Re: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL_MAPPING

[Ard Biesheuvel]

On Mon, 1 Nov 2021 at 13:21, Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
>
> Hi Ard,
>
> On Mon, 2021-11-01 at 09:34 +0100, Ard Biesheuvel wrote:
> > On Mon, 1 Nov 2021 at 04:17, Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> > wrote:
> > >
> > > DMA_ATTR_NO_KERNEL_MAPPING is to avoid creating a kernel mapping
> > > for the allocated buffer, but current implementation is that
> > > PTE of allocated buffer in kernel page table is valid. So we
> > > should set invalid for PTE of allocate buffer so that there are
> > > no kernel mapping for the allocated buffer.
> > >
> > > In some cases, we don't hope the allocated buffer to be read
> > > by cpu or speculative execution, so we use
> > > DMA_ATTR_NO_KERNEL_MAPPING
> > > to get no kernel mapping in order to achieve this goal.
> > >
> > > Signed-off-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> > > Cc: Christoph Hellwig <hch@xxxxxx>
> > > Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> > > Cc: Robin Murphy <robin.murphy@xxxxxxx>
> > > Cc: Matthias Brugger <matthias.bgg@xxxxxxxxx>
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > ---
> > >  kernel/dma/direct.c | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > >
> > > diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> > > index 4c6c5e0635e3..aa10b4c5d762 100644
> > > --- a/kernel/dma/direct.c
> > > +++ b/kernel/dma/direct.c
> > > @@ -13,6 +13,7 @@
> > >  #include <linux/vmalloc.h>
> > >  #include <linux/set_memory.h>
> > >  #include <linux/slab.h>
> > > +#include <asm/cacheflush.h>
> > >  #include "direct.h"
> > >
> > >  /*
> > > @@ -169,6 +170,9 @@ void *dma_direct_alloc(struct device *dev,
> > > size_t size,
> > >                 if (!PageHighMem(page))
> > >                         arch_dma_prep_coherent(page, size);
> > >                 *dma_handle = phys_to_dma_direct(dev,
> > > page_to_phys(page));
> > > +               /* remove kernel mapping for pages */
> > > +               set_memory_valid((unsigned
> > > long)phys_to_virt(dma_to_phys(dev, *dma_handle)),
> > > +                               size >> PAGE_SHIFT, 0);
> >
> > This only works if the memory is mapped at page granularity in the
> > linear region, and you cannot rely on that. Many architectures prefer
> > block mappings for the linear region, and arm64 will only use page
> > mappings if rodata=full is set (which is set by default but can be
> > overridden on the kernel command line)
> >
>
> We mainly want to solve arm64 arch.

Solve what?

> RODATA_FULL_DEFAULT_ENABLED should
> be the arm64 config. If we use CONFIG_RODATA_FULL_DEFAULT_ENABLED to
> check whether it call set_memory_valid(). It should avoid other
> architectures. Do you think this method is work?
>

No. Try it with rodata=off (or rodata=on for that matter) and see what happens.

> Thanks for your explaination and suggestion.
>

YW

>
> >
> > >                 /* return the page pointer as the opaque cookie */
> > >                 return page;
> > >         }
> > > @@ -278,6 +282,10 @@ void dma_direct_free(struct device *dev,
> > > size_t size,
> > >
> > >         if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
> > >             !force_dma_unencrypted(dev) &&
> > > !is_swiotlb_for_alloc(dev)) {
> > > +               size = PAGE_ALIGN(size);
> > > +               /* create kernel mapping for pages */
> > > +               set_memory_valid((unsigned
> > > long)phys_to_virt(dma_to_phys(dev, dma_addr)),
> > > +                               size >> PAGE_SHIFT, 1);
> > >                 /* cpu_addr is a struct page cookie, not a kernel
> > > address */
> > >                 dma_free_contiguous(dev, cpu_addr, size);
> > >                 return;
> > > --
> > > 2.18.0
> > >
> > >
> > > _______________________________________________
> > > linux-arm-kernel mailing list
> > > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > >
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> > >
>