Date:	Fri, 4 Sep 2015 15:24:21 +0200
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Borislav Petkov <bp@...e.de>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Peter Jones <pjones@...hat.com>,
	James Bottomley <JBottomley@...n.com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	"H. Peter Anvin" <hpa@...or.com>, Dave Young <dyoung@...hat.com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/efi: Map EFI memmap entries in-order at runtime

On 4 September 2015 at 15:14, Matt Fleming <matt@...eblueprint.co.uk> wrote:
> From: Matt Fleming <matt.fleming@...el.com>
>
> Beginning with UEFI v2.5 EFI_PROPERTIES_TABLE was introduced that
> signals that the firmware PE/COFF loader supports splitting code and
> data sections of PE/COFF images into separate EFI memory map entries.
> This allows the kernel to map those regions with strict memory
> protections, e.g. EFI_MEMORY_RO for code, EFI_MEMORY_XP for data, etc.
>
> Unfortunately, an unwritten requirement of this new feature is that
> the regions need to be mapped with the same offsets relative to each
> other as observed in the EFI memory map. If this is not done crashes
> like this may occur,
>
>  [    0.006391] BUG: unable to handle kernel paging request at fffffffefe6086dd
>  [    0.006923] IP: [<fffffffefe6086dd>] 0xfffffffefe6086dd
>  [    0.007000] Call Trace:
>  [    0.007000]  [<ffffffff8104c90e>] efi_call+0x7e/0x100
>  [    0.007000]  [<ffffffff81602091>] ? virt_efi_set_variable+0x61/0x90
>  [    0.007000]  [<ffffffff8104c583>] efi_delete_dummy_variable+0x63/0x70
>  [    0.007000]  [<ffffffff81f4e4aa>] efi_enter_virtual_mode+0x383/0x392
>  [    0.007000]  [<ffffffff81f37e1b>] start_kernel+0x38a/0x417
>  [    0.007000]  [<ffffffff81f37495>] x86_64_start_reservations+0x2a/0x2c
>  [    0.007000]  [<ffffffff81f37582>] x86_64_start_kernel+0xeb/0xef
>
> Here 0xfffffffefe6086dd refers to an address the firmware expects to
> be mapped but which the OS never claimed was mapped. The issue is that
> included in these regions are relative addresses to other regions
> which were emitted by the firmware toolchain before the "splitting" of
> sections occurred at runtime.
>
> Needless to say, we don't satisfy this unwritten requirement on x86_64
> and instead map the EFI memory map entries in reverse order. The above
> crash is almost certainly triggerable with any kernel newer than v3.13
> because that's when we rewrote the EFI runtime region mapping code, in
> commit d2f7cbe7b26a ("x86/efi: Runtime services virtual mapping"). For
> kernel versions before v3.13 things may work by pure luck depending on
> the fragmentation of the kernel virtual address space at the time we
> map the EFI regions.
>
> Instead of mapping the EFI memory map entries in reverse order, where
> entry N has a higher virtual address than entry N+1, map them in the
> same order as they appear in the EFI memory map to preserve this
> relative offset between regions.
>

Since the UEFI spec does not mandate an enumeration order for
GetMemoryMap(), it seems to me that you still need to sort its output
before laying out the VA space. Since you need to sort it anyway, why
not simply sort it in reverse order and keep all the original code?
Considering that this is meant for stable, that would keep the delta
*much* smaller.

-- 
Ard.

> This patch has been kept as small as possible with the intention that
> it should be applied aggressively to stable and distribution kernels.
> It is very much a bugfix rather than support for a new feature, since
> when EFI_PROPERTIES_TABLE is enabled we must map things as outlined
> above to even boot - we have no way of asking the firmware not to
> split the code/data regions.
>
> In fact, this patch doesn't even make use of the more strict memory
> protections available in UEFI v2.5. That will come later.
