Subject: CVE-2024-46838: userfaultfd: don't BUG_ON() if khugepaged yanks our page table

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

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

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

userfaultfd: don't BUG_ON() if khugepaged yanks our page table

Since khugepaged was changed to allow retracting page tables in file
mappings without holding the mmap lock, these BUG_ON()s are wrong - get
rid of them.

We could also remove the preceding "if (unlikely(...))" block, but then we
could reach pte_offset_map_lock() with transhuge pages not just for file
mappings but also for anonymous mappings - which would probably be fine
but I think is not necessarily expected.

The Linux kernel CVE team has assigned CVE-2024-46838 to this issue.

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

	Issue introduced in 6.6 with commit 1d65b771bc08 and fixed in 6.6.51 with commit 4a594acc12d5
	Issue introduced in 6.6 with commit 1d65b771bc08 and fixed in 6.10.10 with commit db978287e908
	Issue introduced in 6.6 with commit 1d65b771bc08 and fixed in 6.11 with commit 4828d207dc51

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-2024-46838
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/userfaultfd.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/4a594acc12d5954cdc71d4450a386748bf3d136a
	https://git.kernel.org/stable/c/db978287e908d48b209e374b00d847b2d785e0a9
	https://git.kernel.org/stable/c/4828d207dc5161dc7ddf9a4f6dcfd80c7dd7d20a