CVE-2022-49769

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.

This patch addresses an issue in GFS2 where the sb_bsize_shift field in the superblock was not validated before use. A corrupted or maliciously modified value could cause shift-out-of-bounds errors, leading to a kernel panic or undefined behavior during mount operations. By validating sb_bsize_shift, the kernel can fail gracefully with an error message instead of crashing.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.