Re: [PATCH 1/2] x86/efi: Map EFI memmap entries in-order at runtime
From: Ard Biesheuvel
Date: Sat Sep 26 2015 - 14:16:06 EST
On 26 September 2015 at 10:20, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> I think it "works" because the affected BIOSes don't put spaces between the chunks. I have discussed this with Matt.
>
Forgive the ASCII art but perhaps an illustration might help:
before the 2.5 feature, PE/COFF runtime images were remapped as
illustrated here:
PA VA
+---------------+ +---------------+
| | | |
| PE/COFF .text | | EFI |
| | | Runtime |
+- - - - - - - -+ => | Services |----+
| | | Code | | : :
| PE/COFF .data | | | | : :
| | | | | +---------------+
+---------------+ +---------------+ | | |
| | | | | | EFI |
: : : : | | Runtime |
: : : : +--->| Services |
| | | | | Code |
+---------------+ +---------------+ | |
| | | | | |
| PE/COFF .text | | EFI | +---------------+
| | | Runtime | : gap :
+- - - - - - - -+ => | Services |---+ +---------------+
| | | Code | | | |
| PE/COFF .data | | | | | EFI |
| | | | | | Runtime |
+---------------+ +---------------+ +---->| Services |
| | | | | Code |
: : : : | |
: : : : | |
: : : : +---------------+
: : : : : :
Since the affected symbol references only exist between PE/COFF .text
and PE/COFF .data, there is never a problem since each is PE/COFF
image is mapped as a single region.
However, with the new feature enabled, this no longer holds:
PA VA
+---------------+ +---------------+
| | | |
| PE/COFF .text | | RtServices |----+
| | | Code | |
+- - - - - - - -+ => +---------------+ | +---------------+
| | | RtServices | +--->| RtServices |
| PE/COFF .data | | Data | | Code |
| | | |----+ +---------------+
+---------------+ +---------------+ | : gap :
| | | | | +---------------+
: : : : +--->| RtServices |
: : : : | Data |
| | | | +---------------+
+---------------+ +---------------+ : gap :
| | | | +---------------+
| PE/COFF .text | | RtServices |-------->| RtServices |
| | | Code | | Code |
+- - - - - - - -+ => +---------------+ +---------------+
| | | RtServices | : gap :
| PE/COFF .data | | Data |---+ +---------------+
| | | | | | RtServices |
+---------------+ +---------------+ +---->| Data |
| | | | | |
: : : : +---------------+
: : : : : :
: : : : : :
The illustration uses gaps, but obviously, this applies equally to
inverting the mapping order, since the PE/COFF .text and .data
sections will end up out of order.
--
Ard.
> On September 26, 2015 10:01:14 AM PDT, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>On Fri, Sep 25, 2015 at 10:56 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>>>
>>> So this commit worries me.
>>>
>>> This bug is a good find, and the fix is obviously needed and urgent,
>>but I'm not
>>> sure about the implementation at all. (I've Cc:-ed a few more x86 low
>>level
>>> gents.)
>>>
>>> * Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
>>>> + /*
>>>> + * Starting in UEFI v2.5 the EFI_PROPERTIES_TABLE
>>>> + * config table feature requires us to map all entries
>>>> + * in the same order as they appear in the EFI memory
>>>> + * map. That is to say, entry N must have a lower
>>>> + * virtual address than entry N+1. This is because the
>>>> + * firmware toolchain leaves relative references in
>>>> + * the code/data sections, which are split and become
>>>> + * separate EFI memory regions. Mapping things
>>>> + * out-of-order leads to the firmware accessing
>>>> + * unmapped addresses.
>>>> + *
>>
>>I'm clearly missing something. What is EFI doing that it doesn't care
>>how big the gap between sections is but it still requires them to be
>>in order? It's not as though x86_64 has an addressing mode that
>>allows only non-negative offsets.
>>
>>--Andy
>
> --
> Sent from my Android device with K-9 Mail. Please excuse my brevity.
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