Re: [PATCH 17/17] efi/libstub/arm64: handle randomized TEXT_OFFSET

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 14 May 2018 at 08:47, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
>
>> From: Mark Rutland <mark.rutland@arm.com>
>>
>> When CONFIG_RANDOMIZE_TEXT_OFFSET is selected, TEXT_OFFSET is an
>> arbitrary multiple of PAGE_SIZE in the interval [0, 2MB).
>>
>> The EFI stub does not account for the potential misalignment of
>> TEXT_OFFSET relative to EFI_KIMG_ALIGN, and produces a randomized
>> physical offset which is always a round multiple of EFI_KIMG_ALIGN.
>> This may result in statically allocated objects whose alignment exceeds
>> PAGE_SIZE to appear misaligned in memory. This has been observed to
>> result in spurious stack overflow reports and failure to make use of
>> the IRQ stacks, and theoretically could result in a number of other
>> issues.
>>
>> We can OR in the low bits of TEXT_OFFSET to ensure that we have the
>> necessary offset (and hence preserve the misalignment of TEXT_OFFSET
>> relative to EFI_KIMG_ALIGN), so let's do that.
>>
>> Fixes: 6f26b3671184c36d ("arm64: kaslr: increase randomization granularity")
>> Cc: <stable@vger.kernel.org> # v4.7+
>> Reported-by: Kim Phillips <kim.phillips@arm.com>
>> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
>> Tested-by: Kim Phillips <kim.phillips@arm.com>
>> [ardb: clarify commit log]
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> ---
>>  drivers/firmware/efi/libstub/arm64-stub.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
>> index b9bd827caa22..541b82fdc8a2 100644
>> --- a/drivers/firmware/efi/libstub/arm64-stub.c
>> +++ b/drivers/firmware/efi/libstub/arm64-stub.c
>> @@ -97,6 +97,13 @@ efi_status_t handle_kernel_image(efi_system_table_t *sys_table_arg,
>>               u32 offset = !IS_ENABLED(CONFIG_DEBUG_ALIGN_RODATA) ?
>>                            (phys_seed >> 32) & mask : TEXT_OFFSET;
>>
>> +             /*
>> +              * With CONFIG_RANDOMIZE_TEXT_OFFSET, TEXT_OFFSET may not be a
>> +              * multiple of EFI_KIMG_ALIGN, and we must ensure that we apply
>> +              * the offset below EFI_KIMG_ALIGN.
>> +              */
>
> When referring to config variables in comments and changelogs I'd suggest a bit
> more verbosity:
>
>   s/CONFIG_RANDOMIZE_TEXT_OFFSET
>    /CONFIG_RANDOMIZE_TEXT_OFFSET=y
>
> ... because at first I thought (based on the name) that
> CONFIG_RANDOMIZE_TEXT_OFFSET is an actual integer offset value - while it's a
> bool. The =y makes the bool nature obvious.
>
> ( Similarly, when negated the canonical way to refer to it is
>   !CONFIG_RANDOMIZE_TEXT_OFFSET. )
>

Fair enough.

>> +             offset |= (TEXT_OFFSET % EFI_KIMG_ALIGN);
>
> The parentheses are not needed here I think.
>

Nope.

Will you fix this up when applying? Or should I resend?