[PATCH] fs: Fix UBSAN detected shift-out-bounds error for bad superblock

[Liao Chang]

UBSAN: shift-out-of-bounds in fs/minix/bitmap.c:103:3
shift exponent 8192 is too large for 32-bit type 'unsigned int'
CPU: 1 PID: 32273 Comm: syz-executor.0 Tainted: G        W
6.1.0-rc4-dirty #11
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0xcd/0x134
 ubsan_epilogue+0xb/0x50
 __ubsan_handle_shift_out_of_bounds.cold+0xb1/0x18d
 minix_count_free_blocks.cold+0x16/0x1b
 minix_statfs+0x22a/0x490
 statfs_by_dentry+0x133/0x210
 user_statfs+0xa9/0x160
 __do_sys_statfs+0x7a/0xf0
 do_syscall_64+0x35/0x80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

The superblock stores on disk contains the size of a data zone, which is
too large to used as the shift when kernel try to calculate the total
size of zones, so it needs to check the superblock when kernel mounts
MINIX-FS.

Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx>
---
 fs/minix/inode.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/minix/inode.c b/fs/minix/inode.c
index da8bdd1712a7..f1d1c2312817 100644
--- a/fs/minix/inode.c
+++ b/fs/minix/inode.c
@@ -166,6 +166,12 @@ static bool minix_check_superblock(struct super_block *sb)
 	    sb->s_maxbytes > (7 + 512 + 512*512) * BLOCK_SIZE)
 		return false;
 
+	/* the total size of zones must no exceed the limitation of U32_MAX. */
+	if (sbi->s_log_zone_size && (sbi->s_nzones - sbi->s_firstdatazone) &&
+	    (__builtin_clzl((__u32)(sbi->s_nzones - sbi->s_firstdatazone)) <=
+	     sbi->s_log_zone_size))
+		return false;
+
 	return true;
 }
 
-- 
2.17.1