linux-kernel - Re: [PATCH] mm: adjust max read count in generic_file_buffered

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Message-Id: <20180806155927.4740babd057df9d5078281b1@linux-foundation.org>
Date:   Mon, 6 Aug 2018 15:59:27 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Chengguang Xu <cgxu519@....com>, mgorman@...hsingularity.net,
        jlayton@...hat.com, ak@...ux.intel.com, mawilcox@...rosoft.com,
        tim.c.chen@...ux.intel.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH] mm: adjust max read count in
 generic_file_buffered_read()

On Mon, 6 Aug 2018 12:22:03 +0200 Jan Kara <jack@...e.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@...e.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@....com>
> > > 
> > > Looks good to me. You can add:
> > > 
> > > Reviewed-by: Jan Kara <jack@...e.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.