From: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> commit 3bef198f1b17d1bb89260bad947ef084c0a2d1a6 upstream. syzbot is feeding invalid superblock data to JFS for mount testing. JFS does not check several of the fields -- just assumes that they are good since the JFS_MAGIC and version fields are good. In this case (syzbot reproducer), we have s_l2bsize == 0xda0c, pad == 0xf045, and s_state == 0x50, all of which are invalid IMO. Having s_l2bsize == 0xda0c causes this UBSAN warning: UBSAN: shift-out-of-bounds in fs/jfs/jfs_mount.c:373:25 shift exponent -9716 is negative s_l2bsize can be tested for correctness. pad can be tested for non-0 and punted. s_state can be tested for its valid values and punted. Do those 3 tests and if any of them fails, report the superblock as invalid/corrupt and let fsck handle it. With this patch, chkSuper() says this when JFS_DEBUG is enabled: jfs_mount: Mount Failure: superblock is corrupt! Mount JFS Failure: -22 jfs_mount failed w/return code = -22 The obvious problem with this method is that next week there could be another syzbot test that uses different fields for invalid values, this making this like a game of whack-a-mole. syzkaller link: https://syzkaller.appspot.com/bug?extid=36315852ece4132ec193 Reported-by: syzbot+36315852ece4132ec193@xxxxxxxxxxxxxxxxxxxxxxxxx Reported-by: kernel test robot <lkp@xxxxxxxxx> # v2 Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> Signed-off-by: Dave Kleikamp <dave.kleikamp@xxxxxxxxxx> Cc: jfs-discussion@xxxxxxxxxxxxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/jfs/jfs_filsys.h | 1 + fs/jfs/jfs_mount.c | 10 ++++++++++ 2 files changed, 11 insertions(+) --- a/fs/jfs/jfs_filsys.h +++ b/fs/jfs/jfs_filsys.h @@ -281,5 +281,6 @@ * fsck() must be run to repair */ #define FM_EXTENDFS 0x00000008 /* file system extendfs() in progress */ +#define FM_STATE_MAX 0x0000000f /* max value of s_state */ #endif /* _H_JFS_FILSYS */ --- a/fs/jfs/jfs_mount.c +++ b/fs/jfs/jfs_mount.c @@ -49,6 +49,7 @@ #include <linux/fs.h> #include <linux/buffer_head.h> +#include <linux/log2.h> #include "jfs_incore.h" #include "jfs_filsys.h" @@ -378,6 +379,15 @@ static int chkSuper(struct super_block * sbi->bsize = bsize; sbi->l2bsize = le16_to_cpu(j_sb->s_l2bsize); + /* check some fields for possible corruption */ + if (sbi->l2bsize != ilog2((u32)bsize) || + j_sb->pad != 0 || + le32_to_cpu(j_sb->s_state) > FM_STATE_MAX) { + rc = -EINVAL; + jfs_err("jfs_mount: Mount Failure: superblock is corrupt!"); + goto out; + } + /* * For now, ignore s_pbsize, l2bfactor. All I/O going through buffer * cache.
