linux-kernel - Re: fs: use-after-free in userfaultfd

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Date:   Thu, 2 Mar 2017 00:13:34 +0100
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: fs: use-after-free in userfaultfd_exit

On Wed, Mar 01, 2017 at 07:48:00PM +0100, Dmitry Vyukov wrote:
> Hello,
> 
> I've got the following use-after-free report while running syzkaller
> fuzzer on 86292b33d4b79ee03e2f43ea0381ef85f077c760:

Yes, I posted the fix for this one last Friday, I found it during
stress testing, it triggered the first time post-upstream merging
despite I was running the same stress testing with SLUB poisoning
enabled before.

This affects all apps, also the ones that don't use userfaultfd, it's
a locking issue. Furthermore the cost of userfaultfd_exit was not
acceptable, if something it had to be activated by a flag in mm->flags
(such an optimization would have been absolutely trivial though).

Thankfully I realized another feature (UFFDIO_COPY -ENOSPC retval) can
provide the same information at zero cost so I could drop
userfaultfd_exit as a whole.

https://marc.info/?l=linux-mm&m=148796041217814&w=2

The fix is already included in -mm along with the other fix for
VM_FAULT_NOPAGE.

Thanks,
Andrea