The long version is written up at https://bugzilla.redhat.com/show_bug.cgi?id=1783075 but the short version: There are devices out there which set q->limits.io_opt to small values like 4096 bytes, causing bcache to use that for the stripe size, but the device size could still be large enough that the computed stripe count is 2**32 or more. That value gets stuffed into a 32-bit (unsigned int) field, throwing away the high bits, and then that truncated value is range-checked and used. This can result in memory corruption or faults in some cases. The problem was brought up with us on Red Hat's VDO driver team by a bcache user on a 4.17.8 kernel, has been demonstrated in the Fedora 5.3.15-300.fc31 kernel, and by inspection appears to be present in Linus's tree as of this morning. The easy fix would be to keep the quotient in a 64-bit variable until it's validated, but that would simply limit the size of such devices as bcache backing storage (in this case, limiting VDO volumes to under 8 TB). Is there a way to still be able to use larger devices? Perhaps scale up the stripe size from io_opt to the point where the stripe count falls in the allowed range? Ken Raeburn (Red Hat VDO driver developer)
