Linux Kernel up to 7.2.12 x86/insn-eval get_seg_reg_override_idx infinite loop

Summaryinfo

A vulnerability has been found in Linux Kernel up to 7.2.12 and classified as very critical. Affected by this vulnerability is the function get_seg_reg_override_idx of the component x86/insn-eval. The manipulation leads to infinite loop. This vulnerability is referenced as CVE-2026-99186. The attack can only be performed from a local environment. No exploit is available. The affected component should be upgraded.

Detailsinfo

A vulnerability was found in Linux Kernel up to 7.2.12 and classified as very critical. This issue affects the function get_seg_reg_override_idx of the component x86/insn-eval. The manipulation with an unknown input leads to a infinite loop vulnerability. Using CWE to declare the problem leads to CWE-835. The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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 ]

It is possible to read the advisory at git.kernel.org. The identification of this vulnerability is CVE-2026-99186 since 09/25/2026. The exploitation is known to be easy. Attacking locally is a requirement. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 10/28/2026).

Upgrading to version 7.2.13 eliminates this vulnerability. Applying the patch 0efe927053f7eabd1e8658cbda96ee3d78bd1765 is able to eliminate this problem. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

Version

• 7.2.0
• 7.2.1
• 7.2.2
• 7.2.3
• 7.2.4
• 7.2.5
• 7.2.6
• 7.2.7
• 7.2.8
• 7.2.9
• 7.2.10
• 7.2.11
• 7.2.12

License

• open-source

Website

• Vendor: https://www.kernel.org/

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