Re: [PATCH v5 14/16] dma-direct: Allocate memory from restricted DMA pool if available

[Claire Chang <tientzu@chromium.org>]

On Fri, Apr 23, 2021 at 9:46 PM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2021-04-22 09:15, Claire Chang wrote:
> > The restricted DMA pool is preferred if available.
> >
> > The restricted DMA pools provide a basic level of protection against the
> > DMA overwriting buffer contents at unexpected times. However, to protect
> > against general data leakage and system memory corruption, the system
> > needs to provide a way to lock down the memory access, e.g., MPU.
> >
> > Signed-off-by: Claire Chang <tientzu@chromium.org>
> > ---
> >   kernel/dma/direct.c | 35 ++++++++++++++++++++++++++---------
> >   1 file changed, 26 insertions(+), 9 deletions(-)
> >
> > diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> > index 7a27f0510fcc..29523d2a9845 100644
> > --- a/kernel/dma/direct.c
> > +++ b/kernel/dma/direct.c
> > @@ -78,6 +78,10 @@ static bool dma_coherent_ok(struct device *dev, phys_addr_t phys, size_t size)
> >   static void __dma_direct_free_pages(struct device *dev, struct page *page,
> >                                   size_t size)
> >   {
> > +#ifdef CONFIG_DMA_RESTRICTED_POOL
> > +     if (swiotlb_free(dev, page, size))
> > +             return;
> > +#endif
> >       dma_free_contiguous(dev, page, size);
> >   }
> >
> > @@ -92,7 +96,17 @@ static struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
> >
> >       gfp |= dma_direct_optimal_gfp_mask(dev, dev->coherent_dma_mask,
> >                                          &phys_limit);
> > -     page = dma_alloc_contiguous(dev, size, gfp);
> > +
> > +#ifdef CONFIG_DMA_RESTRICTED_POOL
> > +     page = swiotlb_alloc(dev, size);
> > +     if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) {
> > +             __dma_direct_free_pages(dev, page, size);
> > +             page = NULL;
> > +     }
> > +#endif
> > +
> > +     if (!page)
> > +             page = dma_alloc_contiguous(dev, size, gfp);
> >       if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) {
> >               dma_free_contiguous(dev, page, size);
> >               page = NULL;
> > @@ -148,7 +162,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
> >               gfp |= __GFP_NOWARN;
> >
> >       if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
> > -         !force_dma_unencrypted(dev)) {
> > +         !force_dma_unencrypted(dev) && !is_dev_swiotlb_force(dev)) {
> >               page = __dma_direct_alloc_pages(dev, size, gfp & ~__GFP_ZERO);
> >               if (!page)
> >                       return NULL;
> > @@ -161,8 +175,8 @@ void *dma_direct_alloc(struct device *dev, size_t size,
> >       }
> >
> >       if (!IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED) &&
> > -         !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
> > -         !dev_is_dma_coherent(dev))
> > +         !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) && !dev_is_dma_coherent(dev) &&
> > +         !is_dev_swiotlb_force(dev))
> >               return arch_dma_alloc(dev, size, dma_handle, gfp, attrs);
> >
> >       /*
> > @@ -172,7 +186,9 @@ void *dma_direct_alloc(struct device *dev, size_t size,
> >       if (IS_ENABLED(CONFIG_DMA_COHERENT_POOL) &&
> >           !gfpflags_allow_blocking(gfp) &&
> >           (force_dma_unencrypted(dev) ||
> > -          (IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) && !dev_is_dma_coherent(dev))))
> > +          (IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
> > +           !dev_is_dma_coherent(dev))) &&
> > +         !is_dev_swiotlb_force(dev))
> >               return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
> >
> >       /* we always manually zero the memory once we are done */
> > @@ -253,15 +269,15 @@ void dma_direct_free(struct device *dev, size_t size,
> >       unsigned int page_order = get_order(size);
> >
> >       if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
> > -         !force_dma_unencrypted(dev)) {
> > +         !force_dma_unencrypted(dev) && !is_dev_swiotlb_force(dev)) {
> >               /* cpu_addr is a struct page cookie, not a kernel address */
> >               dma_free_contiguous(dev, cpu_addr, size);
> >               return;
> >       }
> >
> >       if (!IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED) &&
> > -         !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
> > -         !dev_is_dma_coherent(dev)) {
> > +         !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) && !dev_is_dma_coherent(dev) &&
> > +         !is_dev_swiotlb_force(dev)) {
> >               arch_dma_free(dev, size, cpu_addr, dma_addr, attrs);
> >               return;
> >       }
> > @@ -289,7 +305,8 @@ struct page *dma_direct_alloc_pages(struct device *dev, size_t size,
> >       void *ret;
> >
> >       if (IS_ENABLED(CONFIG_DMA_COHERENT_POOL) &&
> > -         force_dma_unencrypted(dev) && !gfpflags_allow_blocking(gfp))
> > +         force_dma_unencrypted(dev) && !gfpflags_allow_blocking(gfp) &&
> > +         !is_dev_swiotlb_force(dev))
> >               return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
>
> Wait, this seems broken for non-coherent devices - in that case we need
> to return a non-cacheable address, but we can't simply fall through into
> the remapping path below in GFP_ATOMIC context. That's why we need the
> atomic pool concept in the first place :/

Sorry for the late reply. I'm not very familiar with this. I wonder if
the memory returned here must be coherent. If yes, could we say for
this case, one must set up another device coherent pool
(shared-dma-pool) and go with dma_alloc_from_dev_coherent()[1]?

[1] https://elixir.bootlin.com/linux/v5.12/source/kernel/dma/mapping.c#L435

>
> Unless I've overlooked something, we're still using the regular
> cacheable linear map address of the dma_io_tlb_mem buffer, no?
>
> Robin.
>
> >
> >       page = __dma_direct_alloc_pages(dev, size, gfp);
> >