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Date: Mon, 30 Mar 2026 13:40:21 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: David Carlier <devnexen@gmail.com>
Cc: Peter Xu <peterx@redhat.com>, Mike Rapoport <rppt@kernel.org>,
 linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mm/userfaultfd: detect VMA replacement after copy
 retry in mfill_copy_folio_retry()
Message-Id: <20260330134021.171441c4c236b03efebc9a77@linux-foundation.org>
In-Reply-To: <20260330202909.136776-1-devnexen@gmail.com>
References: <20260330202909.136776-1-devnexen@gmail.com>
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On Mon, 30 Mar 2026 21:29:09 +0100 David Carlier <devnexen@gmail.com> wrote:

> In mfill_copy_folio_retry(), all locks are dropped to retry
> copy_from_user() with page faults enabled. During this window, the VMA
> can be replaced entirely (e.g. munmap + mmap + UFFDIO_REGISTER by
> another thread), but the caller proceeds with a folio allocated from the
> original VMA's backing store.
> 
> Checking ops alone is insufficient: the replacement VMA could be the
> same type (e.g. shmem -> shmem) with identical flags but a different
> backing inode. Take a snapshot of the VMA's inode and flags before
> dropping locks, and compare after re-acquiring them. If anything
> changed, bail out with -EAGAIN.

Thanks.  What are the userspace-visible runtime effects of the bug?

If they're serious we might be looking at a cc:stable and a
Fixes: tag?