Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 6 Aug 2018 15:59:27 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Jan Kara <jack@suse.cz>
Cc:     Chengguang Xu <cgxu519@gmx.com>, mgorman@techsingularity.net,
        jlayton@redhat.com, ak@linux.intel.com, mawilcox@microsoft.com,
        tim.c.chen@linux.intel.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH] mm: adjust max read count in
 generic_file_buffered_read()
Message-Id: <20180806155927.4740babd057df9d5078281b1@linux-foundation.org>
In-Reply-To: <20180806102203.hmobd26cujmlfcsw@quack2.suse.cz>
References: <20180719081726.3341-1-cgxu519@gmx.com>
        <20180719085812.sjup2odrjyuigt3l@quack2.suse.cz>
        <20180720161429.d63dccb9f66799dc0ff74dba@linux-foundation.org>
        <20180806102203.hmobd26cujmlfcsw@quack2.suse.cz>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, 6 Aug 2018 12:22:03 +0200 Jan Kara <jack@suse.cz> wrote:

> On Fri 20-07-18 16:14:29, Andrew Morton wrote:
> > On Thu, 19 Jul 2018 10:58:12 +0200 Jan Kara <jack@suse.cz> wrote:
> > 
> > > On Thu 19-07-18 16:17:26, Chengguang Xu wrote:
> > > > When we try to truncate read count in generic_file_buffered_read(),
> > > > should deliver (sb->s_maxbytes - offset) as maximum count not
> > > > sb->s_maxbytes itself.
> > > > 
> > > > Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
> > > 
> > > Looks good to me. You can add:
> > > 
> > > Reviewed-by: Jan Kara <jack@suse.cz>
> > 
> > Yup.
> > 
> > What are the runtime effects of this bug?
> 
> Good question. I think ->readpage() could be called for index beyond
> maximum file size supported by the filesystem leading to weird filesystem
> behavior due to overflows in internal calculations.
> 

Sure.  But is it possible for userspace to trigger this behaviour? 
Possibly all callers have already sanitized the arguments by this stage
in which case the statement is arguably redundant.

I guess I'll put a cc:stable on it and send it in for 4.19-rc1, so we
get a bit more time to poke at it.  But we should have a better
understanding of the userspace impact.