Re: [PATCH] dma-direct: zero out DMA_ATTR_NO_KERNEL_MAPPING buf

[James Bottomley]

On Sat, 2020-09-05 at 15:35 +0800, Hillf Danton wrote:
> On Fri, 04 Sep 2020 08:34:39 -0700 James Bottomley wrote:
> > On Fri, 2020-09-04 at 23:25 +0800, Hillf Danton wrote:
> > > The DMA buffer allocated is always cleared in DMA core and this
> > > is making DMA_ATTR_NO_KERNEL_MAPPING non-special.
> > > 
> > > Fixes: d98849aff879 ("dma-direct: handle
> > > DMA_ATTR_NO_KERNEL_MAPPING
> > > in common code")
> > > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> > > Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Hillf Danton <hdanton@xxxxxxxx>
> > > ---
> > > 
> > > --- a/kernel/dma/direct.c
> > > +++ b/kernel/dma/direct.c
> > > @@ -178,9 +178,17 @@ void *dma_direct_alloc_pages(struct devi
> > >  
> > >  	if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
> > >  	    !force_dma_unencrypted(dev)) {
> > > +		int i;
> > > +
> > >  		/* remove any dirty cache lines on the kernel
> > > alias
> > > */
> > >  		if (!PageHighMem(page))
> > >  			arch_dma_prep_coherent(page, size);
> > > +
> > > +		for (i = 0; i < size/PAGE_SIZE; i++) {
> > > +			ret = kmap_atomic(page + i);
> > > +			memset(ret, 0, PAGE_SIZE);
> > > +			kunmap_atomic(ret);
> 
> Hi James
> > 
> > This is massively expensive on PARISC and likely other VIPT/VIVT
> > architectures.
> 
> Correct.
>
> > What's the reason for clearing it?  This could also be
> 
> 	/* we always manually zero the memory once we are done: */
> 	gfp &= ~__GFP_ZERO;
> 	gfp |= dma_direct_optimal_gfp_mask(dev, dev->coherent_dma_mask,
> 					   &phys_limit);

That's not a reason ... that comment was put in for coherent mappings. 
What is the reason we should incur all this expense for clearing pages
which aren't unmapped in the kernel, because we can update the comment?
 The usual rationale for kernel mapped pages is security, because they
may leak information but unmapped pages shouldn't have this problem.

> > really inefficient even on PIPT architectures if the memory is
> > device remote.
> > 
> > If we really have to do this, it should likely be done in the arch
> > or driver hooks because there are potentially more efficient ways
> > we can do this knowing how the architecture behaves.
> 
> I'm open to any vintage ideas in your mind wrt clearing dma buf e.g
> on platforms like PARISC. Or feel free to offload me the work if it
> makes sense to you who are rich of PARISC knowledge.

OK, I've cc'd linux-arch because this is a problem for more than just
parisc.  However, not having to do it is the best solution ... sort of
the doctor, doctor it hurts when I do this answer.

James