gfs2: Check sb_bsize_shift after reading superblock

In the Linux kernel, the following vulnerability has been resolved:

gfs2: Check sb_bsize_shift after reading superblock

Fuzzers like to scribble over sb_bsize_shift but in reality it's very

unlikely that this field would be corrupted on its own. Nevertheless it

should be checked to avoid the possibility of messy mount errors due to

bad calculations. It's always a fixed value based on the block size so

we can just check that it's the expected value.

Tested with:

mkfs.gfs2 -O -p lock_nolock /dev/vdb

for i in 0 -1 64 65 32 33; do

gfs2_edit -p sb field sb_bsize_shift $i /dev/vdb

mount /dev/vdb /mnt/test && umount /mnt/test

done

Before this patch we get a withdraw after

[ 76.413681] gfs2: fsid=loop0.0: fatal: invalid metadata block

[ 76.413681] bh = 19 (type: exp=5, found=4)

[ 76.413681] function = gfs2_meta_buffer, file = fs/gfs2/meta_io.c, line = 492

and with UBSAN configured we also get complaints like

[ 76.373395] UBSAN: shift-out-of-bounds in fs/gfs2/ops_fstype.c:295:19

[ 76.373815] shift exponent 4294967287 is too large for 64-bit type 'long unsigned int'

After the patch, these complaints don't appear, mount fails immediately

and we get an explanation in dmesg.