In the Linux kernel, the following vulnerability has been resolved: Squashfs: check return result of sbminblocksize Syzkaller reports an "UBSAN: shift-out-of-bounds in squashfsbioread" bug. Syzkaller forks multiple processes which after mounting the Squashfs filesystem, issues an ioctl("/dev/loop0", LOOPSETBLOCKSIZE, 0x8000). Now if this ioctl occurs at the same time another process is in the process of mounting a Squashfs filesystem on /dev/loop0, the failure occurs. When this happens the following code in squashfsfillsuper() fails. ---- msblk->devblksize = sbminblocksize(sb, SQUASHFSDEVBLKSIZE); msblk->devblksizelog2 = ffz(~msblk->devblksize); ---- sbminblocksize() returns 0, which means msblk->devblksize is set to 0. As a result, ffz(~msblk->devblksize) returns 64, and msblk->devblksizelog2 is set to 64. This subsequently causes the UBSAN: shift-out-of-bounds in fs/squashfs/block.c:195:36 shift exponent 64 is too large for 64-bit type 'u64' (aka 'unsigned long long') This commit adds a check for a 0 return by sbmin_blocksize().