CVE-2021-4440 (GCVE-0-2021-4440)

Vulnerability from cvelistv5 – Published: 2024-06-25 14:20 – Updated: 2025-05-04 06:59
VLAI?
Title
x86/xen: Drop USERGS_SYSRET64 paravirt call
Summary
In the Linux kernel, the following vulnerability has been resolved: x86/xen: Drop USERGS_SYSRET64 paravirt call commit afd30525a659ac0ae0904f0cb4a2ca75522c3123 upstream. USERGS_SYSRET64 is used to return from a syscall via SYSRET, but a Xen PV guest will nevertheless use the IRET hypercall, as there is no sysret PV hypercall defined. So instead of testing all the prerequisites for doing a sysret and then mangling the stack for Xen PV again for doing an iret just use the iret exit from the beginning. This can easily be done via an ALTERNATIVE like it is done for the sysenter compat case already. It should be noted that this drops the optimization in Xen for not restoring a few registers when returning to user mode, but it seems as if the saved instructions in the kernel more than compensate for this drop (a kernel build in a Xen PV guest was slightly faster with this patch applied). While at it remove the stale sysret32 remnants. [ pawan: Brad Spengler and Salvatore Bonaccorso <carnil@debian.org> reported a problem with the 5.10 backport commit edc702b4a820 ("x86/entry_64: Add VERW just before userspace transition"). When CONFIG_PARAVIRT_XXL=y, CLEAR_CPU_BUFFERS is not executed in syscall_return_via_sysret path as USERGS_SYSRET64 is runtime patched to: .cpu_usergs_sysret64 = { 0x0f, 0x01, 0xf8, 0x48, 0x0f, 0x07 }, // swapgs; sysretq which is missing CLEAR_CPU_BUFFERS. It turns out dropping USERGS_SYSRET64 simplifies the code, allowing CLEAR_CPU_BUFFERS to be explicitly added to syscall_return_via_sysret path. Below is with CONFIG_PARAVIRT_XXL=y and this patch applied: syscall_return_via_sysret: ... <+342>: swapgs <+345>: xchg %ax,%ax <+347>: verw -0x1a2(%rip) <------ <+354>: sysretq ]
Severity ?
8.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
CWE
  • CWE-400 - Uncontrolled Resource Consumption
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: cea750c99d8f6391080c420f811a46b21bad7cf4 , < 1424ab4bb386df9cc590c73afa55f13e9b00dea2 (git)
Create a notification for this product.
    Linux Linux Affected: 5.10.215 , < 5.10.218 (semver)
Create a notification for this product.
Show details on NVD website
To clipboard 

