CVE-2021-47226 x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer

2024-05-2114:19:30

Linux

www.cve.org

cve-2021-47226

x86

linux kernel

fpu

xrstor

vulnerability

amd

intel

microarchitectural conditions

6.4 Medium

AI Score

Confidence

High

0.0004 Low

EPSS

Percentile

15.6%

JSON

In the Linux kernel, the following vulnerability has been resolved:

x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer

Both Intel and AMD consider it to be architecturally valid for XRSTOR to
fail with #PF but nonetheless change the register state. The actual
conditions under which this might occur are unclear [1], but it seems
plausible that this might be triggered if one sibling thread unmaps a page
and invalidates the shared TLB while another sibling thread is executing
XRSTOR on the page in question.

__fpu__restore_sig() can execute XRSTOR while the hardware registers
are preserved on behalf of a different victim task (using the
fpu_fpregs_owner_ctx mechanism), and, in theory, XRSTOR could fail but
modify the registers.

If this happens, then there is a window in which __fpu__restore_sig()
could schedule out and the victim task could schedule back in without
reloading its own FPU registers. This would result in part of the FPU
state that __fpu__restore_sig() was attempting to load leaking into the
victim task’s user-visible state.

Invalidate preserved FPU registers on XRSTOR failure to prevent this
situation from corrupting any state.

[1] Frequent readers of the errata lists might imagine “complex
microarchitectural conditions”.

CNA Affected

[
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "unaffected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "arch/x86/kernel/fpu/signal.c"
    ],
    "versions": [
      {
        "version": "1d731e731c4c",
        "lessThan": "a7748e021b9f",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "1d731e731c4c",
        "lessThan": "002665dcba4b",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "1d731e731c4c",
        "lessThan": "d8778e393afa",
        "status": "affected",
        "versionType": "git"
      }
    ]
  },
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "affected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "arch/x86/kernel/fpu/signal.c"
    ],
    "versions": [
      {
        "version": "5.2",
        "status": "affected"
      },
      {
        "version": "0",
        "lessThan": "5.2",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.10.46",
        "lessThanOrEqual": "5.10.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.12.13",
        "lessThanOrEqual": "5.12.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.13",
        "lessThanOrEqual": "*",
        "status": "unaffected",
        "versionType": "original_commit_for_fix"
      }
    ]
  }
]