linux-kernel - Re: [PATCH] tee: handle lookup of shm with reference count 0

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Message-ID: <5beb196e-bb70-f28c-98c9-9e18df164e94@axis.com>
Date:   Tue, 14 Dec 2021 15:54:11 +0100
From:   Lars Persson <larper@...s.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Jens Wiklander <jens.wiklander@...aro.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "op-tee@...ts.trustedfirmware.org" <op-tee@...ts.trustedfirmware.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Christian König <christian.koenig@....com>,
        Rijo Thomas <Rijo-john.Thomas@....com>,
        Devaraj Rangasamy <Devaraj.Rangasamy@....com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Lars Persson <Lars.Persson@...s.com>,
        Patrik Lantz <Patrik.Lantz@...s.com>
Subject: Re: [PATCH] tee: handle lookup of shm with reference count 0



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