixed in 6.12.94 with commit 7de2d447072be3b1a76793f034432338fc9c494b
	Issue introduced in 4.7 with commit bdabad3e363d825ddf9679dd431cca0b2c30f881 and fixed in 6.18.36 with commit ab269990ed58143a92a263be1bee626d82ac03da
	Issue introduced in 4.7 with commit bdabad3e363d825ddf9679dd431cca0b2c30f881 and fixed in 7.0.13 with commit 3b20ec8f31e8a6a6782243f473b0abd3463621df
	Issue introduced in 4.7 with commit bdabad3e363d825ddf9679dd431cca0b2c30f881 and fixed in 7.1 with commit a2171131ecda1ed61a594a1eb715e75fdad0fef5

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-2026-52947
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/qrtr/af_qrtr.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/2aa4c12723fe432e623462a3be42a197a128722b
	https://git.kernel.org/stable/c/03bfa95e452e2b6ccd76a332060ae4feaf5ad84d
	https://git.kernel.org/stable/c/474293d90880622fde9d2430fb0165767090f7b3
	https://git.kernel.org/stable/c/2047c2aa0963bb2872fd722300a15bcb441a4c00
	https://git.kernel.org/stable/c/7de2d447072be3b1a76793f034432338fc9c494b
	https://git.kernel.org/stable/c/ab269990ed58143a92a263be1bee626d82ac03da
	https://git.kernel.org/stable/c/3b20ec8f31e8a6a6782243f473b0abd3463621df
	https://git.kernel.org/stable/c/a2171131ecda1ed61a594a1eb715e75fdad0fef5