Linux Kernel CVE-2025-68171

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

x86/fpu: Ensure XFD state on signal delivery

Sean reported [1] the following splat when running KVM tests:

WARNING: CPU: 232 PID: 15391 at xfd_validate_state+0x65/0x70 Call Trace: <TASK> fpu__clear_user_states+0x9c/0x100 arch_do_signal_or_restart+0x142/0x210 exit_to_user_mode_loop+0x55/0x100 do_syscall_64+0x205/0x2c0 entry_SYSCALL_64_after_hwframe+0x4b/0x53

Chao further identified [2] a reproducible scenario involving signal delivery: a non-AMX task is preempted by an AMX-enabled task which modifies the XFD MSR.

When the non-AMX task resumes and reloads XSTATE with init values, a warning is triggered due to a mismatch between fpstate::xfd and the CPU's current XFD state. fpu__clear_user_states() does not currently re-synchronize the XFD state after such preemption.

Invoke xfd_update_state() which detects and corrects the mismatch if there is a dynamic feature.

This also benefits the sigreturn path, as fpu__restore_sig() may call fpu__clear_user_states() when the sigframe is inaccessible.

[ dhansen: minor changelog munging ]

The affected product is the Linux kernel on x86-64 platforms that support dynamically enabled XSAVE features (Intel AMX, i.e. Sapphire Rapids and newer Xeon). No CPE list or exact version range was provided in the input; affected versions must be inferred from the five stable-tree fix commits at git.kernel.org (1811c610653c, 388eff894d6b, 3f735419c4b4, 5b2619b488f1, eefbfb722042), which indicate the fix was backported across multiple stable branches. Only systems that actually run AMX-enabled workloads (commonly KVM/virtualization and ML tile-instruction workloads) can reach the vulnerable code path.