[PATCH 3.16 060/245] x86/pti/efi: broken conversion from efi to kernel page table

Ben Hutchings
2020-04-23T23:21:01Z

From: Pavel Tatashin <pasha.tatashin@oracle.com>

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: f9a1666f97b3 ("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>
[bwh: Adjust the Fixes field for 3.16]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/x86/include/asm/kaiser.h | 10 ++++++++++
arch/x86/realmode/init.c | 4 +++-
arch/x86/realmode/rm/trampoline_64.S | 3 ++-
3 files changed, 15 insertions(+), 2 deletions(-)

--- a/arch/x86/include/asm/kaiser.h
+++ b/arch/x86/include/asm/kaiser.h
@@ -19,6 +19,16 @@

#define KAISER_SHADOW_PGD_OFFSET 0x1000

+#ifdef CONFIG_PAGE_TABLE_ISOLATION
+/*
+ * A page table address must have this alignment to stay the same when
+ * KAISER_SHADOW_PGD_OFFSET mask is applied
+ */
+#define KAISER_KERNEL_PGD_ALIGNMENT (KAISER_SHADOW_PGD_OFFSET << 1)
+#else
+#define KAISER_KERNEL_PGD_ALIGNMENT PAGE_SIZE
+#endif
+
#ifdef __ASSEMBLY__
#ifdef CONFIG_PAGE_TABLE_ISOLATION

--- a/arch/x86/realmode/init.c
+++ b/arch/x86/realmode/init.c
@@ -4,6 +4,7 @@
#include <asm/cacheflush.h>
#include <asm/pgtable.h>
#include <asm/realmode.h>
+#include <asm/kaiser.h>

struct real_mode_header *real_mode_header;
u32 *trampoline_cr4_features;
@@ -15,7 +16,8 @@ void __init reserve_real_mode(void)
size_t size = PAGE_ALIGN(real_mode_blob_end - real_mode_blob);

/* Has to be under 1M so we can execute real-mode AP code. */
- mem = memblock_find_in_range(0, 1<<20, size, PAGE_SIZE);
+ mem = memblock_find_in_range(0, 1 << 20, size,
+ KAISER_KERNEL_PGD_ALIGNMENT);
if (!mem)
panic("Cannot allocate trampoline\n");

--- a/arch/x86/realmode/rm/trampoline_64.S
+++ b/arch/x86/realmode/rm/trampoline_64.S
@@ -30,6 +30,7 @@
#include <asm/msr.h>
#include <asm/segment.h>
#include <asm/processor-flags.h>
+#include <asm/kaiser.h>
#include "realmode.h"

.text
@@ -139,7 +140,7 @@ tr_gdt:
tr_gdt_end:

.bss
- .balign PAGE_SIZE
+ .balign KAISER_KERNEL_PGD_ALIGNMENT
GLOBAL(trampoline_pgd) .space PAGE_SIZE

.balign 8