GHSA-CJWF-5VH9-W48G
Vulnerability from github – Published: 2024-10-21 18:30 – Updated: 2024-10-25 15:31In the Linux kernel, the following vulnerability has been resolved:
iommu/vt-d: Fix potential lockup if qi_submit_sync called with 0 count
If qi_submit_sync() is invoked with 0 invalidation descriptors (for instance, for DMA draining purposes), we can run into a bug where a submitting thread fails to detect the completion of invalidation_wait. Subsequently, this led to a soft lockup. Currently, there is no impact by this bug on the existing users because no callers are submitting invalidations with 0 descriptors. This fix will enable future users (such as DMA drain) calling qi_submit_sync() with 0 count.
Suppose thread T1 invokes qi_submit_sync() with non-zero descriptors, while concurrently, thread T2 calls qi_submit_sync() with zero descriptors. Both threads then enter a while loop, waiting for their respective descriptors to complete. T1 detects its completion (i.e., T1's invalidation_wait status changes to QI_DONE by HW) and proceeds to call reclaim_free_desc() to reclaim all descriptors, potentially including adjacent ones of other threads that are also marked as QI_DONE.
During this time, while T2 is waiting to acquire the qi->q_lock, the IOMMU hardware may complete the invalidation for T2, setting its status to QI_DONE. However, if T1's execution of reclaim_free_desc() frees T2's invalidation_wait descriptor and changes its status to QI_FREE, T2 will not observe the QI_DONE status for its invalidation_wait and will indefinitely remain stuck.
This soft lockup does not occur when only non-zero descriptors are submitted.In such cases, invalidation descriptors are interspersed among wait descriptors with the status QI_IN_USE, acting as barriers. These barriers prevent the reclaim code from mistakenly freeing descriptors belonging to other submitters.
Considered the following example timeline: T1 T2 ======================================== ID1 WD1 while(WD1!=QI_DONE) unlock lock WD1=QI_DONE WD2 while(WD2!=QI_DONE) unlock lock WD1==QI_DONE? ID1=QI_DONE WD2=DONE reclaim() ID1=FREE WD1=FREE WD2=FREE unlock soft lockup! T2 never sees QI_DONE in WD2
Where: ID = invalidation descriptor WD = wait descriptor * Written by hardware
The root of the problem is that the descriptor status QI_DONE flag is used for two conflicting purposes: 1. signal a descriptor is ready for reclaim (to be freed) 2. signal by the hardware that a wait descriptor is complete
The solution (in this patch) is state separation by using QI_FREE flag for #1.
Once a thread's invalidation descriptors are complete, their status would be set to QI_FREE. The reclaim_free_desc() function would then only free descriptors marked as QI_FREE instead of those marked as QI_DONE. This change ensures that T2 (from the previous example) will correctly observe the completion of its invalidation_wait (marked as QI_DONE).
{
"affected": [],
"aliases": [
"CVE-2024-49993"
],
"database_specific": {
"cwe_ids": [
"CWE-667"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2024-10-21T18:15:19Z",
"severity": "MODERATE"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\niommu/vt-d: Fix potential lockup if qi_submit_sync called with 0 count\n\nIf qi_submit_sync() is invoked with 0 invalidation descriptors (for\ninstance, for DMA draining purposes), we can run into a bug where a\nsubmitting thread fails to detect the completion of invalidation_wait.\nSubsequently, this led to a soft lockup. Currently, there is no impact\nby this bug on the existing users because no callers are submitting\ninvalidations with 0 descriptors. This fix will enable future users\n(such as DMA drain) calling qi_submit_sync() with 0 count.\n\nSuppose thread T1 invokes qi_submit_sync() with non-zero descriptors, while\nconcurrently, thread T2 calls qi_submit_sync() with zero descriptors. Both\nthreads then enter a while loop, waiting for their respective descriptors\nto complete. T1 detects its completion (i.e., T1\u0027s invalidation_wait status\nchanges to QI_DONE by HW) and proceeds to call reclaim_free_desc() to\nreclaim all descriptors, potentially including adjacent ones of other\nthreads that are also marked as QI_DONE.\n\nDuring this time, while T2 is waiting to acquire the qi-\u003eq_lock, the IOMMU\nhardware may complete the invalidation for T2, setting its status to\nQI_DONE. However, if T1\u0027s execution of reclaim_free_desc() frees T2\u0027s\ninvalidation_wait descriptor and changes its status to QI_FREE, T2 will\nnot observe the QI_DONE status for its invalidation_wait and will\nindefinitely remain stuck.\n\nThis soft lockup does not occur when only non-zero descriptors are\nsubmitted.In such cases, invalidation descriptors are interspersed among\nwait descriptors with the status QI_IN_USE, acting as barriers. These\nbarriers prevent the reclaim code from mistakenly freeing descriptors\nbelonging to other submitters.\n\nConsidered the following example timeline:\n\tT1\t\t\tT2\n========================================\n\tID1\n\tWD1\n\twhile(WD1!=QI_DONE)\n\tunlock\n\t\t\t\tlock\n\tWD1=QI_DONE*\t\tWD2\n\t\t\t\twhile(WD2!=QI_DONE)\n\t\t\t\tunlock\n\tlock\n\tWD1==QI_DONE?\n\tID1=QI_DONE\t\tWD2=DONE*\n\treclaim()\n\tID1=FREE\n\tWD1=FREE\n\tWD2=FREE\n\tunlock\n\t\t\t\tsoft lockup! T2 never sees QI_DONE in WD2\n\nWhere:\nID = invalidation descriptor\nWD = wait descriptor\n* Written by hardware\n\nThe root of the problem is that the descriptor status QI_DONE flag is used\nfor two conflicting purposes:\n1. signal a descriptor is ready for reclaim (to be freed)\n2. signal by the hardware that a wait descriptor is complete\n\nThe solution (in this patch) is state separation by using QI_FREE flag\nfor #1.\n\nOnce a thread\u0027s invalidation descriptors are complete, their status would\nbe set to QI_FREE. The reclaim_free_desc() function would then only\nfree descriptors marked as QI_FREE instead of those marked as\nQI_DONE. This change ensures that T2 (from the previous example) will\ncorrectly observe the completion of its invalidation_wait (marked as\nQI_DONE).",
"id": "GHSA-cjwf-5vh9-w48g",
"modified": "2024-10-25T15:31:26Z",
"published": "2024-10-21T18:30:59Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2024-49993"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/07e4e92f84b7d3018b7064ef8d8438aeb54a2ca5"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/3cf74230c139f208b7fb313ae0054386eee31a81"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/8840dc73ac9e1028291458ef1429ec3c2524ffec"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/92ba5b014d5435dd7a1ee02a2c7f2a0e8fe06c36"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/de9e7f68762585f7532de8a06de9485bf39dbd38"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/dfdbc5ba10fb792c9d6d12ba8cb6e465f97365ed"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/e03f00aa4a6c0c49c17857a4048f586636abdc32"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
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