linux-kernel - Re: [PATCH V3 1/3] x86/mm/pageattr: Introduce helper function to unmap EFI boot services

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Date:   Mon, 5 Nov 2018 14:18:53 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        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>,
        Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH V3 1/3] x86/mm/pageattr: Introduce helper function to
 unmap EFI boot services

On Sun, 4 Nov 2018, Sai Praneeth Prakhya wrote:

> 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. Hence, introduce
> kernel_unmap_pages_in_pgd() which will later be used to unmap EFI boot
> services code/data regions.
> 
> While at it modify kernel_map_pages_in_pgd() by
> 1. Adding __init modifier because it's always used *only* during boot.
> 2. Add a warning if it's used after SMP is initialized because it uses
>    __flush_tlb_all() which flushes mappings only on current CPU.
> 
> 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] efi_reserve_boot_services()
> [2] efi_map_region() -> __map_region() -> kernel_map_pages_in_pgd()
> [3] efi_free_boot_services()
> 
> 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>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>