>
> Reported-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Leif Lindholm <leif.lindholm@...aro.org>
> Cc: Peter Jones <pjones@...hat.com>
> Cc: James Bottomley <JBottomley@...n.com>
> Cc: Matthew Garrett <mjg59@...f.ucam.org>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Dave Young <dyoung@...hat.com>
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Matt Fleming <matt.fleming@...el.com>
> ---
>  arch/x86/include/asm/efi.h     |   1 +
>  arch/x86/platform/efi/efi.c    |   2 +
>  arch/x86/platform/efi/efi_32.c |   1 +
>  arch/x86/platform/efi/efi_64.c | 109 ++++++++++++++++++++++++++++++++++-------
>  4 files changed, 96 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/include/asm/efi.h b/arch/x86/include/asm/efi.h
> index 155162ea0e00..fe988599c5e1 100644
> --- a/arch/x86/include/asm/efi.h
> +++ b/arch/x86/include/asm/efi.h
> @@ -96,6 +96,7 @@ extern void __init efi_call_phys_epilog(pgd_t *save_pgd);
>  extern void __init efi_unmap_memmap(void);
>  extern void __init efi_memory_uc(u64 addr, unsigned long size);
>  extern void __init efi_map_region(efi_memory_desc_t *md);
> +extern void __init efi_map_calculate_base(void);
>  extern void __init efi_map_region_fixed(efi_memory_desc_t *md);
>  extern void efi_sync_low_kernel_mappings(void);
>  extern int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages);
> diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
> index e4308fe6afe8..5276ec6eefef 100644
> --- a/arch/x86/platform/efi/efi.c
> +++ b/arch/x86/platform/efi/efi.c
> @@ -714,6 +714,8 @@ static void * __init efi_map_regions(int *count, int *pg_shift)
>         unsigned long left = 0;
>         efi_memory_desc_t *md;
>
> +       efi_map_calculate_base();
> +
>         for (p = memmap.map; p < memmap.map_end; p += memmap.desc_size) {
>                 md = p;
>                 if (!(md->attribute & EFI_MEMORY_RUNTIME)) {
> diff --git a/arch/x86/platform/efi/efi_32.c b/arch/x86/platform/efi/efi_32.c
> index ed5b67338294..8a80baa877fb 100644
> --- a/arch/x86/platform/efi/efi_32.c
> +++ b/arch/x86/platform/efi/efi_32.c
> @@ -55,6 +55,7 @@ void __init efi_map_region(efi_memory_desc_t *md)
>
>  void __init efi_map_region_fixed(efi_memory_desc_t *md) {}
>  void __init parse_efi_setup(u64 phys_addr, u32 data_len) {}
> +void __init efi_map_calculate_base(void) {}
>
>  pgd_t * __init efi_call_phys_prolog(void)
>  {
> diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> index a0ac0f9c307f..4c1d15984a79 100644
> --- a/arch/x86/platform/efi/efi_64.c
> +++ b/arch/x86/platform/efi/efi_64.c
> @@ -214,14 +214,52 @@ static void __init __map_region(efi_memory_desc_t *md, u64 va)
>                            md->phys_addr, va);
>  }
>
> +/**
> + * efi_map_start_addr - Find an address to map an EFI region
> + * @md: EFI memory map region descriptor
> + * @addr: Begin searching from this address
> + *
> + * Calculate the next highest virtual address at which to map
> + * @md->phys_addr while abiding by any alignment restrictions. For
> + * example, if @md->phys_addr is 2M-aligned we keep the virtual
> + * address 2M-aligned.
> + */
> +static u64 efi_map_start_addr(efi_memory_desc_t *md, u64 addr)
> +{
> +       u64 tail;
> +
> +       tail = md->phys_addr & (PMD_SIZE - 1);
> +
> +       /* Is physical address 2M-aligned? */
> +       if (!tail) {
> +               addr = roundup(addr, PMD_SIZE);
> +       } else {
> +               u64 prev_addr = addr;
> +
> +               /* get us the same offset within this 2M page */
> +               addr = (addr & PMD_MASK) + tail;
> +
> +               /*
> +                * Roll forward to the next PMD.
