{"id":"CVE-2026-99186","kind":"vulnerability","aliases":["CVE-2026-99186"],"related":[],"title":"Linux kernel: segment override bug lets any user hang CPUs","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nx86/insn-eval: Fix shadowed segment index in get_seg_reg_override_idx()\n\nCommit 45e1dccc0653 (\"x86/insn: Simplify for_each_insn_prefix()\") made\nfor_each_insn_prefix() declare its own loop counter named idx.\nget_seg_reg_override_idx() keeps its result in a local of the same name,\nso the assignments in the loop body now write the loop counter:\n\n - the function always returns INAT_SEG_REG_DEFAULT, so FS and GS\n   overrides are ignored;\n\n - a CS override sets the counter to INAT_SEG_REG_CS (2), the increment\n   makes it 3, and prefixes.bytes[3] holds the last prefix, 0x2e again,\n   so the loop never terminates.\n\nThe UMIP emulation reaches this from the #GP handler for user space: an\nSGDT, SIDT or SMSW with a memory operand and a CS prefix leaves the task\nspinning in the kernel, never returning to user space, with a SIGKILL\nthat is never delivered.  Any user can thus tie up CPUs until reboot.\nWith an FS or GS prefix, the emulated result is written to the wrong\naddress.\n\nRename the internal macro iterator variable.\n\n[ Lukas' original patch renamed the use site, I changed it to rename the\n  iterator in the macro instead, but left Lukas' commit message and\n  authorship   - Linus ]","summary":"In the x86 instruction emulation used for UMIP, a loop counter collision makes SGDT, SIDT and SMSW with a CS prefix spin forever in kernel mode. Any local user can tie up CPUs until reboot, and FS or GS prefixed operands are written to the wrong address.","severity":"high","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["infrastructure","linux-kernel","multi-user-hosts","self-hosted","x86"],"not_affected_if":["non-x86-hardware","patched-kernel"],"how_to_check":"- `uname -r` and compare against the kernel version that contains the upstream fix commit, unknown from the sources\n- `grep -i umip /proc/cpuinfo` to see whether the CPU exposes UMIP, which is when the affected emulation path is reachable","fix":"- Update the Linux kernel to a release that includes the upstream commit renaming the for_each_insn_prefix() iterator in get_seg_reg_override_idx(); exact fixed version unknown from the sources\n- Reboot into the patched kernel, since the bug needs a kernel-level fix","status":"open","published_at":"2026-10-28T11:17:39+00:00","first_seen_at":"2026-10-28T11:32:37+00:00","triaged_at":"2026-10-28T11:35:20+00:00","updated_at":"2026-10-28T11:35:20+00:00","category":"infrastructure","confidence":0.6,"triage":{"at":"2026-10-28T11:35:20Z","model":"claude-haiku-5-5","output":{"fix":"- Update the Linux kernel to a release that includes the upstream commit renaming the for_each_insn_prefix() iterator in get_seg_reg_override_idx(); exact fixed version unknown from the sources\n- Reboot into the patched kernel, since the bug needs a kernel-level fix","reach":"local","title":"Linux kernel: segment override bug lets any user hang CPUs","impact":"medium","summary":"In the x86 instruction emulation used for UMIP, a loop counter collision makes SGDT, SIDT and SMSW with a CS prefix spin forever in kernel mode. Any local user can tie up CPUs until reboot, and FS or GS prefixed operands are written to the wrong address.","category":"infrastructure","nicknames":[],"who_cares":["linux-kernel","x86","multi-user-hosts","self-hosted"],"confidence":0.6,"needs_auth":"yes","worst_case":"Local user hangs CPUs until reboot; wrong memory