linux-kernel - Re: [PATCH 0/2] mm/userfaultfd: Fix uninitialized output field for -EAGAIN race

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Message-ID: <dfc6db2d-0af2-44c2-8582-7e783b0292ab@redhat.com>
Date: Fri, 25 Apr 2025 17:58:19 +0200
From: David Hildenbrand <david@...hat.com>
To: James Houghton <jthoughton@...gle.com>, Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Axel Rasmussen <axelrasmussen@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/2] mm/userfaultfd: Fix uninitialized output field for
 -EAGAIN race

On 25.04.25 17:45, James Houghton wrote:
> On Thu, Apr 24, 2025 at 5:57 PM Peter Xu <peterx@...hat.com> wrote:
>>
>> When discussing some userfaultfd issues with Andrea, Andrea pointed out an
>> ABI issue with userfaultfd that existed for years.  Luckily the issue
>> should only be a very corner case one, and the fix (even if changing the
>> kernel ABI) should only be in the good way, IOW there should have no risk
>> breaking any userapp but only fixing.
> 
> FWIW, my userspace basically looks like this:
> 
> struct uffdio_continue uffdio_continue;
> int64_t target_len = /* whatever */;
> int64_t bytes_mapped = 0;
> int ioctl_ret;
> do {
>    uffdio_continue.range = /* whatever */;
>    uffdio_continue.mapped = 0;
>    ioctl_ret = ioctl(uffd, UFFDIO_CONTINUE, &uffdio_continue);
>    if (uffdio_continue.mapped < 0) { break; }
>    bytes_mapped += uffdio_continue.mapped;
> } while (bytes_mapped < target_len && errno == EAGAIN);
> 
> I think your patch would indeed break this. (Perhaps I shouldn't be
> reading from `mapped` without first checking that errno == EAGAIN.)
> 
> Well, that's what I would say, except in practice I never actually hit
> the mmap_changing case while invoking UFFDIO_CONTINUE. :)

Hm, but what if mfill_atomic_continue() would already return -EAGAIN 
when checking mmap_changing etc?

Wouldn't code already run into an issue there?

-- 
Cheers,

David / dhildenb