linux-kernel - Re: [PATCH] x86/efi: Don't unmap EFI boot services code/data regions for EFI_OLD_MEMMAP and EFI_MIXED

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Date:   Sat, 22 Dec 2018 11:54:10 +0100
From:   Ingo Molnar <mingo@...nel.org>
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>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH] x86/efi: Don't unmap EFI boot services code/data regions
 for EFI_OLD_MEMMAP and EFI_MIXED_MODE


* Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com> wrote:

> Commit d5052a7130a6 ("x86/efi: Unmap EFI boot services code/data regions
> from efi_pgd") forgets to take two EFI modes into consideration namely
> EFI_OLD_MEMMAP and EFI_MIXED_MODE.

So the commit sha1 ended up being this one in tip:efi/core:

  08cfb38f3ef4: x86/efi: Unmap EFI boot services code/data regions from efi_pgd

> EFI_OLD_MEMMAP is a legacy way of mapping EFI regions into swapper_pg_dir
> using ioremap() and init_memory_mapping(). This feature can be enabled by
> passing "efi=old_map" as kernel command line argument. But,
> efi_unmap_pages() unmaps EFI boot services code/data regions *only* from
> efi_pgd and hence cannot be used for unmapping EFI boot services code/data
> regions from swapper_pg_dir.
> 
> Introduce a temporary fix to not unmap EFI boot services code/data regions
> when EFI_OLD_MEMMAP is enabled while working on a real fix.
> 
> EFI_MIXED_MODE is another feature where a 64-bit kernel runs on a
> 64-bit platform crippled by a 32-bit firmware. To support EFI_MIXED_MODE,
> all RAM (i.e. namely EFI regions like EFI_CONVENTIONAL_MEMORY,
> EFI_LOADER_<CODE/DATA>, EFI_BOOT_SERVICES_<CODE/DATA> and
> EFI_RUNTIME_CODE/DATA regions) is mapped into efi_pgd all the time to
> facilitate EFI runtime calls access it's arguments in 1:1 mode. Hence,
> don't unmap EFI boot services code/data regions when booted in mixed mode.
> 
> 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: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
>  arch/x86/platform/efi/quirks.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/arch/x86/platform/efi/quirks.c b/arch/x86/platform/efi/quirks.c
> index 09e811b9da26..9c34230aaeae 100644
> --- a/arch/x86/platform/efi/quirks.c
> +++ b/arch/x86/platform/efi/quirks.c
> @@ -380,6 +380,22 @@ static void __init efi_unmap_pages(efi_memory_desc_t *md)
>  	u64 pa = md->phys_addr;
>  	u64 va = md->virt_addr;
>  
> +	/*
> +	 * To Do: Remove this check after adding functionality to unmap EFI boot
> +	 * services code/data regions from direct mapping area because
> +	 * "efi=old_map" maps EFI regions in swapper_pg_dir.
> +	 */
> +	if (efi_enabled(EFI_OLD_MEMMAP))
> +		return;
> +
> +	/*
> +	 * EFI mixed mode has all RAM mapped to access arguments while making
> +	 * EFI runtime calls, hence don't unmap EFI boot services code/data
> +	 * regions.
> +	 */
> +	if (!efi_is_native() && IS_ENABLED(CONFIG_EFI_MIXED))
> +		return;

I suppose old_mmap and mixed mode stopped working altogether after the 
unmapping changes? What are the symptoms, instant reboots, crasher, or 
some more benign behavior like non-working runtime EFI functionality?

If Ard acks this I'll apply it immediately, as these bugs look like 
show-stoppers for merging the EFI tree into v4.21.

Thanks,

	Ingo