{
  "containers": {
    "adp": [
      {
        "affected": [
          {
            "cpes": [
              "cpe:2.3:o:linux:linux_kernel:-:*:*:*:*:*:*:*"
            ],
            "defaultStatus": "unknown",
            "product": "linux_kernel",
            "vendor": "linux",
            "versions": [
              {
                "lessThan": "1424ab4bb386",
                "status": "affected",
                "version": "cea750c99d8f",
                "versionType": "custom"
              }
            ]
          },
          {
            "cpes": [
              "cpe:2.3:o:linux:linux_kernel:-:*:*:*:*:*:*:*"
            ],
            "defaultStatus": "unknown",
            "product": "linux_kernel",
            "vendor": "linux",
            "versions": [
              {
                "lessThan": "5.10.218",
                "status": "affected",
                "version": "5.10.215",
                "versionType": "custom"
              }
            ]
          }
        ],
        "metrics": [
          {
            "cvssV3_1": {
              "attackComplexity": "LOW",
              "attackVector": "LOCAL",
              "availabilityImpact": "HIGH",
              "baseScore": 8.8,
              "baseSeverity": "HIGH",
              "confidentialityImpact": "HIGH",
              "integrityImpact": "HIGH",
              "privilegesRequired": "LOW",
              "scope": "CHANGED",
              "userInteraction": "NONE",
              "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H",
              "version": "3.1"
            }
          },
          {
            "other": {
              "content": {
                "id": "CVE-2021-4440",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "total"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2024-06-26T13:55:14.340611Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "problemTypes": [
          {
            "descriptions": [
              {
                "cweId": "CWE-400",
                "description": "CWE-400 Uncontrolled Resource Consumption",
                "lang": "en",
                "type": "CWE"
              }
            ]
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2024-06-26T14:00:48.356Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      },
      {
        "providerMetadata": {
          "dateUpdated": "2024-08-03T17:30:07.494Z",
          "orgId": "af854a3a-2127-422b-91ae-364da2661108",
          "shortName": "CVE"
        },
        "references": [
          {
            "tags": [
              "x_transferred"
            ],
            "url": "https://git.kernel.org/stable/c/1424ab4bb386df9cc590c73afa55f13e9b00dea2"
          },
          {
            "tags": [
              "x_transferred"
            ],
            "url": "https://grsecurity.net/cve-2021-4440_linux_cna_case_study"
          }
        ],
        "title": "CVE Program Container"
      }
    ],
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "arch/x86/entry/entry_64.S",
            "arch/x86/include/asm/irqflags.h",
            "arch/x86/include/asm/paravirt.h",
            "arch/x86/include/asm/paravirt_types.h",
            "arch/x86/kernel/asm-offsets_64.c",
            "arch/x86/kernel/paravirt.c",
            "arch/x86/kernel/paravirt_patch.c",
            "arch/x86/xen/enlighten_pv.c",
            "arch/x86/xen/xen-asm.S",
            "arch/x86/xen/xen-ops.h"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "1424ab4bb386df9cc590c73afa55f13e9b00dea2",
              "status": "affected",
              "version": "cea750c99d8f6391080c420f811a46b21bad7cf4",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "arch/x86/entry/entry_64.S",
            "arch/x86/include/asm/irqflags.h",
            "arch/x86/include/asm/paravirt.h",
            "arch/x86/include/asm/paravirt_types.h",
            "arch/x86/kernel/asm-offsets_64.c",
            "arch/x86/kernel/paravirt.c",
            "arch/x86/kernel/paravirt_patch.c",
            "arch/x86/xen/enlighten_pv.c",
            "arch/x86/xen/xen-asm.S",
            "arch/x86/xen/xen-ops.h"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "5.10.218",
              "status": "affected",
              "version": "5.10.215",
              "versionType": "semver"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "5.10.218",
                  "versionStartIncluding": "5.10.215",