write","analyst_note":"Find the upstream fix commit and the first fixed stable kernel version for CVE-2026-99186 on kernel.org or the NVD entry.","how_to_check":"- `uname -r` and compare against the kernel version that contains the upstream fix commit, unknown from the sources\n- `grep -i umip /proc/cpuinfo` to see whether the CPU exposes UMIP, which is when the affected emulation path is reachable","needs_analyst":true,"not_affected_if":["non-x86-hardware","patched-kernel"],"who_is_affected":"Any Linux system running a kernel built with the x86 instruction decoder and UMIP emulation from commit 45e1dccc0653 or later, which covers most x86 machines with user-mode instruction emulation. Shared multi-user hosts are most exposed because any local user can trigger the hang. Non-x86 systems are not affected.","user_interaction":"no","needs_special_setup":"no","severity_if_unknown":"high"},"cost_usd":0.00071,"input_tokens":3713,"output_tokens":678,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':12A,15A '-99186':13A,16A '0x2e':184C '2':171C '3':176C,179C '45e1':98C 'address':75B,258C 'alway':147C 'assign':135C 'authorship':289C 'bodi':139C 'bug':5A 'chang':273C 'collis':40B 'commit':97C,286C 'counter':39B,115C,144C,165C 'cpus':10A,61B,241C 'cs':48B,161C,170C,214C 'cve':11A,14A 'dccc0653':99C 'declar':111C 'default':152C 'deliv':234C 'emul':33B,193C,251C 'fix':87C 'follow':81C 'forev':51B 'fs':65B,154C,246C 'function':146C 'get':92C,118C 'gp':198C 'gs':67B,156C,248C 'handler':199C 'hang':9A 'hold':180C 'host':24B,27B 'idx':96C,117C,122C 'ignor':159C 'inat':149C,167C 'increment':173C 'index':90C 'infrastructur':17B 'insn':104C,109C 'instead':282C 'instruct':32B 'intern':261C 'iter':263C,278C 'keep':123C 'kernel':2A,20B,53B,79C,222C 'last':182C 'leav':216C 'left':284C 'let':6A 'linus':290C 'linux':1A,19B,78C 'linux-kernel':18B 'local':56B,128C 'loop':38B,114C,138C,143C,188C 'luka':265C,285C 'macro':262C,281C 'made':106C 'make':41B,174C 'memori':210C 'messag':287C 'mode':54B 'multi':22B 'multi-user-host':21B 'name':116C,132C 'never':189C,223C,233C 'operand':69B,211C 'origin':266C 'overrid':4A,95C,121C,157C,162C 'patch':267C 'prefix':49B,68B,105C,110C,183C,215C,249C 'prefixes.bytes':178C 'reach':194C 'reboot':63B,243C 'reg':94C,120C,151C,169C 'renam':259C,268C,276C 'resolv':85C 'result':125C,252C 'return':148C,224C 'seg':93C,119C,150C,168C 'segment':3A,89C 'self':26B 'self-host':25B 'set':163C 'sgdt':42B,204C 'shadow':88C 'sidt':43B,205C 'sigkil':230C 'simplifi':101C 'site':271C 'smsw':45B,207C 'space':202C,227C 'spin':50B,219C 'task':218C 'termin':190C 'thus':238C 'tie':59B,239C 'umip':36B,192C 'use':34B,270C 'user':8A,23B,57B,201C,226C,236C 'variabl':264C 'vulner':82C 'write':141C 'written':71B,254C 'wrong':74B,257C 'x86':28B,31B 'x86/insn':100C 'x86/insn-eval':86C","nicknames":[],"mitigations":[],"who_affected":"Any Linux system running a kernel built with the x86 instruction decoder and UMIP emulation from commit 45e1dccc0653 or later, which covers most x86 machines with user-mode instruction emulation. Shared multi-user hosts are most exposed because any local user can trigger the hang. Non-x86 systems are not affected.","tier":"B","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-28T11:17:39+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-99186","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-99186","severity":null,"published_at":"2026-10-28T11:17:39+00:00","modified_at":"2026-10-28T11:17:39+00:00","first_seen_at":"2026-10-28T11:32:37+00:00","fetched_at":"2026-10-28T11:32:37+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}