Re: [PATCH] mm: adjust max read count in generic_file_buffered_read()

[Jan Kara]

On Wed 08-08-18 08:57:13, cgxu519 wrote:
> On 08/07/2018 09:54 PM, Jan Kara wrote:
> > On Mon 06-08-18 15:59:27, Andrew Morton wrote:
> > > On Mon, 6 Aug 2018 12:22:03 +0200 Jan Kara <jack@xxxxxxx> wrote:
> > > 
> > > > On Fri 20-07-18 16:14:29, Andrew Morton wrote:
> > > > > On Thu, 19 Jul 2018 10:58:12 +0200 Jan Kara <jack@xxxxxxx> 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@xxxxxxx>
> > > > > > Looks good to me. You can add:
> > > > > > 
> > > > > > Reviewed-by: Jan Kara <jack@xxxxxxx>
> > > > > 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.
> > So I don't think there's any sanitization going on before
> > generic_file_buffered_read(). E.g. I don't see any s_maxbytes check on
> > ksys_read() -> vfs_read() -> __vfs_read() -> new_sync_read() ->
> > call_read_iter() -> generic_file_read_iter() ->
> > generic_file_buffered_read() path... However now thinking about this again:
> > We are guaranteed i_size is within s_maxbytes (places modifying i_size
> > are checking for this) and generic_file_buffered_read() stops when it
> > should read beyond i_size. So in the end I don't think there's any breakage
> > possible and the patch is not necessary?
> > 
> I think most of time i_size is within s_maxbytes in local filesystem,
> but consider network filesystem, write big file in 64bit client and
> read in 32bit client, in this case maybe generic_file_buffered_read()
> can read more than s_maxbytes, right?

I'd consider this an internal problem in the implementation of the
networking filesystem. Not something VFS should care about. It's similar to
a normal filesystem loading corrupted file size from disk...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR