CVE-2024-35801

In the Linux kernel, the following vulnerability has been resolved:
x86/fpu: Keep xfd_state in sync with MSR_IA32_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
xfd_state to keep the MSR_IA32_XFD value cached, in order to avoid
unnecessary writes to the MSR. On CPU hotplug MSR_IA32_XFD 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 xfd_update_state() 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 xfd_set_state() to write xfd_state together
with MSR_IA32_XFD, and use it in all places that set MSR_IA32_XFD.