linux-kernel - Re: [PATCH v2 1/4] userfaultfd: correctly prevent registering VM

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Message-ID: <aERPTlivjt16jCVH@zx2c4.com>
Date: Sat, 7 Jun 2025 08:40:14 -0600
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Tal Zussman <tz2294@...umbia.edu>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.com>,
	David Hildenbrand <david@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] userfaultfd: correctly prevent registering
 VM_DROPPABLE regions

On Sat, Jun 07, 2025 at 02:40:00AM -0400, Tal Zussman 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.
> 
> Link: https://lore.kernel.org/linux-mm/5a875a3a-2243-4eab-856f-bc53ccfec3ea@redhat.com/
> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
> Acked-by: David Hildenbrand <david@...hat.com>
> Acked-by: Peter Xu <peterx@...hat.com>
> Signed-off-by: Tal Zussman <tz2294@...umbia.edu>

Nice catch and thanks for fixing this.

Acked-by: Jason A. Donenfeld <Jason@...c4.com>