[PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3

Sai Praneeth Prakhya
2017-08-15T19:23:01Z

From: Sai Praneeth <sai.praneeth.prakhya@intel.com>

Use helper function (efi_switch_mm()) to switch to/from efi_mm. We
switch to efi_mm before calling
1. efi_set_virtual_address_map() and
2. Invoking any efi_runtime_service()

Likewise, we need to switch back to previous mm (mm context stolen by
efi_mm) after the above calls return successfully. We can use
efi_switch_mm() only with x86_64 kernel and "efi=old_map" disabled
because, x86_32 and efi=old_map doesn't use efi_pgd, rather they use
swapper_pg_dir.

Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Cc: Lee, Chun-Yi <jlee@suse.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Ricardo Neri <ricardo.neri@intel.com>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
---
arch/x86/include/asm/efi.h | 30 +++++++++++++-------------
arch/x86/platform/efi/efi_64.c | 41 ++++++++++++++++++++++++------------
arch/x86/platform/efi/efi_thunk_64.S | 2 +-
3 files changed, 43 insertions(+), 30 deletions(-)

diff --git a/arch/x86/include/asm/efi.h b/arch/x86/include/asm/efi.h
index 2f77bcefe6b4..aa38b546e842 100644
--- a/arch/x86/include/asm/efi.h
+++ b/arch/x86/include/asm/efi.h
@@ -1,10 +1,14 @@
#ifndef _ASM_X86_EFI_H
#define _ASM_X86_EFI_H

+#include <linux/sched/mm.h>
+#include <linux/sched/task.h>
+
#include <asm/fpu/api.h>
#include <asm/pgtable.h>
#include <asm/processor-flags.h>
#include <asm/tlb.h>
+#include <asm/mmu_context.h>

/*
* We map the EFI regions needed for runtime services non-contiguously,
@@ -57,14 +61,14 @@ extern u64 asmlinkage efi_call(void *fp, ...);
#define efi_call_phys(f, args...) efi_call((f), args)

/*
- * Scratch space used for switching the pagetable in the EFI stub
+ * struct efi_scratch - Scratch space used while switching to/from efi_mm
+ * @phys_stack: stack used during EFI Mixed Mode
+ * @prev_mm: store/restore stolen mm_struct while switching
+ * to/from efi_mm
*/
struct efi_scratch {
- u64 r15;
- u64 prev_cr3;
- pgd_t *efi_pgt;
- bool use_pgd;
- u64 phys_stack;
+ u64 phys_stack;
+ struct mm_struct *prev_mm;
} __packed;

#define arch_efi_call_virt_setup() \
@@ -73,11 +77,8 @@ struct efi_scratch {
preempt_disable(); \
__kernel_fpu_begin(); \
\
- if (efi_scratch.use_pgd) { \
- efi_scratch.prev_cr3 = read_cr3(); \
- write_cr3((unsigned long)efi_scratch.efi_pgt); \
- __flush_tlb_all(); \
- } \
+ if (!efi_enabled(EFI_OLD_MEMMAP)) \
+ efi_switch_mm(&efi_mm); \
})

#define arch_efi_call_virt(p, f, args...) \
@@ -85,10 +86,8 @@ struct efi_scratch {

#define arch_efi_call_virt_teardown() \
({ \
- if (efi_scratch.use_pgd) { \
- write_cr3(efi_scratch.prev_cr3); \
- __flush_tlb_all(); \
- } \
+ if (!efi_enabled(EFI_OLD_MEMMAP)) \
+ efi_switch_mm(efi_scratch.prev_mm); \
\
__kernel_fpu_end(); \
preempt_enable(); \
@@ -130,6 +129,7 @@ extern void __init efi_dump_pagetable(void);
extern void __init efi_apply_memmap_quirks(void);
extern int __init efi_reuse_config(u64 tables, int nr_tables);
extern void efi_delete_dummy_variable(void);
+extern void efi_switch_mm(struct mm_struct *mm);

