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Date: Sat, 7 Jun 2025 15:04:38 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Tal Zussman <tz2294@columbia.edu>
Cc: Peter Xu <peterx@redhat.com>, "Jason A. Donenfeld" <Jason@zx2c4.com>,
 David Hildenbrand <david@redhat.com>, Alexander Viro
 <viro@zeniv.linux.org.uk>, Christian Brauner <brauner@kernel.org>, Jan Kara
 <jack@suse.cz>, Andrea Arcangeli <aarcange@redhat.com>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 1/4] userfaultfd: correctly prevent registering
 VM_DROPPABLE regions
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On Sat, 07 Jun 2025 02:40:00 -0400 Tal Zussman <tz2294@columbia.edu> wrote:

> vma_can_userfault() masks off non-userfaultfd VM flags from vm_flags.
> The vm_flags & VM_DROPPABLE test will then always be false, incorrectly
> allowing VM_DROPPABLE regions to be registered with userfaultfd.
> 
> Additionally, vm_flags is not guaranteed to correspond to the actual
> VMA's flags. Fix this test by checking the VMA's flags directly.

Wondering if we should backport this.  afaict we don't know the
userspace impact of this because nobody has tried it!