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 xfdvalidatestate+0x65/0x70 Call Trace: <TASK> fpu__clearuserstates+0x9c/0x100 archdosignalorrestart+0x142/0x210 exittousermodeloop+0x55/0x100 dosyscall64+0x205/0x2c0 entrySYSCALL64afterhwframe+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__clearuserstates() does not currently re-synchronize the XFD state after such preemption.

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

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

[ dhansen: minor changelog munging ]