x86/fpu: Restore the masking out of reserved MXCSR bits

Ser Olmy reported a boot failure:

  init[1] bad frame in sigreturn frame:(ptrval) ip:b7c9fbe6 sp:bf933310 orax:ffffffff \
	  in libc-2.33.so[b7bed000+156000]
  Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
  CPU: 0 PID: 1 Comm: init Tainted: G        W         5.14.9 #1
  Hardware name: Hewlett-Packard HP PC/HP Board, BIOS  JD.00.06 12/06/2001
  Call Trace:
   dump_stack_lvl
   dump_stack
   panic
   do_exit.cold
   do_group_exit
   get_signal
   arch_do_signal_or_restart
   ? force_sig_info_to_task
   ? force_sig
   exit_to_user_mode_prepare
   syscall_exit_to_user_mode
   do_int80_syscall_32
   entry_INT80_32

on an old 32-bit Intel CPU:

  vendor_id       : GenuineIntel
  cpu family      : 6
  model           : 6
  model name      : Celeron (Mendocino)
  stepping        : 5
  microcode       : 0x3

Ser bisected the problem to the commit in Fixes.

tglx suggested reverting the rejection of invalid MXCSR values which
this commit introduced and replacing it with what the old code did -
simply masking them out to zero.

Further debugging confirmed his suggestion:

  fpu->state.fxsave.mxcsr: 0xb7be13b4, mxcsr_feature_mask: 0xffbf
  WARNING: CPU: 0 PID: 1 at arch/x86/kernel/fpu/signal.c:384 __fpu_restore_sig+0x51f/0x540

so restore the original behavior only for 32-bit kernels where you have
ancient machines with buggy hardware. For 32-bit programs on 64-bit
kernels, user space which supplies wrong MXCSR values is considered
malicious so fail the sigframe restoration there.

Fixes: 6f9866a1 ("x86/fpu/signal: Let xrstor handle the features to init")
Reported-by: Ser Olmy <ser.olmy@protonmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Ser Olmy <ser.olmy@protonmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/YVtA67jImg3KlBTw@zn.tnic