linux-kernel - Re: [PATCH] mm/userfaultfd: Reset ptes when close() for wr-protected ones

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Message-Id: <20240422124719.5097e42a736403d306ba7cf0@linux-foundation.org>
Date: Mon, 22 Apr 2024 12:47:19 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, Nadav Amit
 <nadav.amit@...il.com>, David Hildenbrand <david@...hat.com>,
 syzbot+d8426b591c36b21c750e@...kaller.appspotmail.com
Subject: Re: [PATCH] mm/userfaultfd: Reset ptes when close() for
 wr-protected ones

On Mon, 22 Apr 2024 09:33:11 -0400 Peter Xu <peterx@...hat.com> wrote:

> Userfaultfd unregister includes a step to remove wr-protect bits from all
> the relevant pgtable entries, but that only covered an explicit
> UFFDIO_UNREGISTER ioctl, not a close() on the userfaultfd itself.  Cover
> that too.

We should include a description of the userspace-visible effects of the
bug, please.  Always.

I see it triggers a WARN, but so what - why ca't we simply delete the
WARN statement if that's the only effect?  Presumably there are other
consequences - what are they?

Also, a WARN-triggering bug should be fixed in -stable kernels so we'll
need a FIXES:, please?