linux-cve-announce - CVE-2025-40230: mm: prevent poison consumption when splitting THP

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Message-ID: <2025120400-CVE-2025-40230-7b9d@gregkh>
Date: Thu,  4 Dec 2025 16:32:05 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-40230: mm: prevent poison consumption when splitting THP

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

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

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

mm: prevent poison consumption when splitting THP

When performing memory error injection on a THP (Transparent Huge Page)
mapped to userspace on an x86 server, the kernel panics with the following
trace.  The expected behavior is to terminate the affected process instead
of panicking the kernel, as the x86 Machine Check code can recover from an
in-userspace #MC.

  mce: [Hardware Error]: CPU 0: Machine Check Exception: f Bank 3: bd80000000070134
  mce: [Hardware Error]: RIP 10:<ffffffff8372f8bc> {memchr_inv+0x4c/0xf0}
  mce: [Hardware Error]: TSC afff7bbff88a ADDR 1d301b000 MISC 80 PPIN 1e741e77539027db
  mce: [Hardware Error]: PROCESSOR 0:d06d0 TIME 1758093249 SOCKET 0 APIC 0 microcode 80000320
  mce: [Hardware Error]: Run the above through 'mcelog --ascii'
  mce: [Hardware Error]: Machine check: Data load in unrecoverable area of kernel
  Kernel panic - not syncing: Fatal local machine check

The root cause of this panic is that handling a memory failure triggered
by an in-userspace #MC necessitates splitting the THP.  The splitting
process employs a mechanism, implemented in
try_to_map_unused_to_zeropage(), which reads the pages in the THP to
identify zero-filled pages.  However, reading the pages in the THP results
in a second in-kernel #MC, occurring before the initial memory_failure()
completes, ultimately leading to a kernel panic.  See the kernel panic
call trace on the two #MCs.

  First Machine Check occurs // [1]
    memory_failure()         // [2]
      try_to_split_thp_page()
        split_huge_page()
          split_huge_page_to_list_to_order()
            __folio_split()  // [3]
              remap_page()
                remove_migration_ptes()
                  remove_migration_pte()
                    try_to_map_unused_to_zeropage()  // [4]
                      memchr_inv()                   // [5]
                        Second Machine Check occurs  // [6]
                          Kernel panic

[1] Triggered by accessing a hardware-poisoned THP in userspace, which is
    typically recoverable by terminating the affected process.

[2] Call folio_set_has_hwpoisoned() before try_to_split_thp_page().

[3] Pass the RMP_USE_SHARED_ZEROPAGE remap flag to remap_page().

[4] Try to map the unused THP to zeropage.

[5] Re-access pages in the hw-poisoned THP in the kernel.

[6] Triggered in-kernel, leading to a panic kernel.

In Step[2], memory_failure() sets the poisoned flag on the page in the THP
by TestSetPageHWPoison() before calling try_to_split_thp_page().

As suggested by David Hildenbrand, fix this panic by not accessing to the
poisoned page in the THP during zeropage identification, while continuing
to scan unaffected pages in the THP for possible zeropage mapping.  This
prevents a second in-kernel #MC that would cause kernel panic in Step[4].

Thanks to Andrew Zaborowski for his initial work on fixing this issue.

The Linux kernel CVE team has assigned CVE-2025-40230 to this issue.


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

	Issue introduced in 6.12 with commit b1f202060afeb7fcb98473929d26fd3d2093b067 and fixed in 6.12.56 with commit 6fc0a7c99e973a50018c8b4be34914a1b5c7b383
	Issue introduced in 6.12 with commit b1f202060afeb7fcb98473929d26fd3d2093b067 and fixed in 6.17.6 with commit 92acf4b04f255d2f0f6770bb0d0a208d8ffb2b77
	Issue introduced in 6.12 with commit b1f202060afeb7fcb98473929d26fd3d2093b067 and fixed in 6.18 with commit 841a8bfcbad94bb1ba60f59ce34f75259074ae0d

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-2025-40230
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/huge_memory.c
	mm/migrate.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/6fc0a7c99e973a50018c8b4be34914a1b5c7b383
	https://git.kernel.org/stable/c/92acf4b04f255d2f0f6770bb0d0a208d8ffb2b77
	https://git.kernel.org/stable/c/841a8bfcbad94bb1ba60f59ce34f75259074ae0d