Re: [Patch 0/3] Add more checker for Tianocompress and Ueficompress

["Gao, Liming" <liming.gao@intel.com>]

Laszlo: 
  I try to answer your question. I also include the BZ submitter brent.holtsclaw@intel.com. Holtsclaw, please add your comments if my info is not enough. 

Thanks
Liming
>-----Original Message-----
>From: Laszlo Ersek [mailto:lersek@redhat.com]
>Sent: Friday, October 19, 2018 12:01 AM
>To: Gao, Liming <liming.gao@intel.com>; Zeng, Star <star.zeng@intel.com>;
>edk2-devel@lists.01.org; Ard Biesheuvel <ard.biesheuvel@linaro.org>
>Subject: Re: [edk2] [Patch 0/3] Add more checker for Tianocompress and
>Ueficompress
>
>On 10/18/18 15:36, Gao, Liming wrote:
>> Laszlo and Star:
>>   Thank your notes. I will add CVE number in patch subject although it
>>   will make subject long than 80 characters.
>
>I agree the subject will be overlong, but I also think that including
>the CVE numbers is important enough for that.
>
>> Here is my proposed patch subject: CVE-2017-5731..5735 MdePkg: Add
>> more checker in UefiDecompressLib to access the valid buffer only.
>
>I suggest (based on tradition) that we keep the normal subject at the
>front, and then we append the CVE numbers at the end. Also, we should
>spell out all those CVE identifiers individually, if the same patch
>solves them all. It should be possible to search the subject line for
>any one of these CVE numbers in separation, using the official CVE
>number format.
>

So, your proposal is like:  MdePkg: Add more checker in UefiDecompressLib to access the valid buffer only CVE-2017-5731 CVE-2017-5732 CVE-2017-5733 CVE-2017-5734 CVE-2017-5735

>>   In PEI phase, the recovery image is from the external device. If the
>>   recovery image has the corrupt EFI compression section, they will be
>>   handled by EFI Decompression PPI.
>
>In the PEI phase, if the recovery image is crafted, it could cause a
>buffer overflow during decompression. However, if the recovery image is
>crafted, it might as well decompress cleanly, and once it is dispatched,
>do "bad things". Do the decompression and the dispatch occur at
>different privilege levels?
>

This patch focuses on the wrong decompression data that cause the decompression failure or hang.  The data content can be signed and verified.  

>> In DXE phase, UEFI option ROM is the third party code. If it is EFI
>> compression option ROM, EFI decompression protocol will be used to
>> decode its data. I don't think SMM uses EFI decompression protocol.
>> UefiDecompressionLib is used as EFI compression PPI/Protocol. It
>> matches PI EFI compression section instead of GUID section. So, it has
>> no GUID extraction PPI/Protocol.
>
>In the DXE phase, if the option ROM is crafted, it could cause a buffer
>overflow when it is decompressed. But, again, how is that different from
>when a crafted oprom decompresses cleanly, and then does "bad things"
>when it is dispatched?
>
>Here (in the DXE phase), I can imagine two answers myself:
>
>(1) Decompression occurs before Secure Boot validation, but dispatch
>occurs only after. Therefore a crafted UEFI image could cause problems
>via decompression even if it would fail SB verification later.
>
>(2) Decompression of UEFI option ROMs occurs before PlatformBDS locks
>down SMRAM and lockboxes. However, the execution of UEFI option ROMs
>is deferred until after the lockdown.
>
>Do these scenarios apply? Because, if they do, I agree the issue
>qualifies as privilege escalation.
>

Yes. Decompression happen early. After decompression, PE image will be verified. 

>Thank you!
>Laszlo