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

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Message-ID: <CADrL8HXuZkX0CP6apHLw0A0Ax4b4a+-=XEt0dH5mAKiN7hBv3w@mail.gmail.com>
Date: Fri, 25 Apr 2025 11:45:15 -0400
From: James Houghton <jthoughton@...gle.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Mike Rapoport <rppt@...nel.org>, David Hildenbrand <david@...hat.com>, 
	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 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. :)