[PATCH v3] jfs: jfs_dmap: Validate db_l2nbperpage while mounting

[Siddh Raman Pant <code@siddh.me>]

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 blocks per page = Page size / Min block size
=> log2(Max num_block per page) = log2(Page size / Min block size)
				= log2(Page size) - log2(Min block size)

=> Max db_l2nbperpage = L2PSIZE - L2MINBLOCKSIZE

Reported-and-tested-by: syzbot+d2cd27dcf8e04b232eb2@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?id=2a70a453331db32ed491f5cbb07e81bf2d225715
Cc: stable@vger.kernel.org
Suggested-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
Changes in v3:
- Fix typo in commit message (number of pages -> number of blocks per page).

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(+)

diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
index a3eb1e826947..da6a2bc6bf02 100644
--- a/fs/jfs/jfs_dmap.c
+++ b/fs/jfs/jfs_dmap.c
@@ -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;
diff --git a/fs/jfs/jfs_filsys.h b/fs/jfs/jfs_filsys.h
index b5d702df7111..33ef13a0b110 100644
--- a/fs/jfs/jfs_filsys.h
+++ b/fs/jfs/jfs_filsys.h
@@ -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