x86/fpu: Remove fpu->initialized usage in __fpu__restore_sig()

This is a preparation for the removal of the ->initialized member in the
fpu struct.

__fpu__restore_sig() is deactivating the FPU via fpu__drop() and then
setting manually ->initialized followed by fpu__restore(). The result is
that it is possible to manipulate fpu->state and the state of registers
won't be saved/restored on a context switch which would overwrite
fpu->state:

fpu__drop(fpu):
  ...
  fpu->initialized = 0;
  preempt_enable();

  <--- context switch

Don't access the fpu->state while the content is read from user space
and examined/sanitized. Use a temporary kmalloc() buffer for the
preparation of the FPU registers and once the state is considered okay,
load it. Should something go wrong, return with an error and without
altering the original FPU registers.

The removal of fpu__initialize() is a nop because fpu->initialized is
already set for the user task.

 [ bp: Massage a bit. ]

Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Dave Hansen <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Acked-by: Borislav Petkov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: "Jason A. Donenfeld" <[email protected]>
Cc: kvm ML <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Radim Krčmář <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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