Re: [PATCH 7/8 V2] xfs: AGF length has never been bounds checked

Darrick J. Wong
2023-06-29T16:35:40Z

On Thu, Jun 29, 2023 at 12:09:25PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> The AGF verifier does not check that the AGF length field is within
> known good bounds. This has never been checked by runtime kernel
> code (i.e. the lack of verification goes back to 1993) yet we assume
> in many places that it is correct and verify other metdata against
> it.
>
> Add length verification to the AGF verifier. The length of the AGF
> must be equal to the size of the AG specified in the superblock,
> unless it is the last AG in the filesystem. In that case, it must be
> less than or equal to sb->sb_agblocks and greater than
> XFS_MIN_AG_BLOCKS, which is the smallest AG a growfs operation will
> allow to exist.
>
> This requires a bit of rework of the verifier function. We want to
> verify metadata before we use it to verify other metadata. Hence
> we need to verify the AGF sequence numbers before using them to
> verify the length of the AGF. Then we can verify the AGF length
> before we verify AGFL fields. Then we can verifier other fields that
> are bounds limited by the AGF length.
>
> And, finally, by calculating agf_length only once into a local
> variable, we can collapse repeated "if (xfs_has_foo() &&"
> conditionaly checks into single checks. This makes the code much
> easier to follow as all the checks for a given feature are obviously
> in the same place.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Still looks good to me. New question: Do we need to validate agi_length
in the AGI verifier too?

