PT-2026-43722 · Linux · Linux
Published
2026-05-27
·
Updated
2026-05-27
·
CVE-2026-45855
None
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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.
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Related Identifiers
Affected Products
Linux