linux-kernel - Re: [PATCH] virtio: 9p: correctly pass physical address to userspace for high pages

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Message-ID: <20121017161738.GK5973@mudshark.cambridge.arm.com>
Date:	Wed, 17 Oct 2012 17:17:38 +0100
From:	Will Deacon <will.deacon@....com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Marc Zyngier <Marc.Zyngier@....com>
Subject: Re: [PATCH] virtio: 9p: correctly pass physical address to userspace
 for high pages

On Wed, Oct 17, 2012 at 05:05:00PM +0100, Sasha Levin wrote:
> On Wed, Oct 17, 2012 at 6:14 AM, Will Deacon <will.deacon@....com> wrote:
> > When using a virtio transport, the 9p net device allocates pages to back
> > the descriptors inserted into the virtqueue. These allocations may be
> > performed from atomic context (under the channel lock) and can therefore
> > return high mappings which aren't suitable for virt_to_phys.
> >
> > This patch ensures that virtual addresses for virtio queue descriptors
> > are converted to physical addresses using kmap_to_page, which handles
> > high memory correctly.
> >
> > Cc: Rusty Russell <rusty@...tcorp.com.au>
> > Cc: Sasha Levin <levinsasha928@...il.com>
> > Signed-off-by: Marc Zyngier <marc.zyngier@....com>
> > Signed-off-by: Will Deacon <will.deacon@....com>
> > ---
> 
> One of the things I was seeing when fuzzing within lkvm were random
> crashes resulting
> from "odd" addresses used by virtio-9p queue descriptions. I
> attributed that to memory
> corruption as a result of something else failing the fuzz tests
> before, but this patch
> explains what might have gone wrong.

This is *exactly* what I have observed. More specifically, I hacked kvmtool
to support memory banks above 0x0 (I'll send some patches soon) so my
PHYS_OFFSET was 0x80000000. virtio-9p then gave me physical addresses at
0x7fe02000, which if linearly mapped would be 0xffe02000 -- the pkmap!

> I'll plug it in and run tests on it.

The kernel-janitors list pointed out to me that we need to EXPORT_SYMBOL on
kmap_to_page, so if you're building the virtio ring as a module you'll need
to do that.

Cheers,

Will
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