Live › vulnerability › CVE-2026-99186
Go to dashboardIn 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.
Summary written from the sources below. Check the originals before acting.
First seen by InfraStack 2026-10-28 11:32 UTC. All times on this page are UTC.
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.
No product ranges from the sources yet.
uname -r and compare against the kernel version that contains the upstream fix commit, unknown from the sourcesgrep -i umip /proc/cpuinfo to see whether the CPU exposes UMIP, which is when the affected emulation path is reachableEach row is the first time that source carried this signal. The analyst’s rows are its own findings.
In the Linux kernel, the following vulnerability has been resolved:
x86/insn-eval: Fix shadowed segment index in get_seg_reg_override_idx()
Commit 45e1dccc0653 ("x86/insn: Simplify for_each_insn_prefix()") made for_each_insn_prefix() declare its own loop counter named idx. get_seg_reg_override_idx() keeps its result in a local of the same name, so the assignments in the loop body now write the loop counter:
the function always returns INAT_SEG_REG_DEFAULT, so FS and GS overrides are ignored;
a CS override sets the counter to INAT_SEG_REG_CS (2), the increment makes it 3, and prefixes.bytes[3] holds the last prefix, 0x2e again, so the loop never terminates.
The UMIP emulation reaches this from the #GP handler for user space: an SGDT, SIDT or SMSW with a memory operand and a CS prefix leaves the task spinning in the kernel, never returning to user space, with a SIGKILL that is never delivered. Any user can thus tie up CPUs until reboot. With an FS or GS prefix, the emulated result is written to the wrong address.
Rename the internal macro iterator variable.
[ Lukas' original patch renamed the use site, I changed it to rename the iterator in the macro instead, but left Lukas' commit message and authorship - Linus ]
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