diff mbox series

[13/24] dma-buf: drop the DAG approach for the dma_resv object

Message ID 20211207123411.167006-14-christian.koenig@amd.com
State New
Headers show
Series [01/24] dma-buf: add dma_resv_replace_fences | expand

Commit Message

Christian König Dec. 7, 2021, 12:34 p.m. UTC
So far we had the approach of using a directed acyclic
graph with the dma_resv obj.

This turned out to have many downsides, especially it means
that every single driver and user of this interface needs
to be aware of this restriction when adding fences. If the
rules for the DAG are not followed then we end up with
potential hard to debug memory corruption, information
leaks or even elephant big security holes because we allow
userspace to access freed up memory.

Since we already took a step back from that by always
looking at all fences we now go a step further and stop
dropping the shared fences when a new exclusive one is
added.

Signed-off-by: Christian König <christian.koenig@amd.com>
---
 drivers/dma-buf/dma-resv.c | 13 -------------
 1 file changed, 13 deletions(-)

Comments

Christian König Jan. 4, 2022, 3:08 p.m. UTC | #1
Am 22.12.21 um 22:43 schrieb Daniel Vetter:
> On Tue, Dec 07, 2021 at 01:34:00PM +0100, Christian König wrote:
>> So far we had the approach of using a directed acyclic
>> graph with the dma_resv obj.
>>
>> This turned out to have many downsides, especially it means
>> that every single driver and user of this interface needs
>> to be aware of this restriction when adding fences. If the
>> rules for the DAG are not followed then we end up with
>> potential hard to debug memory corruption, information
>> leaks or even elephant big security holes because we allow
>> userspace to access freed up memory.
>>
>> Since we already took a step back from that by always
>> looking at all fences we now go a step further and stop
>> dropping the shared fences when a new exclusive one is
>> added.
>>
>> Signed-off-by: Christian König <christian.koenig@amd.com>
>> ---
>>   drivers/dma-buf/dma-resv.c | 13 -------------
>>   1 file changed, 13 deletions(-)
>>
>> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
>> index 9acceabc9399..ecb2ff606bac 100644
>> --- a/drivers/dma-buf/dma-resv.c
>> +++ b/drivers/dma-buf/dma-resv.c
> No doc update at all!

Scratching my head I'm pretty sure I've updated at least the kerneldoc 
for dma_resv_add_excl_fence(). Must have gone lost in some rebase.

>
> I checked, we're not that shitty with docs,

Well I wouldn't say shitty, but they are not perfect either.

>   Minimally the DOC: section
> header and also the struct dma_resv kerneldoc. Also there's maybe more
> references and stuff I've missed on a quick look, please check for them
> (e.g. dma_buf.resv kerneldoc is rather important to keep correct too).
>
> Code itself does what it says in the commit message, but we really need
> the most accurate docs we can get for this stuff, or the confusion will
> persist :-/

Yeah completely agree, going to fix that.

Thanks,
Christian.

>
> Cheers, Daniel
>
>> @@ -383,29 +383,16 @@ EXPORT_SYMBOL(dma_resv_replace_fences);
>>   void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
>>   {
>>   	struct dma_fence *old_fence = dma_resv_excl_fence(obj);
>> -	struct dma_resv_list *old;
>> -	u32 i = 0;
>>   
>>   	dma_resv_assert_held(obj);
>>   
>> -	old = dma_resv_shared_list(obj);
>> -	if (old)
>> -		i = old->shared_count;
>> -
>>   	dma_fence_get(fence);
>>   
>>   	write_seqcount_begin(&obj->seq);
>>   	/* write_seqcount_begin provides the necessary memory barrier */
>>   	RCU_INIT_POINTER(obj->fence_excl, fence);
>> -	if (old)
>> -		old->shared_count = 0;
>>   	write_seqcount_end(&obj->seq);
>>   
>> -	/* inplace update, no shared fences */
>> -	while (i--)
>> -		dma_fence_put(rcu_dereference_protected(old->shared[i],
>> -						dma_resv_held(obj)));
>> -
>>   	dma_fence_put(old_fence);
>>   }
>>   EXPORT_SYMBOL(dma_resv_add_excl_fence);
>> -- 
>> 2.25.1
>>
diff mbox series

Patch

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 9acceabc9399..ecb2ff606bac 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -383,29 +383,16 @@  EXPORT_SYMBOL(dma_resv_replace_fences);
 void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
 {
 	struct dma_fence *old_fence = dma_resv_excl_fence(obj);
-	struct dma_resv_list *old;
-	u32 i = 0;
 
 	dma_resv_assert_held(obj);
 
-	old = dma_resv_shared_list(obj);
-	if (old)
-		i = old->shared_count;
-
 	dma_fence_get(fence);
 
 	write_seqcount_begin(&obj->seq);
 	/* write_seqcount_begin provides the necessary memory barrier */
 	RCU_INIT_POINTER(obj->fence_excl, fence);
-	if (old)
-		old->shared_count = 0;
 	write_seqcount_end(&obj->seq);
 
-	/* inplace update, no shared fences */
-	while (i--)
-		dma_fence_put(rcu_dereference_protected(old->shared[i],
-						dma_resv_held(obj)));
-
 	dma_fence_put(old_fence);
 }
 EXPORT_SYMBOL(dma_resv_add_excl_fence);