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

[Weiming Shi <bestswngs@gmail.com>]

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)
+		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