Re: binfmt_misc & different PE binaries

Kees Cook
2023-08-07T14:45:15Z

On August 6, 2023 9:23:46 AM PDT, "Pali Rohár" <pali@kernel.org> wrote:
>Hello, I would like to remind this email about binfmt_misc for PE.
>
>On Thursday 06 July 2023 13:55:50 Pali Rohár wrote:
>> Hello,
>>
>> I would like to ask how to properly register binfmt_misc for different
>> PE binaries, so kernel could execute the correct loader for them.
>>
>> I mean, how to register support for Win32 (console/gui) PE binaries and
>> also for CLR PE binaries (dotnet). Win32 needs to be executed under wine
>> and CLR ideally under dotnet core (or mono).
>>
>> I have read kernel documentation files admin-guide/binfmt-misc.rst
>> and admin-guide/mono.rst. But seems that they are in conflicts as both
>> wants to registers its own handler for the same magic:
>>
>> echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register
>>
>> echo ':CLR:M::MZ::/usr/bin/mono:' > /proc/sys/fs/binfmt_misc/register
>>
>> Not mentioning the fact that they register DOS MZ handler, which matches
>> not only all PE binaries (including EFI, libraries, other processors),
>> but also all kind of other NE/LE/LX binaries and different DOS extenders.
>>
>> From documentation it looks like that even registering PE binaries is
>> impossible by binfmt_misc as PE is detected by checking that indirect
>> reference from 0x3C is PE\0\0. And distinguish between Win32 and CLR
>> needs to parse PE COM descriptor directory.
>>
>> Or it is possible to write binfmt_misc pattern match based on indirect
>> offset?

Normally a single userspace program will be registered and it can do whatever it needs to do to further distinguish the binary and hand it off to the appropriate loader.

--
Kees Cook