[PATCH] arm64: dma-mapping: clear buffers allocated with FORCE_CONTIGUOUS flag

[will.deacon@arm.com (Will Deacon)]

On Mon, Jun 11, 2018 at 09:55:54AM +0200, Geert Uytterhoeven wrote:
> Hi Marek,
> 
> Thanks for your patch!
> 
> On Mon, Jun 11, 2018 at 7:14 AM Marek Szyprowski
> <m.szyprowski@samsung.com> wrote:
> > dma_alloc_*() buffers might be exposed to userspace via mmap() call, so
> > they should be cleared on allocation. In case of IOMMU-based dma-mapping
> > implementation such buffer clearing was missing in the code path for
> > DMA_ATTR_FORCE_CONTIGUOUS flag handling. This patch fixes this issue. For
> 
> Is it? The memory is allocated using dma_alloc_from_contiguous(..., gfp),
> and __iommu_alloc_attrs() has
> 
>         /*
>          * Some drivers rely on this, and we probably don't want the
>          * possibility of stale kernel data being read by devices anyway.
>          */
>         gfp |= __GFP_ZERO;
> 
> at the top, before the allocation.
> 
> If cma_alloc() (called from dma_alloc_from_contiguous()) doesn't honor
> __GFP_ZERO, I think cma_alloc() should be fixed instead.

Agreed. We tried to fix this in 7132813c3845 ("arm64: Honor __GFP_ZERO in
dma allocations"). Has something broken that?

Will