> +                */
> +               if (addr < prev_addr)
> +                       addr += PMD_SIZE;
> +       }
> +
> +       return addr;
> +}
> +
>  void __init efi_map_region(efi_memory_desc_t *md)
>  {
>         unsigned long size = md->num_pages << PAGE_SHIFT;
> -       u64 pa = md->phys_addr;
>
>         if (efi_enabled(EFI_OLD_MEMMAP))
>                 return old_map_region(md);
>
> +       /* Has efi_map_calculate_base() been called? */
> +       if (WARN_ON_ONCE(efi_va == EFI_VA_START))
> +               return;
> +
>         /*
>          * Make sure the 1:1 mappings are present as a catch-all for b0rked
>          * firmware which doesn't update all internal pointers after switching
> @@ -239,23 +277,9 @@ void __init efi_map_region(efi_memory_desc_t *md)
>                 return;
>         }
>
> -       efi_va -= size;
> -
> -       /* Is PA 2M-aligned? */
> -       if (!(pa & (PMD_SIZE - 1))) {
> -               efi_va &= PMD_MASK;
> -       } else {
> -               u64 pa_offset = pa & (PMD_SIZE - 1);
> -               u64 prev_va = efi_va;
> -
> -               /* get us the same offset within this 2M page */
> -               efi_va = (efi_va & PMD_MASK) + pa_offset;
> -
> -               if (efi_va > prev_va)
> -                       efi_va -= PMD_SIZE;
> -       }
> +       efi_va = efi_map_start_addr(md, efi_va);
>
> -       if (efi_va < EFI_VA_END) {
> +       if (efi_va > EFI_VA_START) {
>                 pr_warn(FW_WARN "VA address range overflow!\n");
>                 return;
>         }
> @@ -263,6 +287,57 @@ void __init efi_map_region(efi_memory_desc_t *md)
>         /* Do the VA map */
>         __map_region(md, efi_va);
>         md->virt_addr = efi_va;
> +       efi_va += size;
> +}
> +
> +/**
> + * efi_map_calculate_base - Find the base address to map EFI regions
> + *
> + * This function calculates how much virtual address space is required
> + * to map all the EFI regions for runtime. We map those regions as
> + * close to each other as possible while sticking with the PMD
> + * alignment and ensuring we end at EFI_VA_START.
> + *
> + * On return we set 'efi_va' to the start address of the virtual
> + * address space where efi_map_region() will begin mapping things.
> + *
> + * Beginning in UEFI v2.5 the EFI_PROPERTIES_TABLE config table
> + * feature requires us to map all entries in the same order as they
> + * appear in the EFI memory map. That is to say, entry N must have a
> + * lower virtual address than entry N+1. This is because the firmware
> + * toolchain leaves relative references in the code/data sections,
> + * which are split and become separate EFI memory regions. Mapping
> + * things out-of-order leads to the firmware accessing unmapped
> + * addresses.
> + *
> + * We need to map things this way whether or not we actually make use
> + * of the EFI_PROPERTIES_TABLE feature.
> + *
> + * Call this function before invoking efi_map_region().
> + */
> +void __init efi_map_calculate_base(void)
> +{
> +       efi_memory_desc_t *md;
> +       u64 size = 0;
> +
> +       /*
> +        * We don't need to place the mappings this carefully for the
> +        * old mapping scheme.
> +        */
> +       if (efi_enabled(EFI_OLD_MEMMAP))
> +               return;
> +
> +       for_each_efi_memory_desc(&memmap, md) {
> +               if (!(md->attribute & EFI_MEMORY_RUNTIME) &&
> +                   md->type != EFI_BOOT_SERVICES_CODE &&
> +                   md->type != EFI_BOOT_SERVICES_DATA)
> +                       continue;
> +
> +               size = efi_map_start_addr(md, size);
> +               size += md->num_pages << PAGE_SHIFT;
> +       }
> +
> +       efi_va -= size;
>  }
>
>  /*
> --
> 2.1.0
>
--
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