[PATCH v2] ocfs2: validate dx entry list counts on read

[BUG]
A corrupt indexed directory dx leaf can carry de_num_used larger than
de_count. When unlinkat() removes an indexed entry, that corrupt count
reaches ocfs2_dx_list_remove_entry() and drives an oversized memmove():

BUG: KASAN: use-after-free in ocfs2_dx_list_remove_entry fs/ocfs2/dir.c:1246 [inline]
BUG: KASAN: use-after-free in ocfs2_delete_entry_dx fs/ocfs2/dir.c:1363 [inline]
BUG: KASAN: use-after-free in ocfs2_delete_entry+0xb47/0x1070 fs/ocfs2/dir.c:1417
Read of size 401648 at addr ffff888020b8bd20 by task syz.0.1/347
Call Trace:
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0xbe/0x130 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0xd1/0x650 mm/kasan/report.c:482
 kasan_report+0xfb/0x140 mm/kasan/report.c:595
 check_region_inline mm/kasan/generic.c:186 [inline]
 kasan_check_range+0x11c/0x200 mm/kasan/generic.c:200
 __asan_memmove+0x23/0x80 mm/kasan/shadow.c:94
 ocfs2_dx_list_remove_entry fs/ocfs2/dir.c:1246 [inline]
 ocfs2_delete_entry_dx fs/ocfs2/dir.c:1363 [inline]
 ocfs2_delete_entry+0xb47/0x1070 fs/ocfs2/dir.c:1417
 ocfs2_unlink+0xb79/0x1900 fs/ocfs2/namei.c:991
 vfs_unlink+0x2c2/0x900 fs/namei.c:4673
 do_unlinkat+0x474/0x680 fs/namei.c:4737
 __do_sys_unlinkat fs/namei.c:4778 [inline]
 __se_sys_unlinkat fs/namei.c:4771 [inline]
 __x64_sys_unlinkat+0xae/0x130 fs/namei.c:4771
 ...

[CAUSE]
DX leaf and inline dx root blocks validate signatures and ECC, but they
do not validate the geometry of struct ocfs2_dx_entry_list. Corrupt
de_count/de_num_used values therefore survive block read and are later
used as traversal bounds and memmove/memset lengths in lookup, delete
and rebalance paths. In the unlink case, lookup can still find the
target entry before the bad bound matters, and the same corrupt
de_num_used then reaches ocfs2_dx_list_remove_entry().

[FIX]
Validate dx entry-list geometry in the metadata read callbacks. de_count
must match the block layout for the current superblock, and de_num_used
must not exceed de_count. Do this for both dx leaves and inline dx
roots so corrupted metadata is rejected before any caller walks or
rewrites de_entries[].

Signed-off-by: ZhengYuan Huang <[email protected]>
---
No Fixes tag is included because this is a missing metadata validation
check, not a regression attributable to a single introducing commit.

v2:
- Reframe the bug report around the unlinkat-triggered UAF
- Keep the code change unchanged because it fixes the same root cause
---
fs/ocfs2/dir.c | 40 ++++++++++++++++++++++++++++++++++++++--
1 file changed, 38 insertions(+), 2 deletions(-)

diff --git a/fs/ocfs2/dir.c b/fs/ocfs2/dir.c
index 8e6b03238327..a89634e20b32 100644
--- a/fs/ocfs2/dir.c
+++ b/fs/ocfs2/dir.c
@@ -578,6 +578,29 @@ static int ocfs2_read_dir_block_direct(struct inode *dir, u64 phys,
 	return ret;
 }
 
+static int ocfs2_validate_dx_entry_list(struct super_block *sb, u64 blkno,
+					struct ocfs2_dx_entry_list *entry_list,
+					unsigned int expected,
+					const char *owner)
+{
+	unsigned int count = le16_to_cpu(entry_list->de_count);
+	unsigned int num_used = le16_to_cpu(entry_list->de_num_used);
+
+	if (count != expected)
+		return ocfs2_error(sb,
+				   "%s # %llu has invalid de_count %u (expected %u)\n",
+				   owner, (unsigned long long)blkno, count,
+				   expected);
+
+	if (num_used > count)
+		return ocfs2_error(sb,
+				   "%s # %llu has invalid de_num_used %u (de_count %u)\n",
+				   owner, (unsigned long long)blkno, num_used,
+				   count);
+
+	return 0;
+}
+
 static int ocfs2_validate_dx_root(struct super_block *sb,
 				  struct buffer_head *bh)
 {
@@ -604,7 +625,14 @@ static int ocfs2_validate_dx_root(struct super_block *sb,
 		goto bail;
 	}
 
-	if (!(dx_root->dr_flags & OCFS2_DX_FLAG_INLINE)) {
+	if (dx_root->dr_flags & OCFS2_DX_FLAG_INLINE) {
+		ret = ocfs2_validate_dx_entry_list(sb, bh->b_blocknr,
+						   &dx_root->dr_entries,
+						   ocfs2_dx_entries_per_root(sb),
+						   "Dir Index Root");
+		if (ret)
+			goto bail;
+	} else {
 		struct ocfs2_extent_list *el = &dx_root->dr_list;
 
 		if (le16_to_cpu(el->l_count) != ocfs2_extent_recs_per_dx_root(sb)) {
@@ -660,13 +688,21 @@ static int ocfs2_validate_dx_leaf(struct super_block *sb,
 		mlog(ML_ERROR,
 		     "Checksum failed for dir index leaf block %llu\n",
 		     (unsigned long long)bh->b_blocknr);
-		return ret;
+		goto bail;
 	}
 
 	if (!OCFS2_IS_VALID_DX_LEAF(dx_leaf)) {
 		ret = ocfs2_error(sb, "Dir Index Leaf has bad signature %.*s\n",
 				  7, dx_leaf->dl_signature);
+		goto bail;
 	}
 
+	ret = ocfs2_validate_dx_entry_list(sb, bh->b_blocknr,
+					   &dx_leaf->dl_list,
+					   ocfs2_dx_entries_per_leaf(sb),
+					   "Dir Index Leaf");
+
+bail:
+
 	return ret;
 }
-- 
2.43.0