Re: [PATCH] xfs: fix off-by-one in rtrefcount btree root level validation

["Darrick J. Wong" <djwong@kernel.org>]

On Sat, Jul 11, 2026 at 05:26:44PM -0700, Xiang Mei wrote:
> xfs_rtrefcountbt_compute_maxlevels() sets
> 
> 	mp->m_rtrefc_maxlevels = min(d_maxlevels, r_maxlevels) + 1;
> 
> where the trailing "+ 1" already accounts for the inode-root level, so the
> deepest valid on-disk root level is m_rtrefc_maxlevels - 1 and a cursor must
> satisfy bc_nlevels <= bc_maxlevels (= m_rtrefc_maxlevels).
> 
> The two on-disk validation paths, xfs_rtrefcountbt_verify() and
> xfs_iformat_rtrefcount(), check the root level with ">" instead of ">=", so a
> crafted rtreflink (metadir + realtime + reflink) image whose
> /rtgroups/N.refcount inode has bb_level == m_rtrefc_maxlevels is accepted on
> mount. xfs_rtrefcountbt_init_cursor() then sets bc_nlevels = bb_level + 1,
> exceeding bc_maxlevels by one. Since the xfs_rtrefcountbt_cur slab object is
> sized for exactly bc_maxlevels entries, the first btree op on such a cursor
> indexes bc_levels[m_rtrefc_maxlevels] past the end of the object. This is
> reached by the first rtrefcount cursor built after mount, via log/CoW
> recovery (xfs_reflink_recover_cow() during xfs_mountfs()) or an
> FS_IOC_GETFSMAP over the realtime device.
> 
> Reject a root level equal to m_rtrefc_maxlevels, matching the ">=" form
> already used by the sibling data-device refcount/rmap verifiers and the
> in-memory rtrmap verifier.
> 
>   BUG: KASAN: slab-out-of-bounds in xfs_btree_lookup (fs/xfs/libxfs/xfs_btree.c:2101)
>   Write of size 2 at addr ffff888018391658 by task exploit/144
>    xfs_btree_lookup (fs/xfs/libxfs/xfs_btree.c:2101)
>    xfs_btree_query_range (fs/xfs/libxfs/xfs_btree.c:5308)
>    xfs_refcount_recover_cow_leftovers (fs/xfs/libxfs/xfs_refcount.c:2113)
>    xfs_reflink_recover_cow (fs/xfs/xfs_reflink.c:1085)
>    xlog_recover_finish (fs/xfs/xfs_log_recover.c:3551)
>    xfs_mountfs (fs/xfs/xfs_mount.c:1158)
>    xfs_fs_fill_super (fs/xfs/xfs_super.c:1940)
>    get_tree_bdev_flags (fs/super.c:1634)
>    vfs_get_tree (fs/super.c:1694)
>    path_mount (fs/namespace.c:4161)
>    __x64_sys_mount (fs/namespace.c:4367)
>    entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
>   The buggy address belongs to the cache xfs_rtrefcountbt_cur of size 216
>   The buggy address is located 8 bytes to the right of
>    allocated 216-byte region [ffff888018391578, ffff888018391650)
>   Kernel panic - not syncing: Fatal exception
> 
> Fixes: 9abe03a0e4f9 ("xfs: introduce realtime refcount btree ondisk definitions")

Just one fixes tag is fine, thanks.

Cc: <stable@vger.kernel.org> # v6.14
Fixes: 9abe03a0e4f978 ("xfs: introduce realtime refcount btree ondisk definitions")

> Fixes: f0415af60f48 ("xfs: wire up a new metafile type for the realtime refcount")
> Reported-by: Weiming Shi <bestswngs@gmail.com>
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Xiang Mei <xmei5@asu.edu>

I think this is correct, so
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> ---
>  fs/xfs/libxfs/xfs_rtrefcount_btree.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_rtrefcount_btree.c b/fs/xfs/libxfs/xfs_rtrefcount_btree.c
> index f27b80a199ba..22acc1411aac 100644
> --- a/fs/xfs/libxfs/xfs_rtrefcount_btree.c
> +++ b/fs/xfs/libxfs/xfs_rtrefcount_btree.c
> @@ -201,7 +201,7 @@ xfs_rtrefcountbt_verify(
>  	if (fa)
>  		return fa;
>  	level = be16_to_cpu(block->bb_level);
> -	if (level > mp->m_rtrefc_maxlevels)
> +	if (level >= mp->m_rtrefc_maxlevels)
>  		return __this_address;
>  
>  	return xfs_btree_fsblock_verify(bp, mp->m_rtrefc_mxr[level != 0]);
> @@ -651,7 +651,7 @@ xfs_iformat_rtrefcount(
>  	numrecs = be16_to_cpu(dfp->bb_numrecs);
>  	level = be16_to_cpu(dfp->bb_level);
>  
> -	if (level > mp->m_rtrefc_maxlevels ||
> +	if (level >= mp->m_rtrefc_maxlevels ||
>  	    xfs_rtrefcount_droot_space_calc(level, numrecs) > dsize) {
>  		xfs_inode_mark_sick(ip, XFS_SICK_INO_CORE);
>  		return -EFSCORRUPTED;
> -- 
> 2.43.0
> 
>