FAILED: patch "[PATCH] mm/damon/sysfs: fix use-after-free in state_show()" failed to apply to 6.6-stable tree

gregkh
2025-09-13T12:26:32Z

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 3260a3f0828e06f5f13fac69fb1999a6d60d9cff
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2025091318-trombone-graduate-3828@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 3260a3f0828e06f5f13fac69fb1999a6d60d9cff Mon Sep 17 00:00:00 2001
From: Stanislav Fort <stanislav.fort@aisle.com>
Date: Fri, 5 Sep 2025 13:10:46 +0300
Subject: [PATCH] mm/damon/sysfs: fix use-after-free in state_show()

state_show() reads kdamond->damon_ctx without holding damon_sysfs_lock.
This allows a use-after-free race:

CPU 0 CPU 1
----- -----
state_show() damon_sysfs_turn_damon_on()
ctx = kdamond->damon_ctx; mutex_lock(&damon_sysfs_lock);
damon_destroy_ctx(kdamond->damon_ctx);
kdamond->damon_ctx = NULL;
mutex_unlock(&damon_sysfs_lock);
damon_is_running(ctx); /* ctx is freed */
mutex_lock(&ctx->kdamond_lock); /* UAF */

(The race can also occur with damon_sysfs_kdamonds_rm_dirs() and
damon_sysfs_kdamond_release(), which free or replace the context under
damon_sysfs_lock.)

Fix by taking damon_sysfs_lock before dereferencing the context, mirroring
the locking used in pid_show().

The bug has existed since state_show() first accessed kdamond->damon_ctx.

Link: https://lkml.kernel.org/r/20250905101046.2288-1-disclosure@aisle.com
Fixes: a61ea561c871 ("mm/damon/sysfs: link DAMON for virtual address spaces monitoring")
Signed-off-by: Stanislav Fort <disclosure@aisle.com>
Reported-by: Stanislav Fort <disclosure@aisle.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 6d2b0dab50cb..7b9254cadd5f 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1260,14 +1260,18 @@ static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
{
struct damon_sysfs_kdamond *kdamond = container_of(kobj,
struct damon_sysfs_kdamond, kobj);
- struct damon_ctx *ctx = kdamond->damon_ctx;
- bool running;
+ struct damon_ctx *ctx;
+ bool running = false;

- if (!ctx)
- running = false;
- else
+ if (!mutex_trylock(&damon_sysfs_lock))
+ return -EBUSY;
+
+ ctx = kdamond->damon_ctx;
+ if (ctx)
running = damon_is_running(ctx);

+ mutex_unlock(&damon_sysfs_lock);
+
return sysfs_emit(buf, "%s\n", running ?
damon_sysfs_cmd_strs[DAMON_SYSFS_CMD_ON] :
damon_sysfs_cmd_strs[DAMON_SYSFS_CMD_OFF]);