CVE-2025-21696

What the bug is

When mremap()ing a memory region previously registered with userfaultfd as write-protected but without UFFD_FEATURE_EVENT_REMAP, an inconsistency in flag clearing leads to a mismatch between the vma flags (which have uffd-wp cleared) and the pte/pmd flags (which do not have uffd-wp cleared). This mismatch causes a subsequent mprotect(PROT_WRITE) to trigger a warning in page_table_check_pte_flags() due to setting the pte to writable while uffd-wp is still set. Fix this by always explicitly clearing the uffd-wp pte/pmd flags on any such mremap() so that the values are consistent with the existing clearing of VM_UFFD_WP. Be careful to clear the logical flag regardless of its physical form; a PTE bit, a swap PTE bit, or a PTE marker. Cover PTE, huge PMD and hugetlb paths.

Where it lives

The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.

include/linux/userfaultfd_k.h
mm/huge_memory.c
mm/hugetlb.c
mm/mremap.c

What this is

MEMORY MANAGEMENT - USERFAULTFDMAINTAINERS section matched on include/linux/userfaultfd_k.h · linux-mm@kvack.org · Maintained

Check it on the machine

grep -w CONFIG_TRANSPARENT_HUGEPAGE /boot/config-$(uname -r)

Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.

Show whether Debian builds this, per release

Debian builds this, but not on every flavour. At least one release below builds the code on some of its kernel flavours and not on others, so the answer depends on the architecture and flavour installed.

• bookwormYnot built on amd64/rt/amd64, arm64/rt/arm64, armhf/armmp, armhf/rt/armmp, i386/rt/686-pae
• trixieYnot built on amd64/rt/amd64, arm64/rt/arm64, armel/rpi, armhf/armmp, armhf/rt/armmp
• forky, sidYnot built on amd64/rt-amd64, arm64/rt-arm64, armhf/armmp

Y

built in where it is built at all; some flavours do not build it

Build switch

CONFIG_TRANSPARENT_HUGEPAGE

From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages

Signals

Published

2025-02-12 (20 months ago)source

CVSS

no vector published, unrated

Attack vector

not published

EPSS

0.19%, higher than 8.2% of all CVEssource

CISA KEV

not listedthe KEV catalogue

Debian urgency

not yet assignedsource

Subsystem

include/linux

Upstream fixes

The versions the CNA records as introducing and fixing this, and the commit for each. source

Introduced	Fixed in	Commit
5.7	6.12.11	310ac886d68d
5.7	6.13	0cef0bb836e3