Re: filemap: Fix bounds checking in filemap_read()

[Mike Snitzer <snitzer@kernel.org>]

Hi Willy,

On Tue, Sep 17, 2024 at 07:55:29AM +1000, Dave Chinner wrote:
> On Fri, Sep 13, 2024 at 07:21:54PM +0100, Matthew Wilcox wrote:
> > On Fri, Sep 13, 2024 at 01:57:04PM -0400, trondmy@kernel.org wrote:
> > > If the caller supplies an iocb->ki_pos value that is close to the
> > > filesystem upper limit, and an iterator with a count that causes us to
> > > overflow that limit, then filemap_read() enters an infinite loop.
> > 
> > Are we guaranteed that ki_pos lies in the range [0..s_maxbytes)?
> > I'm not too familiar with the upper paths of the VFS and what guarantees
> > we can depend on.  If we are guaranteed that, could somebody document
> > it (and indeed create kernel-doc for struct kiocb)?
> 
> filemap_read() checks this itself before doing anything else:
> 
> 	if (unlikely(iocb->ki_pos >= inode->i_sb->s_maxbytes))
>                 return 0;
> 
> i.e. there is no guarantee provided by the upper layers, it's first
> checked right here in any buffered read path...
> 
> -Dave.
> 
> -- 
> Dave Chinner
> david@fromorbit.com

Linus merged the NFS LOCALIO changes via the NFS client tree a couple
days ago.  LOCALIO teased out this filemap_read infinite loop bug, so
it is important to fix this for 6.12 (probably should get marked for
stable@ too):
https://lore.kernel.org/all/c6f35a86fe9ae6aa33b2fd3983b4023c2f4f9c13.1726250071.git.trond.myklebust@hammerspace.com/

(weirdly, Trond's reply to you didn't make it to the linux-nfs or
linux-fsdevel list archives, but Dave's above reply covers the same)

Trond also offered this additional filemap_read negative check:
https://lore.kernel.org/all/482ee0b8a30b62324adb9f7c551a99926f037393.1726257832.git.trond.myklebust@hammerspace.com/

Could be you've been busy with travel or whatever, but for future
reference, should linux-mm and/or Andrew always be cc'd on filemap
fixes?

Thanks,
Mike