ghsa-vc52-cgfp-jw27
Vulnerability from github
Published
2024-06-25 15:31
Modified
2024-07-11 18:31
Severity ?
8.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
Details

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

]

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{
  "affected": [],
  "aliases": [
    "CVE-2021-4440"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-400"
    ],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2024-06-25T15:15:11Z",
    "severity": "HIGH"
  },
  "details": "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  ]",
  "id": "GHSA-vc52-cgfp-jw27",
  "modified": "2024-07-11T18:31:11Z",
  "published": "2024-06-25T15:31:08Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-4440"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/1424ab4bb386df9cc590c73afa55f13e9b00dea2"
    },
    {
      "type": "WEB",
      "url": "https://grsecurity.net/cve-2021-4440_linux_cna_case_study"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ]
}


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