                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nx86/xen: Drop USERGS_SYSRET64 paravirt call\n\ncommit afd30525a659ac0ae0904f0cb4a2ca75522c3123 upstream.\n\nUSERGS_SYSRET64 is used to return from a syscall via SYSRET, but\na Xen PV guest will nevertheless use the IRET hypercall, as there\nis no sysret PV hypercall defined.\n\nSo instead of testing all the prerequisites for doing a sysret and\nthen mangling the stack for Xen PV again for doing an iret just use\nthe iret exit from the beginning.\n\nThis can easily be done via an ALTERNATIVE like it is done for the\nsysenter compat case already.\n\nIt should be noted that this drops the optimization in Xen for not\nrestoring a few registers when returning to user mode, but it seems\nas if the saved instructions in the kernel more than compensate for\nthis drop (a kernel build in a Xen PV guest was slightly faster with\nthis patch applied).\n\nWhile at it remove the stale sysret32 remnants.\n\n  [ pawan: Brad Spengler and Salvatore Bonaccorso \u003ccarnil@debian.org\u003e\n\t   reported a problem with the 5.10 backport commit edc702b4a820\n\t   (\"x86/entry_64: Add VERW just before userspace transition\").\n\n\t   When CONFIG_PARAVIRT_XXL=y, CLEAR_CPU_BUFFERS is not executed in\n\t   syscall_return_via_sysret path as USERGS_SYSRET64 is runtime\n\t   patched to:\n\n\t.cpu_usergs_sysret64    = { 0x0f, 0x01, 0xf8,\n\t\t\t\t    0x48, 0x0f, 0x07 }, // swapgs; sysretq\n\n\t   which is missing CLEAR_CPU_BUFFERS. It turns out dropping\n\t   USERGS_SYSRET64 simplifies the code, allowing CLEAR_CPU_BUFFERS\n\t   to be explicitly added to syscall_return_via_sysret path. Below\n\t   is with CONFIG_PARAVIRT_XXL=y and this patch applied:\n\n\t   syscall_return_via_sysret:\n\t   ...\n\t   \u003c+342\u003e:   swapgs\n\t   \u003c+345\u003e:   xchg   %ax,%ax\n\t   \u003c+347\u003e:   verw   -0x1a2(%rip)  \u003c------\n\t   \u003c+354\u003e:   sysretq\n  ]"
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2025-05-04T06:59:54.176Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/1424ab4bb386df9cc590c73afa55f13e9b00dea2"
        },
        {
          "url": "https://grsecurity.net/cve-2021-4440_linux_cna_case_study"
        }
      ],
      "title": "x86/xen: Drop USERGS_SYSRET64 paravirt call",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2021-4440",
    "datePublished": "2024-06-25T14:20:00.740Z",
    "dateReserved": "2024-06-25T14:16:59.867Z",
    "dateUpdated": "2025-05-04T06:59:54.176Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.1",
  "vulnerability-lookup:meta": {
    "vulnrichment": {
      "containers": "{\"adp\": [{\"title\": \"CVE Program Container\", \"references\": [{\"url\": \"https://git.kernel.org/stable/c/1424ab4bb386df9cc590c73afa55f13e9b00dea2\", \"tags\": [\"x_transferred\"]}, {\"url\": \"https://grsecurity.net/cve-2021-4440_linux_cna_case_study\", \"tags\": [\"x_transferred\"]}], \"providerMetadata\": {\"orgId\": \"af854a3a-2127-422b-91ae-364da2661108\", \"shortName\": \"CVE\", \"dateUpdated\": \"2024-08-03T17:30:07.494Z\"}}, {\"title\": \"CISA ADP Vulnrichment\", \"metrics\": [{\"cvssV3_1\": {\"scope\": \"CHANGED\", \"version\": \"3.1\", \"baseScore\": 8.8, \"attackVector\": \"LOCAL\", \"baseSeverity\": \"HIGH\", \"vectorString\": \"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H\", \"integrityImpact\": \"HIGH\", \"userInteraction\": \"NONE\", \"attackComplexity\": \"LOW\", \"availabilityImpact\": \"HIGH\", \"privilegesRequired\": \"LOW\", \"confidentialityImpact\": \"HIGH\"}}, {\"other\": {\"type\": \"ssvc\", \"content\": {\"id\": \"CVE-2021-4440\", \"role\": \"CISA Coordinator\", \"options\": [{\"Exploitation\": \"none\"}, {\"Automatable\": \"no\"}, {\"Technical Impact\": \"total\"}], \"version\": \"2.0.3\", \"timestamp\": \"2024-06-26T13:55:14.340611Z\"}}}], \"affected\": [{\"cpes\": [\"cpe:2.3:o:linux:linux_kernel:-:*:*:*:*:*:*:*\"], \"vendor\": \"linux\", \"product\": \"linux_kernel\", \"versions\": [{\"status\": \"affected\", \"version\": \"cea750c99d8f\", \"lessThan\": \"1424ab4bb386\", \"versionType\": \"custom\"}], \"defaultStatus\": \"unknown\"}, {\"cpes\": [\"cpe:2.3:o:linux:linux_kernel:-:*:*:*:*:*:*:*\"], \"vendor\": \"linux\", \"product\": \"linux_kernel\", \"versions\": [{\"status\": \"affected\", \"version\": \"5.10.215\", \"lessThan\": \"5.10.218\", \"versionType\": \"custom\"}], \"defaultStatus\": \"unknown\"}], \"problemTypes\": [{\"descriptions\": [{\"lang\": \"en\", \"type\": \"CWE\", \"cweId\": \"CWE-400\", \"description\": \"CWE-400 Uncontrolled Resource Consumption\"}]}], \"providerMetadata\": {\"orgId\": \"134c704f-9b21-4f2e-91b3-4a467353bcc0\", \"shortName\": \"CISA-ADP\", \"dateUpdated\": \"2024-06-26T13:57:16.433Z\"}}], \"cna\": {\"title\": \"x86/xen: Drop USERGS_SYSRET64 paravirt