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Date: Thu, 12 Nov 2015 19:44:32 +0100
From: Borislav Petkov <bp@...en8.de>
To: Matt Fleming <matt@...eblueprint.co.uk>
Cc: Ingo Molnar <mingo@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
"H . Peter Anvin" <hpa@...or.com>, Toshi Kani <toshi.kani@...com>,
linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Dave Jones <davej@...emonkey.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>,
Andy Lutomirski <luto@...nel.org>,
Denys Vlasenko <dvlasenk@...hat.com>,
Stephen Smalley <sds@...ho.nsa.gov>
Subject: Re: [PATCH 4/6] x86/efi: Hoist page table switching code into
efi_call_virt()
On Thu, Nov 12, 2015 at 03:40:21PM +0000, Matt Fleming wrote:
> This change is a prerequisite for pending patches that switch to a
> dedicated EFI page table, instead of using 'trampoline_pgd' which
> shares PGD entries with 'swapper_pg_dir'. The pending patches make it
> impossible to dereference the runtime service function pointer without
> first switching %cr3.
>
> It's true that we now have duplicated switching code in
> efi_call_virt() and efi_call_phys_{prolog,epilog}() but we are
> sacrificing code duplication for a little more clarity and the ease of
> writing the page table switching code in C instead of asm.
>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Dave Jones <davej@...emonkey.org.uk>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Denys Vlasenko <dvlasenk@...hat.com>,
> Cc: Stephen Smalley <sds@...ho.nsa.gov>
> Signed-off-by: Matt Fleming <matt@...eblueprint.co.uk>
> ---
> arch/x86/include/asm/efi.h | 25 +++++++++++++++++++++
> arch/x86/platform/efi/efi_64.c | 24 ++++++++++-----------
> arch/x86/platform/efi/efi_stub_64.S | 43 -------------------------------------
> 3 files changed, 36 insertions(+), 56 deletions(-)
>
> diff --git a/arch/x86/include/asm/efi.h b/arch/x86/include/asm/efi.h
> index cfee9d4b02af..f9d99d4e7b1a 100644
> --- a/arch/x86/include/asm/efi.h
> +++ b/arch/x86/include/asm/efi.h
> @@ -3,6 +3,7 @@
>
> #include <asm/fpu/api.h>
> #include <asm/pgtable.h>
> +#include <asm/tlb.h>
>
> /*
> * We map the EFI regions needed for runtime services non-contiguously,
> @@ -64,6 +65,17 @@ 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 {
> + u64 r15;
> + u64 prev_cr3;
> + pgd_t *efi_pgt;
> + bool use_pgd;
> + u64 phys_stack;
> +} __packed;
> +
> #define efi_call_virt(f, ...) \
> ({ \
> efi_status_t __s; \
> @@ -71,7 +83,20 @@ extern u64 asmlinkage efi_call(void *fp, ...);
> efi_sync_low_kernel_mappings(); \
> 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(); \
> + } \
> + \
> __s = efi_call((void *)efi.systab->runtime->f, __VA_ARGS__); \
> + \
> + if (efi_scratch.use_pgd) { \
> + write_cr3(efi_scratch.prev_cr3); \
> + __flush_tlb_all(); \
> + } \
> + \
> __kernel_fpu_end(); \
> preempt_enable(); \
> __s; \
> diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> index 634536034e32..ab5f14a886cc 100644
> --- a/arch/x86/platform/efi/efi_64.c
> +++ b/arch/x86/platform/efi/efi_64.c
> @@ -47,16 +47,7 @@
> */
> static u64 efi_va = EFI_VA_START;
>
> -/*
> - * Scratch space used for switching the pagetable in the EFI stub
> - */
> -struct efi_scratch {
> - u64 r15;
> - u64 prev_cr3;
> - pgd_t *efi_pgt;
> - bool use_pgd;
> - u64 phys_stack;
> -} __packed;
> +struct efi_scratch efi_scratch;
>
> static void __init early_code_mapping_set_exec(int executable)
> {
> @@ -83,8 +74,11 @@ pgd_t * __init efi_call_phys_prolog(void)
> int pgd;
> int n_pgds;
>
> - if (!efi_enabled(EFI_OLD_MEMMAP))
> - return NULL;
> + if (!efi_enabled(EFI_OLD_MEMMAP)) {
> + save_pgd = (pgd_t *)read_cr3();
> + write_cr3((unsigned long)efi_scratch.efi_pgt);
> + goto out;
> + }
>
> early_code_mapping_set_exec(1);
>
So this one is called in phys_efi_set_virtual_address_map() like this:
----
save_pgd = efi_call_phys_prolog();
/* Disable interrupts around EFI calls: */
local_irq_save(flags);
<--- MARKER
status = efi_call_phys(efi_phys.set_virtual_address_map,
memory_map_size, descriptor_size,
descriptor_version, virtual_map);
local_irq_restore(flags);
efi_call_phys_epilog(save_pgd);
---
Now, if you look at MARKER, the asm looks like this here:
.loc 1 91 0
call efi_call_phys_prolog #
movq %rax, %r15 #, save_pgd
.file 6 "./arch/x86/include/asm/irqflags.h"
.loc 6 20 0
#APP
# 20 "./arch/x86/include/asm/irqflags.h" 1
# __raw_save_flags
pushf ; pop %r14 # flags
That PUSHF implicitly pushes on the stack pointed by %rsp. But(!) we
have switched the pagetable (i.e., %cr3 has efi_scratch.efi_pgt) and
we're pushing to the VA where the stack *was* but is not anymore.
Or maybe it is because you're copying all the PUDs. It is still not 100%
clean, IMHO.
Can you do the prolog/epilog calls inside the IRQs-off section?
Btw, it was crap like that why I wanted to do SWITCH_PGT in asm...
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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