CVE-2026-43388: mm/damon/core: clear walk_control on inactive context in damos_walk()

CVE-2026-43388: mm/damon/core: clear walk_control on inactive context in damos_walk()

[Greg Kroah-Hartman]

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

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

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

mm/damon/core: clear walk_control on inactive context in damos_walk()

damos_walk() sets ctx->walk_control to the caller-provided control
structure before checking whether the context is running.  If the context
is inactive (damon_is_running() returns false), the function returns
-EINVAL without clearing ctx->walk_control.  This leaves a dangling
pointer to a stack-allocated structure that will be freed when the caller
returns.

This is structurally identical to the bug fixed in commit f9132fbc2e83
("mm/damon/core: remove call_control in inactive contexts") for
damon_call(), which had the same pattern of linking a control object and
returning an error without unlinking it.

The dangling walk_control pointer can cause:
1. Use-after-free if the context is later started and kdamond
   dereferences ctx->walk_control (e.g., in damos_walk_cancel()
   which writes to control->canceled and calls complete())
2. Permanent -EBUSY from subsequent damos_walk() calls, since the
   stale pointer is non-NULL

Nonetheless, the real user impact is quite restrictive.  The
use-after-free is impossible because there is no damos_walk() callers who
starts the context later.  The permanent -EBUSY can actually confuse
users, as DAMON is not running.  But the symptom is kept only while the
context is turned off.  Turning it on again will make DAMON internally
uses a newly generated damon_ctx object that doesn't have the invalid
damos_walk_control pointer, so everything will work fine again.

Fix this by clearing ctx->walk_control under walk_control_lock before
returning -EINVAL, mirroring the fix pattern from f9132fbc2e83.

The Linux kernel CVE team has assigned CVE-2026-43388 to this issue.


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

	Issue introduced in 6.14 with commit bf0eaba0ff9c9c8e6fd58ddfa1a8b6df4b813f61 and fixed in 6.18.19 with commit ce0aa47c963b8c3e5beace89e2b5a665a64b5b6b
	Issue introduced in 6.14 with commit bf0eaba0ff9c9c8e6fd58ddfa1a8b6df4b813f61 and fixed in 6.19.9 with commit 9320c77134ab8d7701e20608bbf08517df4fa321
	Issue introduced in 6.14 with commit bf0eaba0ff9c9c8e6fd58ddfa1a8b6df4b813f61 and fixed in 7.0 with commit d210fdcac9c0d1380eab448aebc93f602c1cd4e6

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-2026-43388
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/damon/core.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/ce0aa47c963b8c3e5beace89e2b5a665a64b5b6b
	https://git.kernel.org/stable/c/9320c77134ab8d7701e20608bbf08517df4fa321
	https://git.kernel.org/stable/c/d210fdcac9c0d1380eab448aebc93f602c1cd4e6