Re: [PATCH next v2] dma-buf/sync-file: do not allow zero size allocation

["Christian König" <christian.koenig@amd.com>]

Am 01.04.22 um 23:31 schrieb Pavel Skripkin:
> num_fences is user-controlled value and it can be equal to 0. Code
> should not pass 0 to kcalloc(), since it will cause kcalloc() to return
> ZERO_PTR. ZERO_PTR will pass `!fences` check and kernel will panic
> because of dereferencing ZERO_PTR in add_fence()
>
> Fix it by validating num_fences and bail out early if it is equal to 0

Well there are multiple issues with this patch. First of all as I wrote 
before it shouldn't be possible that num_fences is zero.

We could still just add this as a precaution, but then bailing out is 
the wrong thing to do here. Instead we should then make sure to allocate 
at least one slot for a fence in the array.

But I think the cleanest would just be to not add a fence into the array 
in the first place when num_fences is zero.

Regards,
Christian.

>
> Fixes: 519f490db07e ("dma-buf/sync-file: fix warning about fence containers")
> Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
> ---
>
> Changes since v1:
> 	- Dropped already merged part
> 	- Removed syzkaller's tag
>
> ---
>   drivers/dma-buf/sync_file.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
> index b8dea4ec123b..024f22193e0c 100644
> --- a/drivers/dma-buf/sync_file.c
> +++ b/drivers/dma-buf/sync_file.c
> @@ -212,7 +212,7 @@ static struct sync_file *sync_file_merge(const char *name, struct sync_file *a,
>   	dma_fence_unwrap_for_each(b_fence, &b_iter, b->fence)
>   		++num_fences;
>   
> -	if (num_fences > INT_MAX)
> +	if (num_fences > INT_MAX || !num_fences)
>   		goto err_free_sync_file;
>   
>   	fences = kcalloc(num_fences, sizeof(*fences), GFP_KERNEL);