linux-kernel - Re: [PATCH V3 0/3] Unmap EFI boot services code/data regions after boot.

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Date:   Mon, 5 Nov 2018 13:47:57 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
Cc:     linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH V3 0/3] Unmap EFI boot services code/data regions after boot.

On 4 November 2018 at 21:10, Sai Praneeth Prakhya
<sai.praneeth.prakhya@...el.com> wrote:
> CC'ing x86 folks because this patch set touches x86/mm which I am no expert of.
>
> Ideally, after kernel assumes control of the platform, firmware shouldn't access
> EFI boot services code/data regions. But, it's noticed that this is not so true
> in many x86 platforms. Hence, during boot, kernel reserves EFI boot services
> code/data regions [1] and maps [2] them to efi_pgd so that call to
> set_virtual_address_map() doesn't fail. After returning from
> set_virtual_address_map(), kernel frees the reserved regions [3] but they still
> remain mapped.
>
> This means that any code that's running in efi_pgd address space (e.g: any EFI
> runtime service) would still be able to access EFI boot services code/data
> regions but the contents of these regions would have long been over written by
> someone else as they are freed by efi_free_boot_services(). So, it's important
> to unmap these regions. After unmapping EFI boot services code/data regions, any
> illegal access by buggy firmware to these regions would result in page fault
> which will be handled by efi specific fault handler.
>
> Unmapping EFI boot services code/data regions will result in clearing
> PAGE_PRESENT bit and it shouldn't bother L1TF cases because it's already handled
> by protnone_mask() at arch/x86/include/asm/pgtable-invert.h.
>
> [1] Please see efi_reserve_boot_services()
> [2] Please see efi_map_region() -> __map_region() -> kernel_map_pages_in_pgd()
> [3] Please see efi_free_boot_services()
>
> Testing the patch set:
> ----------------------
> 1. Download buggy firmware (which accesses boot regions even after kernel has
> booted) from here [1].
> 2. Without the patch set, you shouldn't see any kernel warning/error messages
> (i.e. kernel allows accesses to EFI boot services code/data regions even after
> call to set_virtual_address_map()).
> 3. With the patch set, you should see a kernel warning about buggy firmware,
> efi_rts_wq beeing freezed and disabling runtime services forever.
>
> Please note that this patch will change kernel's existing behavior for some EFI
> runtime services but I think it's OK because kernel should have never allowed
> those accesses in the first place.
>
> Also please note that this patch set needs lot of real time trashing as I just
> tested it out with OVMF.
>
> Note:
> -----
> Patch set based on "next" branch in efi tree.
>
> Changes from V2 -> V3:
> ----------------------
> 1. Expliclty set pfn to 0 in kernel_unmap_pages_in_pgd().
> 2. Add __init modifier to kernel_<map/unmap>_pages_in_pgd().
> 3. Warn if kernel_<map/unmap>_pages_in_pgd() are called after smp_init().
> 4. Split efi_unmap_pages() into a separate patch.
>
> Changes from V1 -> V2:
> ----------------------
> 1. Rewrite the cpa initializer in a more readable fashion.
> 2. Don't use cpa->pfn while unmapping, as it's not useful.
> 3. Unmap regions before freeing them up.
> 4. Fix spelling nits.
>
> Sai Praneeth (3):
>   x86/mm/pageattr: Introduce helper function to unmap EFI boot services
>   x86/efi: Unmap EFI boot services code/data regions from efi_pgd
>   x86/efi: Move efi_<reserve/free>_boot_services() to arch/x86
>
>  arch/x86/include/asm/efi.h           |  2 ++
>  arch/x86/include/asm/pgtable_types.h |  8 ++++++--
>  arch/x86/mm/pageattr.c               | 40 ++++++++++++++++++++++++++++++++++--
>  arch/x86/platform/efi/efi.c          |  2 ++
>  arch/x86/platform/efi/quirks.c       | 25 ++++++++++++++++++++++
>  include/linux/efi.h                  |  3 ---
>  init/main.c                          |  4 ----
>  7 files changed, 73 insertions(+), 11 deletions(-)
>
> Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: Bhupesh Sharma <bhsharma@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>

Thanks Sai.

I have queued these up in efi/next for now, but I'd still like a fresh
ack from the x86 maintainers.