[PATCH v3] xfs: fix heap buffer overflow in xfs_bmbt_to_bmdr

The xfs_bmbt_to_bmdr() function converts an in-memory btree root to
on-disk form during log recovery. It uses bb_numrecs from the source
to determine memcpy size without validating against the destination
fork capacity.

Current code:
  dmxr = xfs_bmdr_maxrecs(dblocklen, 0);           // destination capacity
  ...
  dmxr = be16_to_cpu(dblock->bb_numrecs);          // overwrite with source value!
  memcpy(..., * dmxr);                              // may overflow destination!

If bb_numrecs is larger than the destination fork capacity,
memcpy writes beyond the fork buffer, causing heap overflow.

Fix by validating bb_numrecs against the destination capacity
before using it for memcpy operations.

Cc: [email protected]
Fixes: 8ea5682d0711 ("xfs: refactor log recovery item dispatch for pass2 readhead functions")
Signed-off-by: Hongling Zeng <[email protected]>
---
Change in v3:
- Use dmxr for tpp calculation (on-disk format) and nrecs for memcpy (actual records)
---
Change in v2:
- Ensure memcpy operations consistently use nrecs instead of dmxr.
---
 fs/xfs/libxfs/xfs_bmap_btree.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_bmap_btree.c b/fs/xfs/libxfs/xfs_bmap_btree.c
index 758a4b1ccf5b..a119053d67b5 100644
--- a/fs/xfs/libxfs/xfs_bmap_btree.c
+++ b/fs/xfs/libxfs/xfs_bmap_btree.c
@@ -151,6 +151,7 @@ xfs_bmbt_to_bmdr(
 	int			dmxr;
 	xfs_bmbt_key_t		*fkp;
 	__be64			*fpp;
+	int                     nrecs;
 	xfs_bmbt_key_t		*tkp;
 	__be64			*tpp;
 
@@ -167,14 +168,19 @@ xfs_bmbt_to_bmdr(
 	ASSERT(rblock->bb_level != 0);
 	dblock->bb_level = rblock->bb_level;
 	dblock->bb_numrecs = rblock->bb_numrecs;
+
+	/* Validate record count against destination fork capacity */
+	nrecs = be16_to_cpu(rblock->bb_numrecs);
 	dmxr = xfs_bmdr_maxrecs(dblocklen, 0);
+	if (nrecs > dmxr)
+		return;
+
 	fkp = xfs_bmbt_key_addr(mp, rblock, 1);
 	tkp = xfs_bmdr_key_addr(dblock, 1);
 	fpp = xfs_bmap_broot_ptr_addr(mp, rblock, 1, rblocklen);
 	tpp = xfs_bmdr_ptr_addr(dblock, 1, dmxr);
-	dmxr = be16_to_cpu(dblock->bb_numrecs);
-	memcpy(tkp, fkp, sizeof(*fkp) * dmxr);
-	memcpy(tpp, fpp, sizeof(*fpp) * dmxr);
+	memcpy(tkp, fkp, sizeof(*fkp) * nrecs);
+	memcpy(tpp, fpp, sizeof(*fpp) * nrecs);
 }
 
 STATIC struct xfs_btree_cur *
-- 
2.25.1