--D

> ---
>
> Version 2:
> - growfs will write the new AGFs before the superblock has been
> updated, so we have to skip the new runt AGF seqno check otherwise
> it will fail.
>
> fs/xfs/libxfs/xfs_alloc.c | 92 +++++++++++++++++++++++++++++------------------
> 1 file changed, 57 insertions(+), 35 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index 1e72b91daff6..fe7d5ea47b90 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -2974,6 +2974,7 @@ xfs_agf_verify(
> {
> struct xfs_mount *mp = bp->b_mount;
> struct xfs_agf *agf = bp->b_addr;
> + uint32_t agf_length = be32_to_cpu(agf->agf_length);
>
> if (xfs_has_crc(mp)) {
> if (!uuid_equal(&agf->agf_uuid, &mp->m_sb.sb_meta_uuid))
> @@ -2985,18 +2986,49 @@ xfs_agf_verify(
> if (!xfs_verify_magic(bp, agf->agf_magicnum))
> return __this_address;
>
> - if (!(XFS_AGF_GOOD_VERSION(be32_to_cpu(agf->agf_versionnum)) &&
> - be32_to_cpu(agf->agf_freeblks) <= be32_to_cpu(agf->agf_length) &&
> - be32_to_cpu(agf->agf_flfirst) < xfs_agfl_size(mp) &&
> - be32_to_cpu(agf->agf_fllast) < xfs_agfl_size(mp) &&
> - be32_to_cpu(agf->agf_flcount) <= xfs_agfl_size(mp)))
> + if (!XFS_AGF_GOOD_VERSION(be32_to_cpu(agf->agf_versionnum)))
> return __this_address;
>
> - if (be32_to_cpu(agf->agf_length) > mp->m_sb.sb_dblocks)
> + /*
> + * Both agf_seqno and agf_length need to validated before anything else
> + * block number related in the AGF or AGFL can be checked.
> + *
> + * During growfs operations, the perag is not fully initialised,
> + * so we can't use it for any useful checking. growfs ensures we can't
> + * use it by using uncached buffers that don't have the perag attached
> + * so we can detect and avoid this problem.
> + */
> + if (bp->b_pag && be32_to_cpu(agf->agf_seqno) != bp->b_pag->pag_agno)
> + return __this_address;
> +
> + /*
> + * Only the last AGF in the filesytsem is allowed to be shorter
> + * than the AG size recorded in the superblock.
> + */
> + if (agf_length != mp->m_sb.sb_agblocks) {
> + /*
> + * During growfs, the new last AGF can get here before we
> + * have updated the superblock. Give it a pass on the seqno
> + * check.
> + */
> + if (bp->b_pag &&
> + be32_to_cpu(agf->agf_seqno) != mp->m_sb.sb_agcount - 1)
> + return __this_address;
> + if (agf_length < XFS_MIN_AG_BLOCKS)
> + return __this_address;
> + if (agf_length > mp->m_sb.sb_agblocks)
> + return __this_address;
> + }
> +
> + if (be32_to_cpu(agf->agf_flfirst) >= xfs_agfl_size(mp))
> + return __this_address;
> + if (be32_to_cpu(agf->agf_fllast) >= xfs_agfl_size(mp))
> + return __this_address;
> + if (be32_to_cpu(agf->agf_flcount) > xfs_agfl_size(mp))
> return __this_address;
>
> if (be32_to_cpu(agf->agf_freeblks) < be32_to_cpu(agf->agf_longest) ||
> - be32_to_cpu(agf->agf_freeblks) > be32_to_cpu(agf->agf_length))
> + be32_to_cpu(agf->agf_freeblks) > agf_length)
> return __this_address;
>
> if (be32_to_cpu(agf->agf_levels[XFS_BTNUM_BNO]) < 1 ||
> @@ -3007,38 +3039,28 @@ xfs_agf_verify(
> mp->m_alloc_maxlevels)
> return __this_address;
>
> - if (xfs_has_rmapbt(mp) &&
> - (be32_to_cpu(agf->agf_levels[XFS_BTNUM_RMAP]) < 1 ||
> - be32_to_cpu(agf->agf_levels[XFS_BTNUM_RMAP]) >
> - mp->m_rmap_maxlevels))
> - return __this_address;
> -
> - if (xfs_has_rmapbt(mp) &&
> - be32_to_cpu(agf->agf_rmap_blocks) > be32_to_cpu(agf->agf_length))
> - return __this_address;
> -
> - /*
> - * during growfs operations, the perag is not fully initialised,
> - * so we can't use it for any useful checking. growfs ensures we can't
> - * use it by using uncached buffers that don't have the perag attached
> - * so we can detect and avoid this problem.
> - */
> - if (bp->b_pag && be32_to_cpu(agf->agf_seqno) != bp->b_pag->pag_agno)
> - return __this_address;
> -
> if (xfs_has_lazysbcount(mp) &&
> - be32_to_cpu(agf->agf_btreeblks) > be32_to_cpu(agf->agf_length))
> + be32_to_cpu(agf->agf_btreeblks) > agf_length)
> return __this_address;
>
> - if (xfs_has_reflink(mp) &&
> - be32_to_cpu(agf->agf_refcount_blocks) >
> - be32_to_cpu(agf->agf_length))
> - return __this_address;
> + if (xfs_has_rmapbt(mp)) {
> + if (be32_to_cpu(agf->agf_rmap_blocks) > agf_length)
> + return __this_address;
>
> - if (xfs_has_reflink(mp) &&
> - (be32_to_cpu(agf->agf_refcount_level) < 1 ||
> - be32_to_cpu(agf->agf_refcount_level) > mp->m_refc_maxlevels))
> - return __this_address;
> + if (be32_to_cpu(agf->agf_levels[XFS_BTNUM_RMAP]) < 1 ||
> + be32_to_cpu(agf->agf_levels[XFS_BTNUM_RMAP]) >
> + mp->m_rmap_maxlevels)
> + return __this_address;
> + }
> +
> + if (xfs_has_reflink(mp)) {
> + if (be32_to_cpu(agf->agf_refcount_blocks) > agf_length)
> + return __this_address;
> +
> + if (be32_to_cpu(agf->agf_refcount_level) < 1 ||
> + be32_to_cpu(agf->agf_refcount_level) > mp->m_refc_maxlevels)
> + return __this_address;
> + }
>
> return NULL;
> }