linux-kernel - Re: [PATCH 5.15 v3] mm/filemap: fix UAF in find_lock

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Message-ID: <YsWXeLo9gXTEv3pw@casper.infradead.org>
Date:   Wed, 6 Jul 2022 15:08:56 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Liu Shixin <liushixin2@...wei.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        William Kucharski <william.kucharski@...cle.com>,
        Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 5.15 v3] mm/filemap: fix UAF in find_lock_entries

On Wed, Jul 06, 2022 at 03:45:27PM +0800, Liu Shixin wrote:
>  	rcu_read_lock();
>  	while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
> +		unsigned long next_idx = xas.xa_index;

It's confusing to have next_idx not be the actual next index.
That was why I made it 'xas.xa_index + 1'.  I know it's somewhat
used as an indicator that we don't need to call xas_set(), and so
it doesn't really matter, but let's say what we mean.