The trampoline_pgd only maps the 0xfffffff000000000-0xffffffffffffffff
range of kernel memory (with 4-level paging). This range contains the
kernel's text+data+bss mappings and the module mapping space but not the
direct mapping and the vmalloc area.

This is enough to get the application processors out of real-mode, but
for code that switches back to real-mode the trampoline_pgd is missing
important parts of the address space. For example, consider this code
from arch/x86/kernel/reboot.c, function machine_real_restart() for a
64-bit kernel:

  #ifdef CONFIG_X86_32
  	load_cr3(initial_page_table);
  #else
  	write_cr3(real_mode_header->trampoline_pgd);

  	/* Exiting long mode will fail if CR4.PCIDE is set. */
  	if (boot_cpu_has(X86_FEATURE_PCID))
  		cr4_clear_bits(X86_CR4_PCIDE);
  #endif

  	/* Jump to the identity-mapped low memory code */
  #ifdef CONFIG_X86_32
  	asm volatile("jmpl *%0" : :
  		     "rm" (real_mode_header->machine_real_restart_asm),
  		     "a" (type));
  #else
  	asm volatile("ljmpl *%0" : :
  		     "m" (real_mode_header->machine_real_restart_asm),
  		     "D" (type));
  #endif

The code switches to the trampoline_pgd, which unmaps the direct mapping
and also the kernel stack. The call to cr4_clear_bits() will find no
stack and crash the machine. The real_mode_header pointer below points
into the direct mapping, and dereferencing it also causes a crash.

The reason this does not crash always is only that kernel mappings are
global and the CR3 switch does not flush those mappings. But if theses
mappings are not in the TLB already, the above code will crash before it
can jump to the real-mode stub.

Extend the trampoline_pgd to contain all kernel mappings to prevent
these crashes and to make code which runs on this page-table more
robust.
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20211202153226.22946-5-joro@8bytes.org