linux-kernel - Re: [PULL] topic/iomem-mmap-vs-gup

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Date:   Mon, 10 May 2021 12:34:12 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>
Subject: Re: [PULL] topic/iomem-mmap-vs-gup

On Mon, May 10, 2021 at 04:55:39PM +0200, Daniel Vetter wrote:

> yeah vfio is still broken for the case I care about. I think there's
> also some questions open still about whether kvm really uses
> mmu_notifier in all cases correctly, 

IIRC kvm doesn't either.

> > Daniel I suppose we missed this relation to follow_pte(), so I agree
> > that keeping a unsafe_follow_pfn() around is not good.
> 
> tbh I never really got the additional issue with the missing write
> checks. That users of follow_pfn (or well follow_pte + immediate lock
> dropping like vfio) don't subscribe to the pte updates in general is
> the bug I'm seeing. That v4l also glosses over the read/write access
> stuff is kinda just the icing on the cake :-) It's pretty well broken
> even if it would check that.

It is just severity. Exploiting the use after free bug is somewhat
harder, exploiting the 'you can write to non-page write protected
memory' bug is not so hard.

Jason