CVE-2026-89689: nfsd: don't free session slots that are still in use

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

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

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

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

nfsd: don't free session slots that are still in use

nfsd4_sequence() can free the very slot it is currently processing.
When the session shrinker has reduced se_target_maxslots below
se_fchannel.maxreqs, the shrink path checks three conditions before
calling free_session_slots():

  1. se_target_maxslots < maxreqs  (shrink was advertised)
  2. slot->sl_generation == se_slot_gen  (slot is up-to-date)
  3. seq->maxslots <= se_target_maxslots  (client acknowledges)

However, seq->slotid is never checked against se_target_maxslots.
A client using a slot in the range [se_target_maxslots, maxreqs) can
satisfy all three conditions: its slot has the current generation
(set by a prior SEQUENCE), and it sends sa_highest_slotid <=
se_target_maxslots to acknowledge the reduction.

free_session_slots() then kfrees every slot at index >=
se_target_maxslots, including the caller's own slot. The function
continues to write sl_seqid, sl_flags, sl_generation, and stores the
dangling pointer in cstate->slot. Later, nfsd4_store_cache_entry()
copies up to maxresp_cached bytes of the compound reply into the freed
sl_data[] array, corrupting whatever slab object now occupies that
address.

Additionally, a concurrent thread processing SEQUENCE on a different
high-numbered slot can have its slot freed out from under it.
NFSD4_SLOT_INUSE is set under nn->client_lock before the lock is
released, so any concurrent thread past SEQUENCE will have its slot
marked. However, free_session_slots() does not check NFSD4_SLOT_INUSE
before freeing.

Fix both problems by:
 1. Checking that the current request's slotid is below the shrink
    boundary.
 2. Scanning slots in the to-be-freed range for NFSD4_SLOT_INUSE and
    deferring the shrink if any are active.

The Linux kernel CVE team has assigned CVE-2026-89689 to this issue.


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

	Issue introduced in 6.14 with commit fc8738c68d0bbf5033dd98b4f63d277ecbd49fd7 and fixed in 6.18.50 with commit 72d40b103bb03f0124d3a3c8f71a30cbafaf6afa
	Issue introduced in 6.14 with commit fc8738c68d0bbf5033dd98b4f63d277ecbd49fd7 and fixed in 7.2.4 with commit b449b134e7762ddf98653a4b1d3e76d2d212ea64
	Issue introduced in 6.14 with commit fc8738c68d0bbf5033dd98b4f63d277ecbd49fd7 and fixed in 7.3-rc1 with commit f5d22e372f4ac3eb287037a488c29691d52a6330

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-2026-89689
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/nfsd/nfs4state.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/72d40b103bb03f0124d3a3c8f71a30cbafaf6afa
	https://git.kernel.org/stable/c/b449b134e7762ddf98653a4b1d3e76d2d212ea64
	https://git.kernel.org/stable/c/f5d22e372f4ac3eb287037a488c29691d52a6330