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

Jeffrey Hugo
2017-02-01T22:59:53Z

On 2/1/2017 2:08 PM, Jeffrey Hugo wrote:
> 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@vger.kernel.org>
>> Fixes: abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the
>> kernel")
>> Tested-by: Riku Voipio <riku.voipio@linaro.org>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.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.
>

Sorry, false alarm. The assert appears to have been the result of a bad
tree and a bad target configuration, and was not reproduced on another
setup, nor did the assert make sense in context with this change.

I withdraw my NACK.

--
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.