Re: [PATCH 17/17] efi/libstub/arm64: handle randomized TEXT_OFFSET
From: Ingo Molnar
Date: Mon May 14 2018 - 02:47:10 EST
* Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> From: Mark Rutland <mark.rutland@xxxxxxx>
> 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
> 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@xxxxxxxxxxxxxxx> # v4.7+
> Reported-by: Kim Phillips <kim.phillips@xxxxxxx>
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Tested-by: Kim Phillips <kim.phillips@xxxxxxx>
> [ardb: clarify commit log]
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> 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
... 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
> + offset |= (TEXT_OFFSET % EFI_KIMG_ALIGN);
The parentheses are not needed here I think.