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mm/damon/sysfs: fix use-after-free in state_show()

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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: a61ea561c8 ("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>
This commit is contained in:
Stanislav Fort 2025-09-05 13:10:46 +03:00 committed by Andrew Morton
parent 0ce9398aa0
commit 3260a3f082
1 changed files with 9 additions and 5 deletions
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14
mm/damon/sysfs.c
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@ -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]);