Re: [PATCH] xfs: avoid underflow in xfs_ioc_trim()

Dave Chinner
2012-10-09T19:37:42Z

On Tue, Oct 09, 2012 at 02:15:45PM +0200, Lukas Czerner wrote:
> Currently if len argument in xfs_ioc_trim() is smaller than one BB
> (basic block) the 'end' variable underflow. Avoid that by bailing out if
> len is smaller than BB.
>
> Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> ---
> fs/xfs/xfs_discard.c | 7 ++++++-
> 1 files changed, 6 insertions(+), 1 deletions(-)
>
> diff --git a/fs/xfs/xfs_discard.c b/fs/xfs/xfs_discard.c
> index 69cf4fc..54dc58a 100644
> --- a/fs/xfs/xfs_discard.c
> +++ b/fs/xfs/xfs_discard.c
> @@ -183,8 +183,12 @@ xfs_ioc_trim(
> range.minlen > XFS_FSB_TO_B(mp, XFS_ALLOC_AG_MAX_USABLE(mp)))
> return -XFS_ERROR(EINVAL);
>
> + end = BTOBBT(range.len);
> + if (0 == end)
> + goto out;

Uggh. "if (end == 0)", please.
> +
> start = BTOBB(range.start);
> - end = start + BTOBBT(range.len) - 1;
> + end += start - 1;

Better would be to check if end <= start. That way it also catches
start+len overflows.

> minlen = BTOBB(max_t(u64, granularity, range.minlen));
>
> if (end > XFS_FSB_TO_BB(mp, mp->m_sb.sb_dblocks) - 1)
> @@ -203,6 +207,7 @@ xfs_ioc_trim(
> if (last_error)
> return last_error;
>
> +out:
> range.len = XFS_FSB_TO_B(mp, blocks_trimmed);
> if (copy_to_user(urange, &range, sizeof(range)))
> return -XFS_ERROR(EFAULT);

I think it should return EINVAL, not silently do nothing. If the
user application uses a loop that increments start/len based on the
returned amount of blocks trimmed, returning zero could send it into
an endless loop.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com