CVE-2026-45855: ata: libata-scsi: avoid Non-NCQ command starvation

CVE-2026-45855: ata: libata-scsi: avoid Non-NCQ command starvation

[Greg Kroah-Hartman]

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

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

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

ata: libata-scsi: avoid Non-NCQ command starvation

When a non-NCQ command is issued while NCQ commands are being executed,
ata_scsi_qc_issue() indicates to the SCSI layer that the command issuing
should be deferred by returning SCSI_MLQUEUE_XXX_BUSY.  This command
deferring is correct and as mandated by the ACS specifications since
NCQ and non-NCQ commands cannot be mixed.

However, in the case of a host adapter using multiple submission queues,
when the target device is under a constant load of NCQ commands, there
are no guarantees that requeueing the non-NCQ command will be executed
later and it may be deferred again repeatedly as other submission queues
can constantly issue NCQ commands from different CPUs ahead of the
non-NCQ command. This can lead to very long delays for the execution of
non-NCQ commands, and even complete starvation for these commands in the
worst case scenario.

Since the block layer and the SCSI layer do not distinguish between
queueable (NCQ) and non queueable (non-NCQ) commands, libata-scsi SAT
implementation must ensure forward progress for non-NCQ commands in the
presence of NCQ command traffic. This is similar to what SAS HBAs with a
hardware/firmware based SAT implementation do.

Implement such forward progress guarantee by limiting requeueing of
non-NCQ commands from ata_scsi_qc_issue(): when a non-NCQ command is
received and NCQ commands are in-flight, do not force a requeue of the
non-NCQ command by returning SCSI_MLQUEUE_XXX_BUSY and instead return 0
to indicate that the command was accepted but hold on to the qc using
the new deferred_qc field of struct ata_port.

This deferred qc will be issued using the work item deferred_qc_work
running the function ata_scsi_deferred_qc_work() once all in-flight
commands complete, which is checked with the port qc_defer() callback
return value indicating that no further delay is necessary. This check
is done using the helper function ata_scsi_schedule_deferred_qc() which
is called from ata_scsi_qc_complete(). This thus excludes this mechanism
from all internal non-NCQ commands issued by ATA EH.

When a port deferred_qc is non NULL, that is, the port has a command
waiting for the device queue to drain, the issuing of all incoming
commands (both NCQ and non-NCQ) is deferred using the regular busy
mechanism. This simplifies the code and also avoids potential denial of
service problems if a user issues too many non-NCQ commands.

Finally, whenever ata EH is scheduled, regardless of the reason, a
deferred qc is always requeued so that it can be retried once EH
completes. This is done by calling the function
ata_scsi_requeue_deferred_qc() from ata_eh_set_pending(). This avoids
the need for any special processing for the deferred qc in case of NCQ
error, link or device reset, or device timeout.

The Linux kernel CVE team has assigned CVE-2026-45855 to this issue.


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

	Issue introduced in 5.10 with commit bdb01301f3ea51a59eff252b06643fc1fe843e57 and fixed in 6.12.77 with commit ce22aaed011206fed9cbd8c9c2d44718607f31ee
	Issue introduced in 5.10 with commit bdb01301f3ea51a59eff252b06643fc1fe843e57 and fixed in 6.18.14 with commit 888cd7e40adb2ef4af1b4d3b6e2e83ad409ae8c2
	Issue introduced in 5.10 with commit bdb01301f3ea51a59eff252b06643fc1fe843e57 and fixed in 6.19.4 with commit 5d61a38a60e62750526d94663b69b7ac5c7f07a5
	Issue introduced in 5.10 with commit bdb01301f3ea51a59eff252b06643fc1fe843e57 and fixed in 7.0 with commit 0ea84089dbf62a92dc7889c79e6b18fc89260808

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-2026-45855
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/ata/libata-core.c
	drivers/ata/libata-eh.c
	drivers/ata/libata-scsi.c
	drivers/ata/libata.h
	include/linux/libata.h


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/ce22aaed011206fed9cbd8c9c2d44718607f31ee
	https://git.kernel.org/stable/c/888cd7e40adb2ef4af1b4d3b6e2e83ad409ae8c2
	https://git.kernel.org/stable/c/5d61a38a60e62750526d94663b69b7ac5c7f07a5
	https://git.kernel.org/stable/c/0ea84089dbf62a92dc7889c79e6b18fc89260808