Re: [RFC v2 12/14] xfs: make the calculation generic in xfs_sb_validate_fsb_count()

[Dave Chinner <david@fromorbit.com>]

On Tue, Feb 13, 2024 at 10:48:17PM +0100, Pankaj Raghav (Samsung) wrote:
> On Tue, Feb 13, 2024 at 08:26:11AM -0800, Darrick J. Wong wrote:
> > On Tue, Feb 13, 2024 at 10:37:11AM +0100, Pankaj Raghav (Samsung) wrote:
> > > From: Pankaj Raghav <p.raghav@samsung.com>
> > > 
> > > Instead of assuming that PAGE_SHIFT is always higher than the blocklog,
> > > make the calculation generic so that page cache count can be calculated
> > > correctly for LBS.
> > > 
> > > Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> > > ---
> > >  fs/xfs/xfs_mount.c | 6 +++++-
> > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> > > index aabb25dc3efa..bfbaaecaf668 100644
> > > --- a/fs/xfs/xfs_mount.c
> > > +++ b/fs/xfs/xfs_mount.c
> > > @@ -133,9 +133,13 @@ xfs_sb_validate_fsb_count(
> > >  {
> > >  	ASSERT(PAGE_SHIFT >= sbp->sb_blocklog);
> > >  	ASSERT(sbp->sb_blocklog >= BBSHIFT);
> > > +	unsigned long mapping_count;
> > 
> > Nit: indenting
> > 
> > 	unsigned long		mapping_count;
> 
> I will add this in the next revision.
> > 
> > > +	uint64_t bytes = nblocks << sbp->sb_blocklog;
> > 
> > What happens if someone feeds us a garbage fs with sb_blocklog > 64?
> > Or did we check that previously, so an overflow isn't possible?
> > 
> I was thinking of possibility of an overflow but at the moment the 
> blocklog is capped at 16 (65536 bytes) right? mkfs refuses any block
> sizes more than 64k. And we have check for this in xfs_validate_sb_common()
> in the kernel, which will catch it before this happens?

The sb_blocklog is checked in the superblock verifier when we first read in the
superblock:

	    sbp->sb_blocksize < XFS_MIN_BLOCKSIZE                       ||
            sbp->sb_blocksize > XFS_MAX_BLOCKSIZE                       ||
            sbp->sb_blocklog < XFS_MIN_BLOCKSIZE_LOG                    ||
            sbp->sb_blocklog > XFS_MAX_BLOCKSIZE_LOG                    ||
            sbp->sb_blocksize != (1 << sbp->sb_blocklog)                ||

#define XFS_MAX_BLOCKSIZE_LOG 16

However, we pass mp->m_sb.sb_dblocks or m_sb.sb_rblocks to this
function, and they are validated by the same verifier as invalid
if:

	    sbp->sb_dblocks > XFS_MAX_DBLOCKS(sbp)

#define XFS_MAX_DBLOCKS(s) ((xfs_rfsblock_t)(s)->sb_agcount *
                                             (s)->sb_agblocks)

Which means as long as someone can corrupt some combination of
sb_dblocks, sb_agcount and sb_agblocks that allows sb_dblocks to be
greater than 2^48 on a 64kB fsb fs, then that the above code:

	uint64_t bytes = nblocks << sbp->sb_blocklog;

will overflow.

I also suspect that we can feed a huge rtdev to this new code
and have it overflow without needing to corrupt the superblock in
any way....

-Dave.
-- 
Dave Chinner
david@fromorbit.com