CVE-2026-64064: netfs: Fix netfs_invalidate_folio() to clear dirty bit if all changes gone

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

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

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

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

netfs: Fix netfs_invalidate_folio() to clear dirty bit if all changes gone

If a streaming write is made, this will leave the relevant modified folio
in a not-uptodate, but dirty state with a netfs_folio struct hung off of
folio->private indicating the dirty range.  Subsequently truncating the
file such that the dirty data in the folio is removed, but the first part
of the folio theoretically remains will cause the netfs_folio struct to be
discarded... but will leave the dirty flag set.

If the folio is then read via mmap(), netfs_read_folio() will see that the
page is dirty and jump to netfs_read_gaps() to fill in the missing bits.
netfs_read_gaps(), however, expects there to be a netfs_folio struct
present and can oops because truncate removed it.

Fix this by calling folio_cancel_dirty() in netfs_invalidate_folio() in the
event that all the dirty data in the folio is erased (as nfs does).

Also add some tracepoints to log modifications to a dirty page.

This can be reproduced with something like:

    dd if=/dev/zero of=/xfstest.test/foo bs=1M count=1
    umount /xfstest.test
    mount /xfstest.test
    xfs_io -c "w 0xbbbf 0xf96c" \
           -c "truncate 0xbbbf" \
           -c "mmap -r 0xb000 0x11000" \
           -c "mr 0xb000 0x11000" \
           /xfstest.test/foo

with fscaching disabled (otherwise streaming writes are suppressed) and a
change to netfs_perform_write() to disallow streaming writes if the fd is
open O_RDWR:

	if (//(file->f_mode & FMODE_READ) || <--- comment this out
	    netfs_is_cache_enabled(ctx)) {

It should be reproducible even without this change, but if prevents the
above trivial xfs_io command from reproducing it.

Note that the initial dd is important: the file must start out sufficiently
large that the zero-point logic doesn't just clear the gaps because it
knows there's nothing in the file to read yet.  Unmounting and mounting is
needed to clear the pagecache (there are other ways to do that that may
also work).

This was initially reproduced with the generic/522 xfstest on some patches
that remove the FMODE_READ restriction.

The Linux kernel CVE team has assigned CVE-2026-64064 to this issue.


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

	Issue introduced in 6.8 with commit 9ebff83e648148b9ece97d4e4890dd84ca54d6ce and fixed in 6.12.92 with commit 31ba145faceb378fa01afcb8349e15ea7d95e542
	Issue introduced in 6.8 with commit 9ebff83e648148b9ece97d4e4890dd84ca54d6ce and fixed in 6.18.34 with commit fb6ec883b48b8789e5e690dcd440d2db941e840c
	Issue introduced in 6.8 with commit 9ebff83e648148b9ece97d4e4890dd84ca54d6ce and fixed in 7.0.11 with commit 65ae8717abf36202fef02260b64b781d2d44a9bf
	Issue introduced in 6.8 with commit 9ebff83e648148b9ece97d4e4890dd84ca54d6ce and fixed in 7.1 with commit 156ac2ec2ee77c44c4eb7439d6d165247ba12247

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-2026-64064
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/netfs/misc.c
	include/trace/events/netfs.h


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/31ba145faceb378fa01afcb8349e15ea7d95e542
	https://git.kernel.org/stable/c/fb6ec883b48b8789e5e690dcd440d2db941e840c
	https://git.kernel.org/stable/c/65ae8717abf36202fef02260b64b781d2d44a9bf
	https://git.kernel.org/stable/c/156ac2ec2ee77c44c4eb7439d6d165247ba12247