FKIE_CVE-2026-45855

Vulnerability from fkie_nvd - Published: 2026-05-27 14:16 - Updated: 2026-05-27 14:48
Severity
Summary
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.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/0ea84089dbf62a92dc7889c79e6b18fc89260808
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/5d61a38a60e62750526d94663b69b7ac5c7f07a5
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/888cd7e40adb2ef4af1b4d3b6e2e83ad409ae8c2
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/ce22aaed011206fed9cbd8c9c2d44718607f31ee
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{
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nata: libata-scsi: avoid Non-NCQ command starvation\n\nWhen a non-NCQ command is issued while NCQ commands are being executed,\nata_scsi_qc_issue() indicates to the SCSI layer that the command issuing\nshould be deferred by returning SCSI_MLQUEUE_XXX_BUSY.  This command\ndeferring is correct and as mandated by the ACS specifications since\nNCQ and non-NCQ commands cannot be mixed.\n\nHowever, in the case of a host adapter using multiple submission queues,\nwhen the target device is under a constant load of NCQ commands, there\nare no guarantees that requeueing the non-NCQ command will be executed\nlater and it may be deferred again repeatedly as other submission queues\ncan constantly issue NCQ commands from different CPUs ahead of the\nnon-NCQ command. This can lead to very long delays for the execution of\nnon-NCQ commands, and even complete starvation for these commands in the\nworst case scenario.\n\nSince the block layer and the SCSI layer do not distinguish between\nqueueable (NCQ) and non queueable (non-NCQ) commands, libata-scsi SAT\nimplementation must ensure forward progress for non-NCQ commands in the\npresence of NCQ command traffic. This is similar to what SAS HBAs with a\nhardware/firmware based SAT implementation do.\n\nImplement such forward progress guarantee by limiting requeueing of\nnon-NCQ commands from ata_scsi_qc_issue(): when a non-NCQ command is\nreceived and NCQ commands are in-flight, do not force a requeue of the\nnon-NCQ command by returning SCSI_MLQUEUE_XXX_BUSY and instead return 0\nto indicate that the command was accepted but hold on to the qc using\nthe new deferred_qc field of struct ata_port.\n\nThis deferred qc will be issued using the work item deferred_qc_work\nrunning the function ata_scsi_deferred_qc_work() once all in-flight\ncommands complete, which is checked with the port qc_defer() callback\nreturn value indicating that no further delay is necessary. This check\nis done using the helper function ata_scsi_schedule_deferred_qc() which\nis called from ata_scsi_qc_complete(). This thus excludes this mechanism\nfrom all internal non-NCQ commands issued by ATA EH.\n\nWhen a port deferred_qc is non NULL, that is, the port has a command\nwaiting for the device queue to drain, the issuing of all incoming\ncommands (both NCQ and non-NCQ) is deferred using the regular busy\nmechanism. This simplifies the code and also avoids potential denial of\nservice problems if a user issues too many non-NCQ commands.\n\nFinally, whenever ata EH is scheduled, regardless of the reason, a\ndeferred qc is always requeued so that it can be retried once EH\ncompletes. This is done by calling the function\nata_scsi_requeue_deferred_qc() from ata_eh_set_pending(). This avoids\nthe need for any special processing for the deferred qc in case of NCQ\nerror, link or device reset, or device timeout."
    }
  ],
  "id": "CVE-2026-45855",
  "lastModified": "2026-05-27T14:48:31.480",
  "metrics": {},
  "published": "2026-05-27T14:16:57.543",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/0ea84089dbf62a92dc7889c79e6b18fc89260808"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/5d61a38a60e62750526d94663b69b7ac5c7f07a5"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/888cd7e40adb2ef4af1b4d3b6e2e83ad409ae8c2"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/ce22aaed011206fed9cbd8c9c2d44718607f31ee"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Awaiting Analysis"
}


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