scsi: xen/scsifront: Check for a NULL shadow entry on a backend response

Overview

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

scsi: xen/scsifront: Check for a NULL shadow entry on a backend response

scsifront_do_response() validates the rqid in a backend response against VSCSIIF_MAX_REQS before using it to index info->shadow[], but then dereferences the entry to test ->inflight without checking that the slot is populated. Shadow slots are NULL before a command is submitted (the host private area is zeroed at allocation) and are reset to NULL in _scsifront_put_rqid() once a request completes, so an in-range rqid that does not correspond to an outstanding request makes the frontend dereference a NULL pointer.

A malicious or buggy backend can thus crash the guest by returning a response whose rqid is in range but not in flight -- for example a spurious response before any command has been issued, or a duplicate of one already completed. The ring has only VSCSIIF_MAX_REQS (16) slots, so before the first command every in-range rqid selects a NULL slot.

Reject a response whose shadow slot is not populated.

Affected

• Linux >= 6d1c2f48f3fcf5f42967938c8ba6b42ac6b44b43 < 1d91ec793d6c80eca963d426e0da832d40a062df
• Linux >= 6d1c2f48f3fcf5f42967938c8ba6b42ac6b44b43 < 15791e57f57f7050014d08cfe239d4aa1e6b94b0
• Linux >= 6d1c2f48f3fcf5f42967938c8ba6b42ac6b44b43 < 8c8222c255772713eb36cde695777a1fce224917
• Linux 5.19

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.