CVE-2026-74591: mm/filemap: __filemap_add_folio() restore index before retrying

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

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

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

mm/filemap: __filemap_add_folio() restore index before retrying

In __filemap_add_folio()'s split-a-conflict loop, xas_set_order() is
applied repeatedly: each application modifies xas.xa_index, rounding it
down according to the split_order attempted at that stage: and if all goes
as intended, it eventually (or immediately) converges on an
xas_try_split() to the required folio_order, with xas.xa_index now the
same as index: then xas_store() puts the new folio into the xarray there.

But if a new node was needed, and GFP_NOWAIT allocation did not get one,
the lock is dropped, xas_nomem() used to allocate, and sequence retried. 
If (that part of) the xarray is unchanged when the lock is reacquired, no
problem.  But what if the conflict was meanwhile resolved by another
thread (perhaps even doing the same thing, inserting a folio at that same
index)?  Isn't there a danger of now putting our folio into the xarray at
an intermediate rounded-down index?  With !folio_contains() bug to follow,
when CONFIG_DEBUG_VM=y is checking for that.

Fix this with an xas_set_order() to restore the original xas.xa_index at
the bottom of the loop, so the retry does a full re-evaluation after
reacquiring the lock, and cannot reach xas_store() with the wrong index.

Production was suffering from rare SIGILLs and SIGSEGVs, executable text
found a page away from where it belonged, !folio_contains() bug hit when
debug enabled: symptoms not seen since this patch went in.

The Linux kernel CVE team has assigned CVE-2026-74591 to this issue.


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

	Issue introduced in 6.15 with commit 200a89c159a7a416115e6e309183c82183bf98aa and fixed in 6.18.45 with commit 4917e3ebcab50f0265e8ca01c8567de4c4a47511
	Issue introduced in 6.15 with commit 200a89c159a7a416115e6e309183c82183bf98aa and fixed in 7.1.9 with commit 267ecd2eb7759c26f1a026eb0a5b231071534c9c
	Issue introduced in 6.15 with commit 200a89c159a7a416115e6e309183c82183bf98aa and fixed in 7.2 with commit 86da3f7e1e609e1e8bfbab198af68467c5a015a5

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-2026-74591
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:
	mm/filemap.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/4917e3ebcab50f0265e8ca01c8567de4c4a47511
	https://git.kernel.org/stable/c/267ecd2eb7759c26f1a026eb0a5b231071534c9c
	https://git.kernel.org/stable/c/86da3f7e1e609e1e8bfbab198af68467c5a015a5