linux-kernel - Re: [PATCH] vfio/type1: Empty batch for pfnmap pages

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Date:   Thu, 25 Mar 2021 13:25:51 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     Cornelia Huck <cohuck@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Steven Sistare <steven.sistare@...cle.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfio/type1: Empty batch for pfnmap pages

On Wed, 24 Mar 2021 21:05:52 -0400
Daniel Jordan <daniel.m.jordan@...cle.com> wrote:

> When vfio_pin_pages_remote() returns with a partial batch consisting of
> a single VM_PFNMAP pfn, a subsequent call will unfortunately try
> restoring it from batch->pages, resulting in vfio mapping the wrong page
> and unbalancing the page refcount.
> 
> Prevent the function from returning with this kind of partial batch to
> avoid the issue.  There's no explicit check for a VM_PFNMAP pfn because
> it's awkward to do so, so infer it from characteristics of the batch
> instead.  This may result in occasional false positives but keeps the
> code simpler.
> 
> Fixes: 4d83de6da265 ("vfio/type1: Batch page pinning")
> Link: https://lkml.kernel.org/r/20210323133254.33ed9161@omen.home.shazbot.org/
> Reported-by: Alex Williamson <alex.williamson@...hat.com>
> Suggested-by: Alex Williamson <alex.williamson@...hat.com>
> Signed-off-by: Daniel Jordan <daniel.m.jordan@...cle.com>
> ---
> 
> Alex, I couldn't immediately find a way to trigger this bug, but I can
> run your test case if you like.
> 
> This is the minimal fix, but it should still protect all calls of
> vfio_batch_unpin() from this problem.

Thanks, applied to my for-linus branch for v5.12.  The attached unit
test triggers the issue, I don't have any real world examples and was
only just experimenting with this for another series earlier this week.
Thanks,

Alex

View attachment "alternate-pfnmap.c" of type "text/x-c++src" (8080 bytes)