linux-kernel - Re: [PATCH 1/1] userfaultfd: check VM_MAYWRITE was set after verifying the uffd is registered

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Date:   Fri, 7 Dec 2018 11:43:45 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Jann Horn <jannh@...gle.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH 1/1] userfaultfd: check VM_MAYWRITE was set after
 verifying the uffd is registered

On Thu, Dec 06, 2018 at 04:20:28PM -0500, Andrea Arcangeli wrote:
> Calling UFFDIO_UNREGISTER on virtual ranges not yet registered in uffd
> could trigger an harmless false positive WARN_ON. Check the vma is
> already registered before checking VM_MAYWRITE to shut off the
> false positive warning.
> 
> Cc: <stable@...r.kernel.org>
> Fixes: 29ec90660d68 ("userfaultfd: shmem/hugetlbfs: only allow to register VM_MAYWRITE vmas")
> Reported-by: syzbot+06c7092e7d71218a2c16@...kaller.appspotmail.com
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> ---
>  fs/userfaultfd.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index cd58939dc977..7a85e609fc27 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1566,7 +1566,6 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
>  		cond_resched();
>  
>  		BUG_ON(!vma_can_userfault(vma));
> -		WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
>  
>  		/*
>  		 * Nothing to do: this vma is already registered into this

Maybe we want to fix this comment too some day:

		/*
		 * Nothing to do: this vma is already registered into this
		 * userfaultfd and with the right tracking mode too.
		 */

But I don't think it's anything urgent since it's clear it means the
other way round and it can potentially be touched up in any further
cleanup/fixes of uffd.

Acked-by: Peter Xu <peterx@...hat.com>

> @@ -1575,6 +1574,8 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
>  		if (!vma->vm_userfaultfd_ctx.ctx)
>  			goto skip;
>  
> +		WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
> +
>  		if (vma->vm_start > start)
>  			start = vma->vm_start;
>  		vma_end = min(end, vma->vm_end);

Thanks,

-- 
Peter Xu