CVE-2024-35801

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

x86/fpu: Keep xfdstate in sync with MSRIA32_XFD

Commit 672365477ae8 ("x86/fpu: Update XFD state where required") and commit 8bf26758ca96 ("x86/fpu: Add XFD state to fpstate") introduced a per CPU variable xfdstate to keep the MSRIA32_XFD value cached, in order to avoid unnecessary writes to the MSR.

On CPU hotplug MSRIA32XFD is reset to the init_fpstate.xfd, which wipes out any stale state. But the per CPU cached xfd value is not reset, which brings them out of sync.

As a consequence a subsequent xfdupdatestate() might fail to update the MSR which in turn can result in XRSTOR raising a #NM in kernel space, which crashes the kernel.

To fix this, introduce xfdsetstate() to write xfdstate together with MSRIA32XFD, and use it in all places that set MSRIA32_XFD.