Re: [PATCH 2/2] ext4: Fix stale data exposure when read races with hole punch

[Jan Kara <jack@suse.cz>]

On Wed 05-06-19 11:25:51, Dave Chinner wrote:
> On Mon, Jun 03, 2019 at 03:21:55PM +0200, Jan Kara wrote:
> > Hole puching currently evicts pages from page cache and then goes on to
> > remove blocks from the inode. This happens under both i_mmap_sem and
> > i_rwsem held exclusively which provides appropriate serialization with
> > racing page faults. However there is currently nothing that prevents
> > ordinary read(2) from racing with the hole punch and instantiating page
> > cache page after hole punching has evicted page cache but before it has
> > removed blocks from the inode. This page cache page will be mapping soon
> > to be freed block and that can lead to returning stale data to userspace
> > or even filesystem corruption.
> > 
> > Fix the problem by protecting reads as well as readahead requests with
> > i_mmap_sem.
> > 
> > CC: stable@vger.kernel.org
> > Reported-by: Amir Goldstein <amir73il@gmail.com>
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  fs/ext4/file.c | 35 +++++++++++++++++++++++++++++++----
> >  1 file changed, 31 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> > index 2c5baa5e8291..a21fa9f8fb5d 100644
> > --- a/fs/ext4/file.c
> > +++ b/fs/ext4/file.c
> > @@ -34,6 +34,17 @@
> >  #include "xattr.h"
> >  #include "acl.h"
> >  
> > +static ssize_t ext4_file_buffered_read(struct kiocb *iocb, struct iov_iter *to)
> > +{
> > +	ssize_t ret;
> > +	struct inode *inode = file_inode(iocb->ki_filp);
> > +
> > +	down_read(&EXT4_I(inode)->i_mmap_sem);
> > +	ret = generic_file_read_iter(iocb, to);
> > +	up_read(&EXT4_I(inode)->i_mmap_sem);
> > +	return ret;
> 
> Isn't i_mmap_sem taken in the page fault path? What makes it safe
> to take here both outside and inside the mmap_sem at the same time?
> I mean, the whole reason for i_mmap_sem existing is that the inode
> i_rwsem can't be taken both outside and inside the i_mmap_sem at the
> same time, so what makes the i_mmap_sem different?

Drat, you're right that read path may take page fault which will cause lock
inversion with mmap_sem. Just my xfstests run apparently didn't trigger
this as I didn't get any lockdep splat. Thanks for catching this!

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR