linux-kernel - Re: [PATCH] mm/filemap: Fix handling of THPs in generic_file_buffered

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Date:   Wed, 23 Feb 2022 18:05:38 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        stable@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kirill.shutemov@...ux.intel.com,
        Song Liu <songliubraving@...com>
Cc:     Adam Majer <amajer@...e.com>, Dirk Mueller <dmueller@...e.com>,
        Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH] mm/filemap: Fix handling of THPs in
 generic_file_buffered_read()

On 2/23/22 16:59, Matthew Wilcox (Oracle) wrote:
> When a THP is present in the page cache, we can return it several times,
> leading to userspace seeing the same data repeatedly if doing a read()
> that crosses a 64-page boundary.  This is probably not a security issue
> (since the data all comes from the same file), but it can be interpreted
> as a transient data corruption issue.  Fortunately, it is very rare as
> it can only occur when CONFIG_READ_ONLY_THP_FOR_FS is enabled, and it can
> only happen to executables.  We don't often call read() on executables.
> 
> This bug is fixed differently in v5.17 by commit 6b24ca4a1a8d
> ("mm: Use multi-index entries in the page cache").  That commit is
> unsuitable for backporting, so fix this in the clearest way.  It
> sacrifices a little performance for clarity, but this should never
> be a performance path in these kernel versions.
> 
> Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")
> Cc: stable@...r.kernel.org # v5.15, v5.16
> Link: https://lore.kernel.org/r/df3b5d1c-a36b-2c73-3e27-99e74983de3a@suse.cz/
> Analyzed-by: Adam Majer <amajer@...e.com>
> Analyzed-by: Dirk Mueller <dmueller@...e.com>
> Bisected-by: Takashi Iwai <tiwai@...e.de>
> Reported-by: Vlastimil Babka <vbabka@...e.cz>

Replace with:

Reported-and-tested-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!

> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  mm/filemap.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 82a17c35eb96..1293c3409e42 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2354,8 +2354,12 @@ static void filemap_get_read_batch(struct address_space *mapping,
>  			break;
>  		if (PageReadahead(head))
>  			break;
> -		xas.xa_index = head->index + thp_nr_pages(head) - 1;
> -		xas.xa_offset = (xas.xa_index >> xas.xa_shift) & XA_CHUNK_MASK;
> +		if (PageHead(head)) {
> +			xas_set(&xas, head->index + thp_nr_pages(head));
> +			/* Handle wrap correctly */
> +			if (xas.xa_index - 1 >= max)
> +				break;
> +		}
>  		continue;
>  put_page:
>  		put_page(head);