linux-kernel - Re: [PATCH 07/13] mm: close race in generic_access

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Date:   Wed, 7 Oct 2020 20:21:17 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>, linux-s390@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Rik van Riel <riel@...hat.com>,
        Benjamin Herrensmidt <benh@...nel.crashing.org>,
        Dave Airlie <airlied@...ux.ie>,
        Hugh Dickins <hugh@...itas.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 07/13] mm: close race in generic_access_phys

On Wed, Oct 07, 2020 at 08:01:42PM +0200, Daniel Vetter wrote:
> I think it'd fix the bug, until someone wires ->access up for
> drivers/gpu, or the next subsystem. This is also just for ptrace, so
> we really don't care when we stall the vm badly and other silly
> things. So I figured the somewhat ugly, but full generic solution is
> the better one, so that people who want to be able to ptrace
> read/write their iomem mmaps can just sprinkle this wherever they feel
> like.
> 
> But yeah if we go with most minimal fix, i.e. only trying to fix the
> current users, then your thing should work and is simpler. But it
> leaves the door open for future problems.

The only other idea I had was to fully make the 'vma of __iomem
memory' some generic utility, completely take over the vm_ops.

We did something like this in RDMA, what I found was even just
implementing mmap() using the kernel helpers turned out to be pretty
tricky, many drivers did it wrong in small ways.

Jason