struct efi_setup_data {
u64 fw_vendor;
diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
index 0bb98c35e178..3be94480c1ce 100644
--- a/arch/x86/platform/efi/efi_64.c
+++ b/arch/x86/platform/efi/efi_64.c
@@ -80,9 +80,8 @@ pgd_t * __init efi_call_phys_prolog(void)
int n_pgds, i, j;

if (!efi_enabled(EFI_OLD_MEMMAP)) {
- save_pgd = (pgd_t *)read_cr3();
- write_cr3((unsigned long)efi_scratch.efi_pgt);
- goto out;
+ efi_switch_mm(&efi_mm);
+ return NULL;
}

early_code_mapping_set_exec(1);
@@ -152,8 +151,7 @@ void __init efi_call_phys_epilog(pgd_t *save_pgd)
pud_t *pud;

if (!efi_enabled(EFI_OLD_MEMMAP)) {
- write_cr3((unsigned long)save_pgd);
- __flush_tlb_all();
+ efi_switch_mm(efi_scratch.prev_mm);
return;
}

@@ -336,8 +334,6 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
if (efi_enabled(EFI_OLD_MEMMAP))
return 0;

- efi_scratch.efi_pgt = (pgd_t *)__pa(pgd);
-
/*
* It can happen that the physical address of new_memmap lands in memory
* which is not mapped in the EFI page table. Therefore we need to go
@@ -350,8 +346,6 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
return 1;
}

- efi_scratch.use_pgd = true;
-
/*
* Certain firmware versions are way too sentimential and still believe
* they are exclusive and unquestionable owners of the first physical page,
@@ -596,6 +590,28 @@ void __init efi_dump_pagetable(void)
#endif
}

+/*
+ * Makes the calling kernel thread switch to/from efi_mm context
+ * Can be used from SetVirtualAddressMap() or during efi runtime calls
+ * (Note: This routine is heavily inspired from use_mm)
+ */
+void efi_switch_mm(struct mm_struct *mm)
+{
+ struct task_struct *tsk = current;
+
+ task_lock(tsk);
+ efi_scratch.prev_mm = tsk->active_mm;
+ if (efi_scratch.prev_mm != mm) {
+ mmgrab(mm);
+ tsk->active_mm = mm;
+ }
+ switch_mm(efi_scratch.prev_mm, mm, NULL);
+ task_unlock(tsk);
+
+ if (efi_scratch.prev_mm != mm)
+ mmdrop(efi_scratch.prev_mm);
+}
+
#ifdef CONFIG_EFI_MIXED
extern efi_status_t efi64_thunk(u32, ...);

@@ -649,16 +665,13 @@ efi_status_t efi_thunk_set_virtual_address_map(
efi_sync_low_kernel_mappings();
local_irq_save(flags);

- efi_scratch.prev_cr3 = read_cr3();
- write_cr3((unsigned long)efi_scratch.efi_pgt);
- __flush_tlb_all();
+ efi_switch_mm(&efi_mm);

func = (u32)(unsigned long)phys_set_virtual_address_map;
status = efi64_thunk(func, memory_map_size, descriptor_size,
descriptor_version, virtual_map);

- write_cr3(efi_scratch.prev_cr3);
- __flush_tlb_all();
+ efi_switch_mm(efi_scratch.prev_mm);
local_irq_restore(flags);

return status;
diff --git a/arch/x86/platform/efi/efi_thunk_64.S b/arch/x86/platform/efi/efi_thunk_64.S
index ff85d28c50f2..5cdc72ebbc82 100644
--- a/arch/x86/platform/efi/efi_thunk_64.S
+++ b/arch/x86/platform/efi/efi_thunk_64.S
@@ -32,7 +32,7 @@ ENTRY(efi64_thunk)
* Switch to 1:1 mapped 32-bit stack pointer.
*/
movq %rsp, efi_saved_sp(%rip)
- movq efi_scratch+25(%rip), %rsp
+ movq efi_scratch(%rip), %rsp

/*
* Calculate the physical address of the kernel text.
--
2.1.4