On 2021-12-14 14:44, Greg KH wrote:
On Tue, Dec 14, 2021 at 01:35:40PM +0100, Jens Wiklander wrote:
Since the tee subsystem does not keep a strong reference to its idle
shared memory buffers, it races with other threads that try to destroy a
shared memory through a close of its dma-buf fd or by unmapping the
memory.
In tee_shm_get_from_id() when a lookup in teedev->idr has been
successful, it is possible that the tee_shm is in the dma-buf teardown
path, but that path is blocked by the teedev mutex. Since we don't have
an API to tell if the tee_shm is in the dma-buf teardown path or not we
must find another way of detecting this condition.
Fix this by doing the reference counting directly on the tee_shm using a
new refcount_t refcount field. dma-buf is replaced by using
anon_inode_getfd() instead, this separates the life-cycle of the
underlying file from the tee_shm. tee_shm_put() is updated to hold the
mutex when decreasing the refcount to 0 and then remove the tee_shm from
teedev->idr before releasing the mutex. This means that the tee_shm can
never be found unless it has a refcount larger than 0.
So you are dropping dma-buf support entirely? And anon_inode_getfd()
works instead? Why do more people not do this as well?
Indeed, thinking about it, does it really makes sense to do mmap() on an
anon_inode_getfd() fd ? It is a singleton inode used there so don't we
breach some contract with the linux mm ? The dma-buf code for creating
the file object is more complex, it creates a unique inode for each object.
I am by no means claiming to understand inodes' interaction with mmap,
just sharing a concern that popped up in my head.
- Lars
