In entry_64.S we have code like this:
/* Unconditionally use kernel CR3 for do_nmi() */
/* %rax is saved above, so OK to clobber here */
ALTERNATIVE "jmp 2f", "movq %cr3, %rax", X86_FEATURE_KAISER
/* If PCID enabled, NOFLUSH now and NOFLUSH on return */
ALTERNATIVE "", "bts $63, %rax", X86_FEATURE_PCID
pushq %rax
/* mask off "user" bit of pgd address and 12 PCID bits: */
andq $(~(X86_CR3_PCID_ASID_MASK | KAISER_SHADOW_PGD_OFFSET)), %rax
movq %rax, %cr3
2:
/* paranoidentry do_nmi, 0; without TRACE_IRQS_OFF */
call do_nmi
With this instruction:
andq $(~(X86_CR3_PCID_ASID_MASK | KAISER_SHADOW_PGD_OFFSET)), %rax
We unconditionally switch from whatever our CR3 was to kernel page table.
But, in arch/x86/platform/efi/efi_64.c We temporarily set a different page
table, that does not have the kernel page table with 0x1000 offset from it.
Look in efi_thunk() and efi_thunk_set_virtual_address_map().
So, while CR3 points to the other page table, we get an NMI interrupt,
and clear 0x1000 from CR3, resulting in a bogus CR3 if the 0x1000 bit was
set.
The efi page table comes from realmode/rm/trampoline_64.S:
arch/x86/realmode/rm/trampoline_64.S
141 .bss
142 .balign PAGE_SIZE
143 GLOBAL(trampoline_pgd) .space PAGE_SIZE
Notice: alignment is PAGE_SIZE, so after applying KAISER_SHADOW_PGD_OFFSET
which equal to PAGE_SIZE, we can get a different page table.
But, even if we fix alignment, here the trampoline binary is later copied
into dynamically allocated memory in reserve_real_mode(), so we need to
fix that place as well.
Fixes: 8a43ddfb93 ("KAISER: Kernel Address Isolation")
Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Reviewed-by: Steven Sistare <steven.sistare@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
7ec5d87df3