linux-kernel - Re: [PATCH] efi: fdt: avoid FDT manipulation after ExitBootServices()

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Message-ID: <ac1bad94-ffa3-4acf-a719-a8e8f4c7a283@codeaurora.org>
Date:   Wed, 1 Feb 2017 14:08:23 -0700
From:   Jeffrey Hugo <jhugo@...eaurora.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        leif.lindholm@...aro.org, mark.rutland@....com,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     linux-arm-kernel@...ts.infradead.org, riku.voipio@...aro.org,
        matt@...eblueprint.co.uk, linux-kernel@...r.kernel.org,
        linux-efi@...r.kernel.org
Subject: Re: [PATCH] efi: fdt: avoid FDT manipulation after ExitBootServices()

On 2/1/2017 10:45 AM, Ard Biesheuvel wrote:
> Some AArch64 UEFI implementations disable the MMU in ExitBootServices(),
> after which unaligned accesses to RAM are no longer supported.
>
> Commit abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the
> kernel") fixed an issue in the memory map handling of the stub FDT code,
> but inadvertently created an issue with such firmwares, by moving some
> of the FDT manipulation to after the invocation of ExitBootServices().
> Given that the stub's libfdt implementation uses the ordinary, accelerated
> string functions, which rely on hardware handling of unaligned accesses,
> manipulating the FDT with the MMU off may result in alignment faults.
>
> So fix the situation by moving the update_fdt_memmap() call into the
> callback function invoked by efi_exit_boot_services() right before it
> calls the ExitBootServices() UEFI service (which is arguably a better
> place for it anyway)
>
> Note that disabling the MMU in ExitBootServices() is not compliant with
> the UEFI spec, and carries great risk due to the fact that switching from
> cached to uncached memory accesses halfway through compiler generated code
> (i.e., involving a stack) can never be done in a way that is architecturally
> safe.
>
> Cc: <stable@...r.kernel.org>
> Fixes: abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the kernel")
> Tested-by: Riku Voipio <riku.voipio@...aro.org>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>

NACK, please.  This causes a regression on my platform, in the form of 
an assert in UEFI once ExitBootServices() is called, per initial 
testing.  I'll do more testing to determine why.

-- 
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.