Re: [PATCH] xfs: reject attr leaf blocks with inconsistent usedbytes

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

On Fri, Jul 03, 2026 at 08:15:44AM -0700, Weiming Shi wrote:
> xfs_attr3_leaf_verify() checks each attr leaf entry on its own, but never
> checks that the entries' nameval regions are disjoint. A crafted leaf can
> point several entries at overlapping offsets: every entry passes the
> per-entry check, yet the summed entry sizes far exceed the nameval region.
> 
> ichdr.usedbytes is kept as the exact sum of the entries'
> xfs_attr_leaf_entsize() (see xfs_attr3_leaf_add()), so for such a leaf the
> real sum no longer matches usedbytes. When the leaf is later repacked,
> xfs_attr3_leaf_compact() resets firstused to blksize and calls
> xfs_attr3_leaf_moveents(), which subtracts each entry size from firstused;
> the oversized sum underflows the 32-bit firstused and the following memmove
> writes out of bounds. The same repack runs from xfs_attr3_leaf_rebalance()
> and xfs_attr3_leaf_unbalance(). The only guard is an ASSERT, which is
> compiled out on production kernels.
> 
> A single setxattr() on a file with such a leaf, after mounting a crafted
> image, triggers the write:
> 
>   BUG: KASAN: use-after-free in xfs_attr3_leaf_moveents (fs/xfs/libxfs/xfs_attr_leaf.c:2788)
>   Write of size 400 at addr ffff88802b187f98 by task exploit
>    xfs_attr3_leaf_moveents (fs/xfs/libxfs/xfs_attr_leaf.c:2788)
>    xfs_attr3_leaf_compact (fs/xfs/libxfs/xfs_attr_leaf.c:1790)
>    xfs_attr3_leaf_add (fs/xfs/libxfs/xfs_attr_leaf.c:1563)
>    xfs_attr_set_iter (fs/xfs/libxfs/xfs_attr.c:556)
>    xfs_attr_set (fs/xfs/libxfs/xfs_attr.c:1244)
>    xfs_xattr_set (fs/xfs/xfs_xattr.c:186)
>    __vfs_setxattr (fs/xattr.c:218)
>    vfs_setxattr (fs/xattr.c:339)
>    __x64_sys_fsetxattr (fs/xattr.c:774)
> 
> Sum the entry sizes while verifying and reject the leaf unless the sum
> equals usedbytes and usedbytes fits in [firstused, blksize).  The online
> scrubber already validates this in xchk_xattr_block(); this brings the
> read/write verifier in line with it so the bad leaf is rejected before any
> reshape can run.
> 
> Fixes: c84760659dcf ("xfs: check attribute leaf block structure")
> Reported-by: Xiang Mei <xmei5@asu.edu>
> Assisted-by: Claude:claude-opus-4-8
> Cc: stable@vger.kernel.org
> Signed-off-by: Weiming Shi <bestswngs@gmail.com>
> ---
>  fs/xfs/libxfs/xfs_attr_leaf.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_attr_leaf.c b/fs/xfs/libxfs/xfs_attr_leaf.c
> index 86c5c09a5db4..9814dcfbd7ac 100644
> --- a/fs/xfs/libxfs/xfs_attr_leaf.c
> +++ b/fs/xfs/libxfs/xfs_attr_leaf.c
> @@ -300,7 +300,8 @@ xfs_attr3_leaf_verify_entry(
>  	struct xfs_attr3_icleaf_hdr		*leafhdr,
>  	struct xfs_attr_leaf_entry		*ent,
>  	int					idx,
> -	__u32					*last_hashval)
> +	__u32					*last_hashval,
> +	unsigned int				*usedbytes)
>  {
>  	struct xfs_attr_leaf_name_local		*lentry;
>  	struct xfs_attr_leaf_name_remote	*rentry;
> @@ -344,6 +345,7 @@ xfs_attr3_leaf_verify_entry(
>  	if (name_end > buf_end)
>  		return __this_address;
>  
> +	*usedbytes += namesize;
>  	return NULL;
>  }
>  
> @@ -376,6 +378,7 @@ xfs_attr3_leaf_verify(
>  	char				*buf_end;
>  	uint32_t			end;	/* must be 32bit - see below */
>  	__u32				last_hashval = 0;
> +	unsigned int			usedbytes = 0;
>  	int				i;
>  	xfs_failaddr_t			fa;
>  
> @@ -410,11 +413,21 @@ xfs_attr3_leaf_verify(
>  	buf_end = (char *)bp->b_addr + mp->m_attr_geo->blksize;
>  	for (i = 0, ent = entries; i < ichdr.count; ent++, i++) {
>  		fa = xfs_attr3_leaf_verify_entry(mp, buf_end, leaf, &ichdr,
> -				ent, i, &last_hashval);
> +				ent, i, &last_hashval, &usedbytes);
>  		if (fa)
>  			return fa;
>  	}
>  
> +	/*
> +	 * usedbytes must equal the summed entry sizes and fit in the
> +	 * nameval region; otherwise a later repack underflows firstused
> +	 * in xfs_attr3_leaf_moveents().
> +	 */
> +	if (usedbytes != ichdr.usedbytes)
> +		return __this_address;
> +	if (ichdr.usedbytes > mp->m_attr_geo->blksize - ichdr.firstused)

Interesting ... where did this new logic come from?  xchk_xattr_block
doesn't perform this test.

--D

> +		return __this_address;
> +
>  	/*
>  	 * Quickly check the freemap information.  Attribute data has to be
>  	 * aligned to 4-byte boundaries, and likewise for the free space.
> -- 
> 2.43.0
> 
>