ghsa-gjg2-r77w-jx7q
Vulnerability from github
Published
2025-01-19 12:31
Modified
2025-01-19 12:31
Details

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

x86/fpu: Ensure shadow stack is active before "getting" registers

The x86 shadow stack support has its own set of registers. Those registers are XSAVE-managed, but they are "supervisor state components" which means that userspace can not touch them with XSAVE/XRSTOR. It also means that they are not accessible from the existing ptrace ABI for XSAVE state. Thus, there is a new ptrace get/set interface for it.

The regset code that ptrace uses provides an ->active() handler in addition to the get/set ones. For shadow stack this ->active() handler verifies that shadow stack is enabled via the ARCH_SHSTK_SHSTK bit in the thread struct. The ->active() handler is checked from some call sites of the regset get/set handlers, but not the ptrace ones. This was not understood when shadow stack support was put in place.

As a result, both the set/get handlers can be called with XFEATURE_CET_USER in its init state, which would cause get_xsave_addr() to return NULL and trigger a WARN_ON(). The ssp_set() handler luckily has an ssp_active() check to avoid surprising the kernel with shadow stack behavior when the kernel is not ready for it (ARCH_SHSTK_SHSTK==0). That check just happened to avoid the warning.

But the ->get() side wasn't so lucky. It can be called with shadow stacks disabled, triggering the warning in practice, as reported by Christina Schimpe:

WARNING: CPU: 5 PID: 1773 at arch/x86/kernel/fpu/regset.c:198 ssp_get+0x89/0xa0 [...] Call Trace: ? show_regs+0x6e/0x80 ? ssp_get+0x89/0xa0 ? __warn+0x91/0x150 ? ssp_get+0x89/0xa0 ? report_bug+0x19d/0x1b0 ? handle_bug+0x46/0x80 ? exc_invalid_op+0x1d/0x80 ? asm_exc_invalid_op+0x1f/0x30 ? __pfx_ssp_get+0x10/0x10 ? ssp_get+0x89/0xa0 ? ssp_get+0x52/0xa0 __regset_get+0xad/0xf0 copy_regset_to_user+0x52/0xc0 ptrace_regset+0x119/0x140 ptrace_request+0x13c/0x850 ? wait_task_inactive+0x142/0x1d0 ? do_syscall_64+0x6d/0x90 arch_ptrace+0x102/0x300 [...]

Ensure that shadow stacks are active in a thread before looking them up in the XSAVE buffer. Since ARCH_SHSTK_SHSTK and user_ssp[SHSTK_EN] are set at the same time, the active check ensures that there will be something to find in the XSAVE buffer.

[ dhansen: changelog/subject tweaks ]

Show details on source website


{
  "affected": [],
  "aliases": [
    "CVE-2025-21632"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2025-01-19T11:15:08Z",
    "severity": null
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nx86/fpu: Ensure shadow stack is active before \"getting\" registers\n\nThe x86 shadow stack support has its own set of registers. Those registers\nare XSAVE-managed, but they are \"supervisor state components\" which means\nthat userspace can not touch them with XSAVE/XRSTOR.  It also means that\nthey are not accessible from the existing ptrace ABI for XSAVE state.\nThus, there is a new ptrace get/set interface for it.\n\nThe regset code that ptrace uses provides an -\u003eactive() handler in\naddition to the get/set ones. For shadow stack this -\u003eactive() handler\nverifies that shadow stack is enabled via the ARCH_SHSTK_SHSTK bit in the\nthread struct. The -\u003eactive() handler is checked from some call sites of\nthe regset get/set handlers, but not the ptrace ones. This was not\nunderstood when shadow stack support was put in place.\n\nAs a result, both the set/get handlers can be called with\nXFEATURE_CET_USER in its init state, which would cause get_xsave_addr() to\nreturn NULL and trigger a WARN_ON(). The ssp_set() handler luckily has an\nssp_active() check to avoid surprising the kernel with shadow stack\nbehavior when the kernel is not ready for it (ARCH_SHSTK_SHSTK==0). That\ncheck just happened to avoid the warning.\n\nBut the -\u003eget() side wasn\u0027t so lucky. It can be called with shadow stacks\ndisabled, triggering the warning in practice, as reported by Christina\nSchimpe:\n\nWARNING: CPU: 5 PID: 1773 at arch/x86/kernel/fpu/regset.c:198 ssp_get+0x89/0xa0\n[...]\nCall Trace:\n\u003cTASK\u003e\n? show_regs+0x6e/0x80\n? ssp_get+0x89/0xa0\n? __warn+0x91/0x150\n? ssp_get+0x89/0xa0\n? report_bug+0x19d/0x1b0\n? handle_bug+0x46/0x80\n? exc_invalid_op+0x1d/0x80\n? asm_exc_invalid_op+0x1f/0x30\n? __pfx_ssp_get+0x10/0x10\n? ssp_get+0x89/0xa0\n? ssp_get+0x52/0xa0\n__regset_get+0xad/0xf0\ncopy_regset_to_user+0x52/0xc0\nptrace_regset+0x119/0x140\nptrace_request+0x13c/0x850\n? wait_task_inactive+0x142/0x1d0\n? do_syscall_64+0x6d/0x90\narch_ptrace+0x102/0x300\n[...]\n\nEnsure that shadow stacks are active in a thread before looking them up\nin the XSAVE buffer. Since ARCH_SHSTK_SHSTK and user_ssp[SHSTK_EN] are\nset at the same time, the active check ensures that there will be\nsomething to find in the XSAVE buffer.\n\n[ dhansen: changelog/subject tweaks ]",
  "id": "GHSA-gjg2-r77w-jx7q",
  "modified": "2025-01-19T12:31:24Z",
  "published": "2025-01-19T12:31:24Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-21632"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/0a3a872214188e4268d31581ed0cd44508e038cf"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/6bfe1fc22f462bec87422cdcbec4d7a2f43ff01d"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/a9d9c33132d49329ada647e4514d210d15e31d81"
    }
  ],
  "schema_version": "1.4.0",
  "severity": []
}


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