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

[Ingo Molnar]

* Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:

> On 14 May 2018 at 08:47, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > * 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
> >> 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@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
> > 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?

Since this was at the tail with no dependencies I'll skip this for now I think - 
mind sending the refreshed version in the next batch?

Thanks,

	Ingo