[PATCH] media: mc: Fix potential media_device lifetime race

[Haotian Zhang <vulab@iscas.ac.cn>]

__media_device_get() traverses the global media_device_list under
media_device_lock and takes a reference on a matching instance with an
unconditional kref_get(). media_device_delete() drops the last
reference with kref_put() outside the lock, so the refcount can reach
zero while the instance is still on the list; if a concurrent
allocate() attempts to acquire a reference from such an instance,
kref_get() operates on a zero refcount while release() is already in
progress. The allocator may then return an instance that release()
subsequently frees, leading to a use-after-free.

Use kref_get_unless_zero() in __media_device_get() when acquiring the
reference. If the reference count has already dropped to zero, release
is in progress, so skip the entry and allocate a fresh instance
instead.

Fixes: 6e1d824e7a1d ("media: Media Device Allocator API")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
---
 drivers/media/mc/mc-dev-allocator.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/media/mc/mc-dev-allocator.c b/drivers/media/mc/mc-dev-allocator.c
index ae17887dec59..8b97372adc83 100644
--- a/drivers/media/mc/mc-dev-allocator.c
+++ b/drivers/media/mc/mc-dev-allocator.c
@@ -68,7 +68,8 @@ static struct media_device *__media_device_get(struct device *dev,
 		if (mdi->mdev.dev != dev)
 			continue;
 
-		kref_get(&mdi->refcount);
+		if (!kref_get_unless_zero(&mdi->refcount))
+			continue;
 
 		/* get module reference for the media_device owner */
 		if (owner != mdi->owner && !try_module_get(mdi->owner))
-- 
2.43.0