CVE-2026-74599: mm/ptdump: always stabilise against page table freeing using init_mm

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

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

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

mm/ptdump: always stabilise against page table freeing using init_mm

Previous commits have established the invariant that kernel page table
freeing is performed while an mmap read lock on init_mm is held, which
fixes races between ptdump and kernel page table freeing over init_mm.

However, x86 and arm64 can perform a ptdump over an mm other than init_mm
via ptdump_walk_pgd() and since kernel memory ranges are shared across
non-kernel mm's, this means that the race still exists for these cases.

Fix this by acquiring a nested mmap write lock for init_mm in
ptdump_walk_pgd().

This is safe as we take this after mmap write locking the mm, and nothing
acquires the init_mm lock first before locking an arbitrary mm, so no
deadlock is possible.

Also update walk_page_range_debug() to assert that init_mm is write
locked, add a comment explaining why and remove some redundant code, and
eliminate the unnecessary and confusing invocation of
walk_kernel_page_table_range().

We can safely remove the non-NULL check for walk.mm, as the mmap lock
asserts would NULL pointer deref if it was (and of course no callers do
this).

The first point at which ptdump can race kernel page table freeing is
commit b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page
table"), so we target this in the Fixes tag.

The Linux kernel CVE team has assigned CVE-2026-74599 to this issue.


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

	Issue introduced in 4.16 with commit b6bdb7517c3d3f41f20e5c2948d6bc3f8897394e and fixed in 6.18.45 with commit 7f740664aec1f832953c2e6d9b8920cd6c8bcc0c
	Issue introduced in 4.16 with commit b6bdb7517c3d3f41f20e5c2948d6bc3f8897394e and fixed in 7.1.9 with commit 4adc4c9a9a43d61fe476dfe10811f3df2e7e4106
	Issue introduced in 4.16 with commit b6bdb7517c3d3f41f20e5c2948d6bc3f8897394e and fixed in 7.2 with commit 27c32e5538344b13c1505a08861e04620c125d47
	Issue introduced in 4.4.125 with commit 31895cfd79564111cdd5a9f48c5d491ae26a238e
	Issue introduced in 4.9.91 with commit 9c7f7bdb1932f8c1e5f80d32c717184701afe701
	Issue introduced in 4.14.31 with commit acdb4981644c8e31ccee294bdefff475c0cf587b
	Issue introduced in 4.15.14 with commit 0454e2fad9306961540ee7e84da47a8e345b7d22

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-2026-74599
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/pagewalk.c
	mm/ptdump.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/7f740664aec1f832953c2e6d9b8920cd6c8bcc0c
	https://git.kernel.org/stable/c/4adc4c9a9a43d61fe476dfe10811f3df2e7e4106
	https://git.kernel.org/stable/c/27c32e5538344b13c1505a08861e04620c125d47