Re: [PATCH v15 01/23] drm/shmem-helper: Fix UAF in error path when freeing SGT of imported GEM

[Dmitry Osipenko]

On 8/28/23 14:16, Boris Brezillon wrote:
> On Sun, 27 Aug 2023 20:54:27 +0300
> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> 
>> Freeing drm-shmem GEM right after creating it using
>> drm_gem_shmem_prime_import_sg_table() frees SGT of the imported dma-buf
>> and then dma-buf frees this SGT second time.
>>
>> The v3d_prime_import_sg_table() is example of a error code path where
>> dma-buf's SGT is freed by drm-shmem and then it's freed second time by
>> dma_buf_unmap_attachment() in drm_gem_prime_import_dev().
>>
>> Add drm-shmem GEM flag telling that this is imported SGT shall not be
>> treated as own SGT, fixing the use-after-free bug.
>>
>> Cc: stable@xxxxxxxxxxxxxxx
>> Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
>> ---
>>  drivers/gpu/drm/drm_gem_shmem_helper.c | 3 ++-
>>  include/drm/drm_gem_shmem_helper.h     | 7 +++++++
>>  2 files changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> index a783d2245599..78d9cf2355a5 100644
>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> @@ -141,7 +141,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>>  
>>  	if (obj->import_attach) {
>>  		drm_prime_gem_destroy(obj, shmem->sgt);
>> -	} else {
>> +	} else if (!shmem->imported_sgt) {
>>  		dma_resv_lock(shmem->base.resv, NULL);
>>  
>>  		drm_WARN_ON(obj->dev, shmem->vmap_use_count);
>> @@ -758,6 +758,7 @@ drm_gem_shmem_prime_import_sg_table(struct drm_device *dev,
>>  		return ERR_CAST(shmem);
>>  
>>  	shmem->sgt = sgt;
>> +	shmem->imported_sgt = true;
> 
> 
> I feel like adding more fields that can be used to do the is_imported()
> check is going to be even more confusing. Can we instead have
> 
> 	/* drm_gem_shmem_prime_import_sg_table() can be called from a
> 	 * driver specific ->import_sg_table() implementations that
> 	 * have extra failable initialization steps. Assign
> 	 * drm_gem_object::import_attach here (even though it's
> 	 * assigned in drm_gem_prime_import_dev()), so we don't end up
> 	 * with driver error paths calling drm_gem_shmem_free() with an
> 	 * imported sg_table assigned to drm_gem_shmem_object::sgt and
> 	 * drm_gem_object::import_attach left uninitialized.
> 	 */
> 	shmem->base.import_attach = attach;
> 
> here?

AFAICT, this is not going to work because obj->import_attach will be
released by drm_prime core by the time drm_gem_shmem_free() is invoked
and drm_gem_shmem_free() uses obj->import_attach as well. I'll keep this
patch around unless there will be other suggestions. To me the flag is
good enough, I'll add a clarifying comment to the code in v16.

-- 
Best regards,
Dmitry