powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare()

{
  "id": "e4b1f99ca0251dd08e241dba6822432d1364c2d675e6838ddf46e7a73e7e87f3",
  "url": "https://osv.dev/vulnerability/CVE-2026-98396",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare()",
    "summary": "In the Linux kernel, the following vulnerability has been resolved: powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare() KUAP means kernel cannot touch user memory unless it explicitly is enabled. In the kernel it should stay AMR_KUAP_BLOCKED. While returning to userspace just before RFI, kernel should restore the user AMR value back. Looks like GENERIC_ENTRY might be treating arch_exit_to_user_mode_prepare() as the last architecture step before returning to userspace. commit bee25f97ad24 (\"powerpc: Enable GENERIC_ENTRY feature\") therefore called kuap_user_restore() from that hook. But on PowerPC that is too early. After irqentry_exit() / syscall_exit_to_user_mode() we still run platform specific exit routines. e.g. code snippets showing both exception handling and system call handling as the callers of function arch_exit_to_user_mode_prepare() which does kuap_user_restore(). The below path shows that calling kuap_user_restore() is too early when called from arch_exit_to_user_mode_prepare(). Exception handling in exceptions-64s.S ======================================= bl CFUNC(do_page_fault) ..DEFINE_INTERRUPT_HANDLER_ASYNC(do_page_fault) arch_interrupt_async_enter_prepare(regs); state = irqentry_enter(regs); instrumentation_begin(); irq_enter_rcu(); handler(regs); nap_adjust_return(regs); irq_exit_rcu(); instrumentation_end(); arch_interrupt_async_exit_prepare(regs); irqentry_exit(regs, state); <<< too early irqentry_exit_to_user_mode() __exit_to_user_mode_prepa",
    "evidence": "osv:CVE-2026-98396",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "7.2.7",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "unknown",
    "structured": {
      "id": "CVE-2026-98396",
      "aliases": [
        "CVE-2026-98396"
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
                {
                  "introduced": "02565a782c1ee7e8ada38ad24a698e01d6ea9ff8"
                },
                {
                  "fixed": "c8e80c19c3b9c8db83a14fab00445a6064992442"
                },
                {
                  "fixed": "c2549d749539487239475fbc8c614a1f9244d655"
                }
              ]
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98396.json"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "7.2.0"
                },
                {
                  "fixed": "7.2.7"
                }
              ]
            }
          ],
          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98396.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:38.282875684Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/c2549d749539487239475fbc8c614a1f9244d655",
        "https://git.kernel.org/stable/c/c8e80c19c3b9c8db83a14fab00445a6064992442",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98396.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-98396",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
      ],
      "provider_metadata": {
        "cna_assigner": "Linux",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98396.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-98396",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": null
  },
  "source_id": "core-osv",
  "document_id": 505807,
  "published_at": "2026-10-14T13:25:58.49+00:00"
}