Re: [PATCH v2] mm/damon/sysfs-schemes: fix use-after-free on memcg_path and goal path

SeongJae Park
2026-04-21T01:20:41Z

Hello Junxi,

When you send a new version of a patch, please send it as a new mail with
changes log [1], rather than as a reply to the old version.

On Mon, 20 Apr 2026 20:54:05 +0800 Junxi Qian <qjx1298677004@gmail.com> wrote:

> memcg_path_store() and path_store() free and replace their respective
> string pointers without holding damon_sysfs_lock. This creates a race
> with the kdamond thread, which reads these pointers in
> damon_sysfs_add_scheme_filters() and damos_sysfs_add_quota_score()
> via damon_call() during a "commit" operation.
>
> The race window is as follows:
> Thread A (commit): holds damon_sysfs_lock, triggers damon_call()
> kdamond thread: reads sysfs_filter->memcg_path in
> damon_sysfs_memcg_path_to_id()
> Thread B (sysfs): calls memcg_path_store() WITHOUT lock,
> kfree(filter->memcg_path), replaces pointer
>
> Since Thread B does not hold damon_sysfs_lock, the kdamond thread can
> observe a freed pointer, resulting in a use-after-free when
> damon_sysfs_memcg_path_eq() dereferences it for string comparison.

Thank you for finding and sharing this bug!
>
> Similarly, memcg_path_show() and path_show() read the string pointers
> without holding the lock, so a concurrent store can free the string
> just before sysfs_emit() dereferences it, causing a use-after-free.

Is this correct? Isn't sysfs prohibiting such concurrent read/write using
kernfs_open_file->mutex? Were you able to trigger this?

>
> KASAN reports the following on v7.0.0-rc5 with CONFIG_KASAN=y:
>
> ==================================================================
> BUG: KASAN: double-free in memcg_path_store+0x6e/0xc0
>
> Free of addr ffff888100a086c0 by task exp/149
>
> CPU: 1 UID: 0 PID: 149 Comm: exp Not tainted 7.0.0-rc5-gd38efd7c139a #18 PREEMPT(lazy)
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
> Call Trace:
> <TASK>
> dump_stack_lvl+0x55/0x70
> print_report+0xcb/0x5d0
> kasan_report_invalid_free+0x94/0xc0
> check_slab_allocation+0xde/0x110
> kfree+0x114/0x3b0
> memcg_path_store+0x6e/0xc0
> kernfs_fop_write_iter+0x2fc/0x490
> vfs_write+0x8e1/0xcc0
> ksys_write+0xee/0x1c0
> do_syscall_64+0xfc/0x580
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> </TASK>
>
> Allocated by task 147 on cpu 0 at 12.364295s:
> kasan_save_stack+0x24/0x50
> kasan_save_track+0x17/0x60
> __kasan_kmalloc+0x7f/0x90
> __kmalloc_noprof+0x191/0x490
> memcg_path_store+0x32/0xc0
> kernfs_fop_write_iter+0x2fc/0x490
> vfs_write+0x8e1/0xcc0
> ksys_write+0xee/0x1c0
>
> Freed by task 150 on cpu 0 at 13.373810s:
> kasan_save_stack+0x24/0x50
> kasan_save_track+0x17/0x60
> kasan_save_free_info+0x3b/0x60
> __kasan_slab_free+0x43/0x70
> kfree+0x137/0x3b0
> memcg_path_store+0x6e/0xc0
> kernfs_fop_write_iter+0x2fc/0x490
> vfs_write+0x8e1/0xcc0
> ksys_write+0xee/0x1c0
>
> The buggy address belongs to the object at ffff888100a086c0
> which belongs to the cache kmalloc-8 of size 8
> ==================================================================
>
> Fix this by holding damon_sysfs_lock while swapping the path pointer
> in both memcg_path_store() and path_store(),

I'd like to avoid use of the lock in long term. But this sounds good and
simple for hotfixes.

> and while reading the
> path pointer in memcg_path_show() and path_show().

As I commented above, I'm not sure if this is really needed.

> The actual kfree()
> is moved outside the lock since the old pointer is no longer reachable
> once replaced.
>
> Fixes: 490a43d07f16 ("mm/damon/sysfs-schemes: free old damon_sysfs_scheme_filter->memcg_path on write")

I think this deserves Cc-ing stable@.

Also, this patch is fixing two bugs. For making backport of the fixes easy,
could we split this patch into two patches, one for memcg_path_{show,store}(),
and the other one for path_{show,store}()?

> Signed-off-by: Junxi Qian <qjx1298677004@gmail.com>
>
> ---
> Changes in v2:
> - Also protect memcg_path_show() and path_show() with damon_sysfs_lock,
> since sysfs show and store callbacks can run concurrently and a
> lockless read in show can race with the kfree in store. (Sashiko AI)

As I abovely commented, I'm not sure that Sashiko comment is correct.

>
> mm/damon/sysfs-schemes.c | 36 ++++++++++++++++++++++++++++--------
> 1 file changed, 28 insertions(+), 8 deletions(-)
>
> diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
> index 5186966da..1a890e2a4 100644
> --- a/mm/damon/sysfs-schemes.c
> +++ b/mm/damon/sysfs-schemes.c
> @@ -533,9 +533,14 @@ static ssize_t memcg_path_show(struct kobject *kobj,
> {
> struct damon_sysfs_scheme_filter *filter = container_of(kobj,
> struct damon_sysfs_scheme_filter, kobj);
> + ssize_t len;
>
> - return sysfs_emit(buf, "%s\n",
> + mutex_lock(&damon_sysfs_lock);

I read Sashiko is commenting [2] about possible ABBA deadlock, and suggesting
mutex_trylock(). I think that makes sense. damon_sysfs_lock is always only
mutex_trylock()-ed for the reason.

> + len = sysfs_emit(buf, "%s\n",
> filter->memcg_path ? filter->memcg_path : "");
> + mutex_unlock(&damon_sysfs_lock);
> +
> + return len;
> }

And I'm unsure if this change for memcg_path_show() is really needed.

>
> static ssize_t memcg_path_store(struct kobject *kobj,
> @@ -543,15 +548,20 @@ static ssize_t memcg_path_store(struct kobject *kobj,
> {
> struct damon_sysfs_scheme_filter *filter = container_of(kobj,
> struct damon_sysfs_scheme_filter, kobj);
> - char *path = kmalloc_array(size_add(count, 1), sizeof(*path),
> - GFP_KERNEL);
> + char *path, *old;
>
> + path = kmalloc_array(size_add(count, 1), sizeof(*path), GFP_KERNEL);
> if (!path)
> return -ENOMEM;
>
> strscpy(path, buf, count + 1);
> - kfree(filter->memcg_path);
> +
> + mutex_lock(&damon_sysfs_lock);

Let's use mutex_trylock().

> + old = filter->memcg_path;
> filter->memcg_path = path;
> + mutex_unlock(&damon_sysfs_lock);
> +
> + kfree(old);
> return count;
> }

Same comments for path_show() and path_store() changes.

[1] https://docs.kernel.org/process/submitting-patches.html#commentary
[2] https://lore.kernel.org/20260420183146.0DA7AC2BCB0@smtp.kernel.org

Thanks,
SJ …