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Subject[PATCH v2] mm/damon/sysfs: fix use-after-free in state_show()
Date
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.

Fixes: a61ea561c871 ("mm/damon/sysfs: link DAMON for virtual address spaces monitoring")
Reported-by: Stanislav Fort <disclosure@aisle.com>
Closes: N/A # non-publicly reported
Signed-off-by: Stanislav Fort <disclosure@aisle.com>
---
mm/damon/sysfs.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

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]);
--
2.39.3 (Apple Git-146)

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