linux-kernel - Re: [PATCH 1/3] userfaultfd: correctly prevent registering VM

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Message-ID: <5de3da6b-7c5f-4d40-b3a7-f6a0f5cbc6da@redhat.com>
Date: Wed, 4 Jun 2025 15:19:34 +0200
From: David Hildenbrand <david@...hat.com>
To: Tal Zussman <tz2294@...umbia.edu>,
 Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.com>,
 "Jason A. Donenfeld" <Jason@...c4.com>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Pavel Emelyanov <xemul@...allels.com>, Andrea Arcangeli <aarcange@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/3] userfaultfd: correctly prevent registering
 VM_DROPPABLE regions

On 04.06.25 00:14, 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")
> Signed-off-by: Tal Zussman <tz2294@...umbia.edu>
> ---
>   include/linux/userfaultfd_k.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index 75342022d144..f3b3d2c9dd5e 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -218,7 +218,7 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
>   {
>   	vm_flags &= __VM_UFFD_FLAGS;
>   
> -	if (vm_flags & VM_DROPPABLE)
> +	if (vma->vm_flags & VM_DROPPABLE)
>   		return false;
>   
>   	if ((vm_flags & VM_UFFD_MINOR) &&

Nice catch!

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb