Re: [PATCH] ArmPkg/ArmMmuLib ARM: fix Mva to use idx instead of table base

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

On 19 June 2018 at 22:52, Chris Co <Christopher.Co@microsoft.com> wrote:
> Hi,
>
> Just checking if there is anything needed on my end to get this patch merged in.
>

Well, the patch looks obviously correct, but I just tested it and it
breaks ArmVirtQemu running in 32-bit mode.

I will investigate

>> -----Original Message-----
>> From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> Sent: Thursday, April 19, 2018 5:30 AM
>> To: Chris Co <Christopher.Co@microsoft.com>
>> Cc: Leif Lindholm <leif.lindholm@linaro.org>; edk2-devel@lists.01.org
>> Subject: Re: [PATCH] ArmPkg/ArmMmuLib ARM: fix Mva to use idx instead
>> of table base
>>
>> On 16 April 2018 at 21:45, Chris Co <Christopher.Co@microsoft.com> wrote:
>> > Hi Leif,
>> >
>> >> -----Original Message-----
>> >> From: Leif Lindholm <leif.lindholm@linaro.org>
>> >> Sent: Monday, April 16, 2018 3:44 AM
>> >> To: Chris Co <Christopher.Co@microsoft.com>
>> >> Cc: edk2-devel@lists.01.org; Ard Biesheuvel
>> >> <ard.biesheuvel@linaro.org>
>> >> Subject: Re: [PATCH] ArmPkg/ArmMmuLib ARM: fix Mva to use idx
>> instead
>> >> of table base
>> >>
>> >> On Fri, Apr 13, 2018 at 11:43:27PM +0000, Chris Co wrote:
>> >> > Mva address calculation should use the left-shifted current section
>> >> > index instead of the left-shifted table base address.
>> >> >
>> >> > Using the table base address here has the side-effect of
>> >> > potentially causing an access violation depending on the base address
>> value.
>> >> >
>> >> > Cc: Leif Lindholm <leif.lindholm@linaro.org>
>> >> > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> >> > Contributed-under: TianoCore Contribution Agreement 1.1
>> >> > Signed-off-by: Christopher Co <christopher.co@microsoft.com>
>> >> > ---
>> >> >  ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibCore.c | 2 +-
>> >> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >> >
>> >> > diff --git a/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibCore.c
>> >> > b/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibCore.c
>> >> > index 774a7ccf59..9bf4ba03fd 100644
>> >> > --- a/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibCore.c
>> >> > +++ b/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibCore.c
>> >> > @@ -716,7 +716,7 @@ UpdateSectionEntries (
>> >> >        Descriptor |= EntryValue;
>> >> >
>> >> >        if (CurrentDescriptor  != Descriptor) {
>> >> > -        Mva = (VOID *)(UINTN)(((UINTN)FirstLevelTable) <<
>> >> TT_DESCRIPTOR_SECTION_BASE_SHIFT);
>> >> > +        Mva = (VOID *)(UINTN)(((UINTN)FirstLevelIdx + i) <<
>> >> > + TT_DESCRIPTOR_SECTION_BASE_SHIFT);
>> >>
>> >> So, this clearly looks like you've found a bug - thanks!
>> >>
>> >> But I am a little bit confused about the patch - should this not need
>> >> to incorporate the descriptor size in some way?
>> >> I.e. something like
>> >>   Mva = (VOID *)(UINTN)(((UINTN)FirstLevelIdx + (i * sizeof(UINTN)))
>> >> << TT_DESCRIPTOR_SECTION_BASE_SHIFT);
>> >> or
>> >>   ...                           &FirstLevelTable[FirstLevelIndex + i] ...
>> >>
>> >> ?
>> >>
>> >> Regards,
>> >>
>> >> Leif
>> >>
>> > I don't think descriptor size is needed here.
>> >
>> > My understanding is that Mva is the base address of the current section.
>> >
>> > FirstLevelidx is derived by the first section's BaseAddress >> 20.
>> > The current section index is then (FirstLevelIdx + i), which makes the
>> > base address of the current section (FirstLeveLidx + i) << 20.
>> >
>>
>> Indeed. 'Index' is a bit misleading here, given that it is the top level index into
>> the entire VA space, and so it is congruent with the virtual base address
>> itself. The use of 'FirstLevelTable' in this context is obviously incorrect, given
>> that it refers to the [physical] address of the page tables itself, not to the
>> virtual region they describe.
>>
>> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>