linux-kernel - Re: [PATCH] mm/swapfile: unuse_pte can map random data if swap read fails

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Date:   Mon, 4 Apr 2022 15:37:36 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/swapfile: unuse_pte can map random data if swap read
 fails

On 01.04.22 09:29, Miaohe Lin wrote:
> There is a bug in unuse_pte(): when swap page happens to be unreadable,
> page filled with random data is mapped into user address space. The fix
> is to check for PageUptodate and fail swapoff in case of error.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/swapfile.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 63c61f8b2611..e72a35de7a0f 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1795,6 +1795,10 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>  		ret = 0;
>  		goto out;
>  	}
> +	if (unlikely(!PageUptodate(page))) {
> +		ret = -EIO;
> +		goto out;
> +	}

Yeah, we have the same handling in do_swap_page(), whereby we send a
SIGBUS because we're dealing with an actual access.

Interestingly, folio_test_uptodate() states:

"Anonymous and CoW folios are always uptodate."

@Willy, is that true or is the swapin case not documented there?

-- 
Thanks,

David / dhildenb