CVE-2026-72422: ksmbd: fix use-after-free of conn->preauth_info in concurrent SMB2 NEGOTIATE

CVE-2026-72422: ksmbd: fix use-after-free of conn->preauth_info in concurrent SMB2 NEGOTIATE

[Greg Kroah-Hartman]

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

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

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

ksmbd: fix use-after-free of conn->preauth_info in concurrent SMB2 NEGOTIATE

conn->preauth_info is shared connection state (struct
preauth_integrity_info, kmalloc-96) that is allocated and freed by the
SMB2 NEGOTIATE handler and read by the response send path.

smb2_handle_negotiate() allocates conn->preauth_info, and on a
deassemble_neg_contexts() failure kfrees it and sets it to NULL. Both the
allocation and the free/NULL happen under ksmbd_conn_lock(conn) (the
connection srv_mutex), which is held across the whole handler body.

The response send path smb3_preauth_hash_rsp(), called from the send:
block of __handle_ksmbd_work(), reads conn->preauth_info and dereferences
conn->preauth_info->Preauth_HashValue (via
ksmbd_gen_preauth_integrity_hash()) without taking conn_lock. When a
client drives two SMB2 NEGOTIATE requests on the same connection, one
worker can free conn->preauth_info on the failing-negotiate path while a
concurrent send-path worker is reading it, producing a slab
use-after-free read (KASAN-confirmed).

The send-path read tested conn->preauth_info for NULL but raced with the
free that occurs between the NULL check and the dereference, so the NULL
guard alone does not close the window.

Serialize the NEGOTIATE-branch read in smb3_preauth_hash_rsp() under
ksmbd_conn_lock(conn) and re-check conn->preauth_info inside the lock.
Because the negotiate handler holds conn_lock across its kfree + NULL
assignment, a reader that also takes conn_lock either runs fully before
the allocation or fully after the NULL store, and can never observe the
freed-but-not-yet-NULLed pointer. ksmbd_gen_preauth_integrity_hash()
takes no locks itself (it only computes a SHA-512 over the buffer), so
no lock-ordering inversion is introduced, and conn_lock is a sleepable
mutex which is safe on this send path (it already performs network I/O).

The Linux kernel CVE team has assigned CVE-2026-72422 to this issue.


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

	Issue introduced in 5.15.61 with commit dd4e4c811898410e6a3ae3b63207b7c542860907 and fixed in 5.15.212 with commit c7bef84740d1d57848c74f6f5b996606e43ea4fe
	Issue introduced in 6.0 with commit aa7253c2393f6dcd6a1468b0792f6da76edad917 and fixed in 6.1.178 with commit d0a469122e7bf8338fec1949fb1e8e1290ed8caa
	Issue introduced in 6.0 with commit aa7253c2393f6dcd6a1468b0792f6da76edad917 and fixed in 6.6.145 with commit 7470511d085af1c7a043a60e53d52b512d5a10b1
	Issue introduced in 6.0 with commit aa7253c2393f6dcd6a1468b0792f6da76edad917 and fixed in 6.12.97 with commit 16a1ecf39c217e3d164bd32ef2a4f650abc067fa
	Issue introduced in 6.0 with commit aa7253c2393f6dcd6a1468b0792f6da76edad917 and fixed in 6.18.40 with commit 77bb0bbfcc4e777ca653174689e5e363f8ee63d1
	Issue introduced in 6.0 with commit aa7253c2393f6dcd6a1468b0792f6da76edad917 and fixed in 7.1.5 with commit 1c89da3baa2b1f269178afa87dc30479b8535776
	Issue introduced in 6.0 with commit aa7253c2393f6dcd6a1468b0792f6da76edad917 and fixed in 7.2-rc1 with commit 0c054227479ed7e36ebccb3a558bc0ef698264f6
	Issue introduced in 5.18.18 with commit 8a8315a5960bd2b5ffc75f44fc089e57c3b17c44
	Issue introduced in 5.19.2 with commit ff20f1875889dbe4a67c9298e609d7c88cf6456d

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-2026-72422
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:
	fs/smb/server/smb2pdu.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/c7bef84740d1d57848c74f6f5b996606e43ea4fe
	https://git.kernel.org/stable/c/d0a469122e7bf8338fec1949fb1e8e1290ed8caa
	https://git.kernel.org/stable/c/7470511d085af1c7a043a60e53d52b512d5a10b1
	https://git.kernel.org/stable/c/16a1ecf39c217e3d164bd32ef2a4f650abc067fa
	https://git.kernel.org/stable/c/77bb0bbfcc4e777ca653174689e5e363f8ee63d1
	https://git.kernel.org/stable/c/1c89da3baa2b1f269178afa87dc30479b8535776
	https://git.kernel.org/stable/c/0c054227479ed7e36ebccb3a558bc0ef698264f6