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    • Ard Biesheuvel's avatar
      efi/fdt: Avoid FDT manipulation after ExitBootServices() · c8f325a5
      Ard Biesheuvel authored
      Some AArch64 UEFI implementations disable the MMU in ExitBootServices(),
      after which unaligned accesses to RAM are no longer supported.
      
      Commit:
      
        abfb7b68 ("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 firmware, 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.
      
      Fixes: abfb7b68 ("efi/libstub/arm*: Pass latest memory map to the kernel")
      Signed-off-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
      Tested-by: default avatarRiku Voipio <riku.voipio@linaro.org>
      Cc: <stable@vger.kernel.org>
      Cc: mark.rutland@arm.com
      Cc: linux-efi@vger.kernel.org
      Cc: matt@codeblueprint.co.uk
      Cc: leif.lindholm@linaro.org
      Cc: linux-arm-kernel@lists.infradead.org
      Link: http://lkml.kernel.org/r/1485971102-23330-2-git-send-email-ard.biesheuvel@linaro.orgSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
      c8f325a5