xfs: fix off-by-one in rtrefcount btree root level validation

Summary

In the Linux kernel, the following vulnerability has been resolved: xfs: fix off-by-one in rtrefcount btree root level validation 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