CVE-2026-99155: scsi: xen/scsifront: Check for a NULL shadow entry on a backend response

CVE-2026-99155: scsi: xen/scsifront: Check for a NULL shadow entry on a backend response

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-99155 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.19 with commit 6d1c2f48f3fcf5f42967938c8ba6b42ac6b44b43 and fixed in 6.18.59 with commit 1d91ec793d6c80eca963d426e0da832d40a062df
	Issue introduced in 5.19 with commit 6d1c2f48f3fcf5f42967938c8ba6b42ac6b44b43 and fixed in 7.2.13 with commit 15791e57f57f7050014d08cfe239d4aa1e6b94b0
	Issue introduced in 5.19 with commit 6d1c2f48f3fcf5f42967938c8ba6b42ac6b44b43 and fixed in 7.3 with commit 8c8222c255772713eb36cde695777a1fce224917

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-99155
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/scsi/xen-scsifront.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/1d91ec793d6c80eca963d426e0da832d40a062df
	https://git.kernel.org/stable/c/15791e57f57f7050014d08cfe239d4aa1e6b94b0
	https://git.kernel.org/stable/c/8c8222c255772713eb36cde695777a1fce224917