Re: [RFC PATCH 0/2] Introduce a way to expose the interpreted file with binfmt_misc

[Eric W. Biederman]

David Hildenbrand <david@xxxxxxxxxx> writes:

> On 13.11.23 19:29, Eric W. Biederman wrote:
>> "Guilherme G. Piccoli" <gpiccoli@xxxxxxxxxx> writes:
>> 
>>> On 09/10/2023 14:37, Kees Cook wrote:
>>>> On Fri, Oct 06, 2023 at 02:07:16PM +0200, David Hildenbrand wrote:
>>>>> On 07.09.23 22:24, Guilherme G. Piccoli wrote:
>>>>>> Currently the kernel provides a symlink to the executable binary, in the
>>>>>> form of procfs file exe_file (/proc/self/exe_file for example). But what
>>>>>> happens in interpreted scenarios (like binfmt_misc) is that such link
>>>>>> always points to the *interpreter*. For cases of Linux binary emulators,
>>>>>> like FEX [0] for example, it's then necessary to somehow mask that and
>>>>>> emulate the true binary path.
>>>>>
>>>>> I'm absolutely no expert on that, but I'm wondering if, instead of modifying
>>>>> exe_file and adding an interpreter file, you'd want to leave exe_file alone
>>>>> and instead provide an easier way to obtain the interpreted file.
>>>>>
>>>>> Can you maybe describe why modifying exe_file is desired (about which
>>>>> consumers are we worrying? ) and what exactly FEX does to handle that (how
>>>>> does it mask that?).
>>>>>
>>>>> So a bit more background on the challenges without this change would be
>>>>> appreciated.
>>>>
>>>> Yeah, it sounds like you're dealing with a process that examines
>>>> /proc/self/exe_file for itself only to find the binfmt_misc interpreter
>>>> when it was run via binfmt_misc?
>>>>
>>>> What actually breaks? Or rather, why does the process to examine
>>>> exe_file? I'm just trying to see if there are other solutions here that
>>>> would avoid creating an ambiguous interface...
>>>>
>>>
>>> Thanks Kees and David! Did Ryan's thorough comment addressed your
>>> questions? Do you have any take on the TODOs?
>>>
>>> I can maybe rebase against 6.7-rc1 and resubmit , if that makes sense!
>>> But would be better having the TODOs addressed, I guess.
>> Currently there is a mechanism in the kernel for changing
>> /proc/self/exe.  Would that be reasonable to use in this case?
>> It came from the checkpoint/restart work, but given that it is
>> already
>> implemented it seems like the path of least resistance to get your
>> binfmt_misc that wants to look like binfmt_elf to use that mechanism.
>
> I had that in mind as well, but
> prctl_set_mm_exe_file()->replace_mm_exe_file() fails if the executable
> is still mmaped (due to denywrite handling); that should be the case
> for the emulator I strongly assume.

Bah yes.  The sanity check that that the old executable is no longer
mapped does make it so that we can't trivially change the /proc/self/exe
using prctl(PR_SET_MM_EXE_FILE).

Eric