call\", \"affected\": [{\"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\", \"vendor\": \"Linux\", \"product\": \"Linux\", \"versions\": [{\"status\": \"affected\", \"version\": \"cea750c99d8f6391080c420f811a46b21bad7cf4\", \"lessThan\": \"1424ab4bb386df9cc590c73afa55f13e9b00dea2\", \"versionType\": \"git\"}], \"programFiles\": [\"arch/x86/entry/entry_64.S\", \"arch/x86/include/asm/irqflags.h\", \"arch/x86/include/asm/paravirt.h\", \"arch/x86/include/asm/paravirt_types.h\", \"arch/x86/kernel/asm-offsets_64.c\", \"arch/x86/kernel/paravirt.c\", \"arch/x86/kernel/paravirt_patch.c\", \"arch/x86/xen/enlighten_pv.c\", \"arch/x86/xen/xen-asm.S\", \"arch/x86/xen/xen-ops.h\"], \"defaultStatus\": \"unaffected\"}, {\"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\", \"vendor\": \"Linux\", \"product\": \"Linux\", \"versions\": [{\"status\": \"affected\", \"version\": \"5.10.215\", \"lessThan\": \"5.10.218\", \"versionType\": \"semver\"}], \"programFiles\": [\"arch/x86/entry/entry_64.S\", \"arch/x86/include/asm/irqflags.h\", \"arch/x86/include/asm/paravirt.h\", \"arch/x86/include/asm/paravirt_types.h\", \"arch/x86/kernel/asm-offsets_64.c\", \"arch/x86/kernel/paravirt.c\", \"arch/x86/kernel/paravirt_patch.c\", \"arch/x86/xen/enlighten_pv.c\", \"arch/x86/xen/xen-asm.S\", \"arch/x86/xen/xen-ops.h\"], \"defaultStatus\": \"unaffected\"}], \"references\": [{\"url\": \"https://git.kernel.org/stable/c/1424ab4bb386df9cc590c73afa55f13e9b00dea2\"}, {\"url\": \"https://grsecurity.net/cve-2021-4440_linux_cna_case_study\"}], \"x_generator\": {\"engine\": \"bippy-1.2.0\"}, \"descriptions\": [{\"lang\": \"en\", \"value\": \"In the Linux kernel, the following vulnerability has been resolved:\\n\\nx86/xen: Drop USERGS_SYSRET64 paravirt call\\n\\ncommit afd30525a659ac0ae0904f0cb4a2ca75522c3123 upstream.\\n\\nUSERGS_SYSRET64 is used to return from a syscall via SYSRET, but\\na Xen PV guest will nevertheless use the IRET hypercall, as there\\nis no sysret PV hypercall defined.\\n\\nSo instead of testing all the prerequisites for doing a sysret and\\nthen mangling the stack for Xen PV again for doing an iret just use\\nthe iret exit from the beginning.\\n\\nThis can easily be done via an ALTERNATIVE like it is done for the\\nsysenter compat case already.\\n\\nIt should be noted that this drops the optimization in Xen for not\\nrestoring a few registers when returning to user mode, but it seems\\nas if the saved instructions in the kernel more than compensate for\\nthis drop (a kernel build in a Xen PV guest was slightly faster with\\nthis patch applied).\\n\\nWhile at it remove the stale sysret32 remnants.\\n\\n  [ pawan: Brad Spengler and Salvatore Bonaccorso \u003ccarnil@debian.org\u003e\\n\\t   reported a problem with the 5.10 backport commit edc702b4a820\\n\\t   (\\\"x86/entry_64: Add VERW just before userspace transition\\\").\\n\\n\\t   When CONFIG_PARAVIRT_XXL=y, CLEAR_CPU_BUFFERS is not executed in\\n\\t   syscall_return_via_sysret path as USERGS_SYSRET64 is runtime\\n\\t   patched to:\\n\\n\\t.cpu_usergs_sysret64    = { 0x0f, 0x01, 0xf8,\\n\\t\\t\\t\\t    0x48, 0x0f, 0x07 }, // swapgs; sysretq\\n\\n\\t   which is missing CLEAR_CPU_BUFFERS. It turns out dropping\\n\\t   USERGS_SYSRET64 simplifies the code, allowing CLEAR_CPU_BUFFERS\\n\\t   to be explicitly added to syscall_return_via_sysret path. Below\\n\\t   is with CONFIG_PARAVIRT_XXL=y and this patch applied:\\n\\n\\t   syscall_return_via_sysret:\\n\\t   ...\\n\\t   \u003c+342\u003e:   swapgs\\n\\t   \u003c+345\u003e:   xchg   %ax,%ax\\n\\t   \u003c+347\u003e:   verw   -0x1a2(%rip)  \u003c------\\n\\t   \u003c+354\u003e:   sysretq\\n  ]\"}], \"cpeApplicability\": [{\"nodes\": [{\"negate\": false, \"cpeMatch\": [{\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.10.218\", \"versionStartIncluding\": \"5.10.215\"}], \"operator\": \"OR\"}]}], \"providerMetadata\": {\"orgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"shortName\": \"Linux\", \"dateUpdated\": \"2025-05-04T06:59:54.176Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2021-4440\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2025-05-04T06:59:54.176Z\", \"dateReserved\": \"2024-06-25T14:16:59.867Z\", \"assignerOrgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"datePublished\": \"2024-06-25T14:20:00.740Z\", \"assignerShortName\": \"Linux\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.1"
    }
  }
}


Log in or create an account to share your comment.




Tags
Taxonomy of the tags.




Sightings

Author Source Type Date

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



Detection rules are retrieved from Rulezet.