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Date: Wed, 7 Oct 2020 20:21:17 -0300
From: Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: [PATCH 07/13] mm: close race in generic_access_phys
Message-ID: <20201007232117.GB5177@ziepe.ca>
References: <20201007164426.1812530-1-daniel.vetter@ffwll.ch>
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To: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: DRI Development <dri-devel@lists.freedesktop.org>, LKML <linux-kernel@vger.kernel.org>, kvm@vger.kernel.org, Linux MM <linux-mm@kvack.org>, Linux ARM <linux-arm-kernel@lists.infradead.org>, linux-samsung-soc <linux-samsung-soc@vger.kernel.org>, "open list:DMA BUFFER SHARING FRAMEWORK" <linux-media@vger.kernel.org>, linux-s390@vger.kernel.org, Dan Williams <dan.j.williams@intel.com>, Kees Cook <keescook@chromium.org>, Rik van Riel <riel@redhat.com>, Benjamin Herrensmidt <benh@kernel.crashing.org>, Dave Airlie <airlied@linux.ie>, Hugh Dickins <hugh@veritas.com>, Andrew Morton <akpm@linux-foundation.org>, John Hubbard <jhubbard@nvidia.com>, =?utf-8?B?SsOpcsO0bWU=?= Glisse <jglisse@redhat.com>, Jan Kara <jack@suse.cz>, Daniel Vetter <daniel.vetter@intel.com>

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