jfs: jfs_dmap: Validate db_l2nbperpage while mounting

In jfs_dmap.c at line 381, BLKTODMAP is used to get a logical block number inside dbFree(). db_l2nbperpage, which is the log2 number of blocks per page, is passed as an argument to BLKTODMAP which uses it for shifting.

Syzbot reported a shift out-of-bounds crash because db_l2nbperpage is too big. This happens because the large value is set without any validation in dbMount() at line 181.

Thus, make sure that db_l2nbperpage is correct while mounting.

Max number of pages = Page size / Min block size
=> log2(Max number of pages) = log2(Page size / Min block size)
= log2(Page size) - log2(Min block size)

=> Max db_l2nbperpage = L2PSIZE - L2MINBLOCKSIZE

---
Changes in v2:
- Fix upper bound as pointed out in v1 by Shaggy.
- Add an explanation for the same in commit message for completeness.

 fs/jfs/jfs_dmap.c   | 6 ++++++
 fs/jfs/jfs_filsys.h | 2 ++
 2 files changed, 8 insertions(+)

index a3eb1e826947..da6a2bc6bf02 100644
@@ -178,7 +178,13 @@ int dbMount(struct inode *ipbmap)
 	dbmp_le = (struct dbmap_disk *) mp->data;
 	bmp->db_mapsize = le64_to_cpu(dbmp_le->dn_mapsize);
 	bmp->db_nfree = le64_to_cpu(dbmp_le->dn_nfree);
+
 	bmp->db_l2nbperpage = le32_to_cpu(dbmp_le->dn_l2nbperpage);
+	if (bmp->db_l2nbperpage > L2PSIZE - L2MINBLOCKSIZE) {
+		err = -EINVAL;
+		goto err_release_metapage;
+	}
+
 	bmp->db_numag = le32_to_cpu(dbmp_le->dn_numag);
 	if (!bmp->db_numag) {
 		err = -EINVAL;

index b5d702df7111..33ef13a0b110 100644
@@ -122,7 +122,9 @@
 #define NUM_INODE_PER_IAG	INOSPERIAG
 
 #define MINBLOCKSIZE		512
+#define L2MINBLOCKSIZE		9
 #define MAXBLOCKSIZE		4096
+#define L2MAXBLOCKSIZE		12
 #define	MAXFILESIZE		((s64)1 << 52)
 
 #define JFS_LINK_MAX		0xffffffff

2.39.2

Sorry for this typo, it should be max number of blocks (per page).

Will fix and send.

Thanks,
Siddh