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

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 occur with other functions that free or replace the context
while holding damon_sysfs_lock, such as damon_sysfs_kdamonds_rm_dirs()
and damon_sysfs_kdamond_release().

Fix this by acquiring damon_sysfs_lock before accessing the context,
mirroring the locking pattern used in pid_show().

This vulnerability was present when state_show() was first introduced to
access kdamond->damon_ctx.

Fixes: a61ea561c871 ("mm/damon/sysfs: link DAMON for virtual address spaces monitoring")
Reported-by: Stanislav Fort <disclosure@aisle.com>
Signed-off-by: Stanislav Fort <disclosure@aisle.com>
---
 mm/damon/sysfs.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 1234567..abcdef0 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1258,17 +1258,24 @@ static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
 		char *buf)
 {
 	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 (!mutex_trylock(&damon_sysfs_lock))
+		return -EBUSY;
+
+	ctx = kdamond->damon_ctx;
+	if (ctx)
+		running = damon_is_running(ctx);
 
-	if (!ctx)
-		running = false;
-	else
-		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]);
 }
 
 static int damon_sysfs_set_attrs(struct damon_ctx *ctx,
-- 
2.34.1

On Thu,  4 Sep 2025 20:55:49 +0300 Stanislav Fort <stanislav.fort@aisle.com> wrote:

> state_show() currently reads kdamond->damon_ctx without holding
> damon_sysfs_lock. This creates a use-after-free race condition:
> 
> 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 occur with other functions that free or replace the context
> while holding damon_sysfs_lock, such as damon_sysfs_kdamonds_rm_dirs()
> and damon_sysfs_kdamond_release().
> 
> Fix this by acquiring damon_sysfs_lock before accessing the context,
> mirroring the locking pattern used in pid_show().
> 
> This vulnerability was present when state_show() was first introduced to
> access kdamond->damon_ctx.

Nice catch, thank you!

checkpatch.pl complains as below, though:

WARNING: Reported-by: should be immediately followed by Closes: with a URL to the report
#34:
Reported-by: Stanislav Fort <disclosure@aisle.com>
Signed-off-by: Stanislav Fort <disclosure@aisle.com>

ERROR: patch seems to be corrupt (line wrapped?)
#77: FILE: mm/damon/sysfs.c:1279:
2.34.1

WARNING: From:/Signed-off-by: email address mismatch: 'From: Stanislav Fort <stanislav.fort@aisle.com>' != 'Signed-off-by: Stanislav Fort <disclosure@aisle.com>'

I know the reporting was made in a non-public mailing list.  Could you please
add a context though, e.g.,

Closes: N/A # non-publicly reported

The second and third ones should be properly fixed.

> 
> Fixes: a61ea561c871 ("mm/damon/sysfs: link DAMON for virtual address spaces monitoring")
> Reported-by: Stanislav Fort <disclosure@aisle.com>
> Signed-off-by: Stanislav Fort <disclosure@aisle.com>
> ---
>  mm/damon/sysfs.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
> index 1234567..abcdef0 100644
> --- a/mm/damon/sysfs.c
> +++ b/mm/damon/sysfs.c
> @@ -1258,17 +1258,24 @@ static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
>  		char *buf)
>  {
>  	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 (!mutex_trylock(&damon_sysfs_lock))
> +		return -EBUSY;
> +
> +	ctx = kdamond->damon_ctx;
> +	if (ctx)
> +		running = damon_is_running(ctx);
>  
> -	if (!ctx)
> -		running = false;
> -	else
> -		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]);
>  }

Other than the checkpatch issue, the change looks good to me.


Thanks,
SJ

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