Re: [PATCH] userfaultfd: fix mmap_changing checking in mfill_atomic_hugetlb

[Axel Rasmussen <axelrasmussen@google.com>]

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On Thu, Jan 18, 2024 at 1:59 PM Andrew Morton <akpm@linux-foundation.org>
wrote:

> On Wed, 17 Jan 2024 14:37:29 -0800 Lokesh Gidra <lokeshgidra@google.com>
> wrote:
>
> > In mfill_atomic_hugetlb(), mmap_changing isn't being checked
> > again if we drop mmap_lock and reacquire it. When the lock is not held,
> > mmap_changing could have been incremented. This is also inconsistent
> > with the behavior in mfill_atomic().
>

The change looks reasonable to me. I'm not sure I can conclusively say
there isn't some other mechanism specific to hugetlbfs which means this
isn't needed, though.


>
> Thanks. Could you and reviewers please consider
>
> - what might be the userspace-visible runtime effects?
>
> - Should the fix be backported into earlier kernels?
>
> - A suitable Fixes: target?
>

Hmm, 60d4d2d2b40e4 added __mcopy_atomic_hugetlb without this. But, at that
point in history, none of the other functions had mmap_changing either.

So, I think the right Fixes: target is df2cc96e77011 ("userfaultfd: prevent
non-cooperative events vs mcopy_atomic races") ? It seems to have missed
the hugetlb path. This was introduced in 4.18.

Based on that commit's message, essentially what can happen if the race
"succeeds" is, memory can be accessed without userfaultfd being notified of
this fact. Depending on what userfaultfd is being used for, from
userspace's perspective this can appear like memory corruption for example.
So, based on that it seems to me reasonable to backport this to stable
kernels (4.19+).

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