x86/fpu: Fix copy_xstate_to_uabi() to copy init states correctly

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

x86/fpu: Fix copy_xstate_to_uabi() to copy init states correctly

When an extended state component is not present in fpstate, but in init

state, the function copies from init_fpstate via copy_feature().

But, dynamic states are not present in init_fpstate because of all-zeros

init states. Then retrieving them from init_fpstate will explode like this:

BUG: kernel NULL pointer dereference, address: 0000000000000000

...

RIP: 0010:memcpy_erms+0x6/0x10

? __copy_xstate_to_uabi_buf+0x381/0x870

fpu_copy_guest_fpstate_to_uabi+0x28/0x80

kvm_arch_vcpu_ioctl+0x14c/0x1460 [kvm]

? __this_cpu_preempt_check+0x13/0x20

? vmx_vcpu_put+0x2e/0x260 [kvm_intel]

kvm_vcpu_ioctl+0xea/0x6b0 [kvm]

? kvm_vcpu_ioctl+0xea/0x6b0 [kvm]

? __fget_light+0xd4/0x130

__x64_sys_ioctl+0xe3/0x910

? debug_smp_processor_id+0x17/0x20

? fpregs_assert_state_consistent+0x27/0x50

do_syscall_64+0x3f/0x90

entry_SYSCALL_64_after_hwframe+0x63/0xcd

Adjust the 'mask' to zero out the userspace buffer for the features that

are not available both from fpstate and from init_fpstate.

The dynamic features depend on the compacted XSAVE format. Ensure it is

enabled before reading XCOMP_BV in init_fpstate.