CVE-2026-23110: scsi: core: Wake up the error handler when final completions race against each other

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

scsi: core: Wake up the error handler when final completions race against each other

The fragile ordering between marking commands completed or failed so that the error handler only wakes when the last running command completes or times out has race conditions. These race conditions can cause the SCSI layer to fail to wake the error handler, leaving I/O through the SCSI host stuck as the error state cannot advance.

First, there is an memory ordering issue within scsidechostbusy(). The write which clears SCMDSTATEINFLIGHT may be reordered with reads counting in scsihostbusy(). While the local CPU will see its own write, reordering can allow other CPUs in scsidechostbusy() or scsiehinchostfailed() to see a raised busy count, causing no CPU to see a host busy equal to the hostfailed count.

This race condition can be prevented with a memory barrier on the error path to force the write to be visible before counting host busy commands.

Second, there is a general ordering issue with scsiehinchostfailed(). By counting busy commands before incrementing hostfailed, it can race with a final command in scsidechostbusy(), such that scsidechostbusy() does not see hostfailed incremented but scsiehinchostfailed() counts busy commands before SCMDSTATEINFLIGHT is cleared by scsidechostbusy(), resulting in neither waking the error handler task.

This needs the call to scsihostbusy() to be moved after hostfailed is incremented to close the race condition.