[PATCH RFC v5] jfs: validate dmap start before block allocation

A corrupted JFS filesystem image can cause a state where the block
allocation map (bmap) and the dmap pages are out of sync. Specifically, if
a dmap page's `dp->start` is corrupted, it can lead to a mismatch during
block allocation. The block allocator calculates the returned block number
based on `dp->start` (i.e., `dp->start + block_index`), while the bitmap
update is derived from the block's relative position within the dmap. If
`dp->start` does not match the expected start block number for that dmap's
position in the bmap, the allocator may return a block number that is
already in use (e.g., as an inode table block or metadata), while marking a
completely different block as allocated. This mismatch leads to duplicate
allocations. When a thread later attempts to lock this duplicate block (for
example, during a directory btree split), `txLock()` detects a metapage
conflict because the block is already locked for another purpose,
triggering a kernel BUG:

kernel BUG at fs/jfs/jfs_txnmgr.c:836!
Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
RIP: 0010:txLock+0x1cc3/0x1d10 fs/jfs/jfs_txnmgr.c:836
Call Trace:
 <TASK>
 dtSplitRoot+0x38d/0x18a0 fs/jfs/jfs_dtree.c:1924
 dtSplitUp fs/jfs/jfs_dtree.c:990 [inline]
 dtInsert+0xeb2/0x5890 fs/jfs/jfs_dtree.c:868
 jfs_create+0x730/0xae0 fs/jfs/namei.c:138
...

To fix this, validate that the `dp->start` of the dmap page matches the
expected start block number during block allocation operations (`dbAlloc`
and `dbAllocCtl`). Rejecting a mismatched start with `-EIO` prevents
duplicate allocations before `txLock()` is reached. Additionally, preserve
the existing independent geometry check (`dp->tree.budmin < 0`) in
`dbAllocCtl()`. Both a mismatched start and a negative `budmin` will now
fail with `-EIO` early, preventing corrupted dmap structures from causing
duplicate allocations and subsequent transaction lock crashes.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=a843f6ae2130a987d63b
Link: https://syzkaller.appspot.com/ai_job?id=601b1802-e8c6-4427-af36-c1b279764dc7
To: <[email protected]>
To: "Dave Kleikamp" <[email protected]>
To: "Linus Torvalds" <[email protected]>
Cc: "Kees Cook" <[email protected]>
Cc: <[email protected]>
Cc: "Yun Zhou" <[email protected]>

---
v5:
- Narrowed the validation of `dp->start` to only the primary block allocation paths (`dbAlloc` and `dbAllocCtl`).
- Removed validation checks from `dbFree`, `dbUpdatePMap`, `dbExtend`, `dbAllocBottomUp`, and secondary loops in `dbAllocCtl`.

v4:
- Renamed dbValidateDmap to db_validate_dmap to follow kernel naming conventions.
- Preserved the existing dp->tree.budmin < 0 check in dbAllocCtl instead of replacing it.
https://lore.kernel.org/all/[email protected]/T/

v3:
- Renamed the patch to "jfs: validate dmap start before block map operations".
- Simplified dbValidateDmap() to only validate that the dmap dp->start matches the expected start block number, removing unproven integrity checks.
- Updated the commit description to explain the exact root cause of the selection/update mismatch and how validating dp->start prevents duplicate allocations before txLock() is reached.
https://lore.kernel.org/all/[email protected]/T/

v2:
- Replaced the approach of propagating errors from txLock() with proactive dmap validation.
- Introduced dbValidateDmap() to check dmap page integrity (nblocks, nfree, start address, and tree structure).
- Integrated dmap validation into dbFree(), dbUpdatePMap(), dbAlloc(), dbExtend(), dbAllocCtl(), dbAllocBottomUp(), and dbExtendFS().
- Reverted return type changes for functions in jfs_dtree.c and jfs_xtree.c from the previous version.
https://lore.kernel.org/all/[email protected]/T/

v1:
https://lore.kernel.org/all/[email protected]/T/
---
diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
index a841cf21d..9bdbc55d8 100644
--- a/fs/jfs/jfs_dmap.c
+++ b/fs/jfs/jfs_dmap.c
@@ -98,6 +98,9 @@ static int blkstol2(s64 nb);
 static int cntlz(u32 value);
 static int cnttz(u32 word);
 
+static bool db_validate_dmap(struct super_block *sb, const struct dmap *dp,
+			     s64 expected_start);
+
 static int dbAllocDmapBU(struct bmap * bmp, struct dmap * dp, s64 blkno,
 			 int nblocks);
 static int dbInitDmap(struct dmap * dp, s64 blkno, int nblocks);
@@ -886,6 +889,12 @@ int dbAlloc(struct inode *ip, s64 hint, s64 nblocks, s64 * results)
 
 		dp = (struct dmap *) mp->data;
 
+		if (!db_validate_dmap(ip->i_sb, dp,
+				      blkno & ~(s64)(BPERDMAP - 1))) {
+			release_metapage(mp);
+			goto read_unlock;
+		}
+
 		/* first, try to satisfy the allocation request with the
 		 * blocks beginning at the hint.
 		 */
@@ -1902,7 +1911,9 @@ dbAllocCtl(struct bmap * bmp, s64 nblocks, int l2nb, s64 blkno, s64 * results)
 			return -EIO;
 		dp = (struct dmap *) mp->data;
 
-		if (dp->tree.budmin < 0) {
+		if (dp->tree.budmin < 0 ||
+		    !db_validate_dmap(bmp->db_ipbmap->i_sb, dp,
+				      blkno & ~(s64)(BPERDMAP - 1))) {
 			release_metapage(mp);
 			return -EIO;
 		}
@@ -2134,6 +2145,18 @@ static int dbAllocDmap(struct bmap * bmp, struct dmap * dp, s64 blkno,
 	return (rc);
 }
 
+static bool db_validate_dmap(struct super_block *sb, const struct dmap *dp,
+			     s64 expected_start)
+{
+	if (le64_to_cpu(dp->start) != expected_start) {
+		jfs_error(sb, "corrupt dmap page: start %lld expected %lld\n",
+			  (long long)le64_to_cpu(dp->start),
+			  (long long)expected_start);
+		return false;
+	}
+
+	return true;
+}
 
 /*
  * NAME:	dbFreeDmap()


base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
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