CVE-2026-43278: dm: clear cloned request bio pointer when last clone bio completes

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

dm: clear cloned request bio pointer when last clone bio completes

Stale rq->bio values have been observed to cause double-initialization of cloned bios in request-based device-mapper targets, leading to use-after-free and double-free scenarios.

One such case occurs when using dm-multipath on top of a PCIe NVMe namespace, where cloned request bios are freed during blkcompleterequest(), but rq->bio is left intact. Subsequent clone teardown then attempts to free the same bios again via blkrqunprepclone().

The resulting double-free path looks like:

nvmepcicompletebatch() nvmecompletebatch() blkmqendrequestbatch() blkcompleterequest() // called on a DM clone request bioendio() // first free of all clone bios ... rq->endio() // endclonerequest() dmcompleterequest(tio->orig) dmsoftirqdone() dmdone() dmendrequest() blkrqunprepclone() // second free of clone bios

Fix this by clearing the clone request's bio pointer when the last cloned bio completes, ensuring that later teardown paths do not attempt to free already-released bios.