Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)

Florian Weimer
2024-07-06T15:32:48Z

* Mickaël Salaün:

> On Fri, Jul 05, 2024 at 08:03:14PM +0200, Florian Weimer wrote:
>> * Mickaël Salaün:
>>
>> > Add a new AT_CHECK flag to execveat(2) to check if a file would be
>> > allowed for execution. The main use case is for script interpreters and
>> > dynamic linkers to check execution permission according to the kernel's
>> > security policy. Another use case is to add context to access logs e.g.,
>> > which script (instead of interpreter) accessed a file. As any
>> > executable code, scripts could also use this check [1].
>>
>> Some distributions no longer set executable bits on most shared objects,
>> which I assume would interfere with AT_CHECK probing for shared objects.
>
> A file without the execute permission is not considered as executable by
> the kernel. The AT_CHECK flag doesn't change this semantic. Please
> note that this is just a check, not a restriction. See the next patch
> for the optional policy enforcement.
>
> Anyway, we need to define the policy, and for Linux this is done with
> the file permission bits. So for systems willing to have a consistent
> execution policy, we need to rely on the same bits.

Yes, that makes complete sense. I just wanted to point out the odd
interaction with the old binutils bug and the (sadly still current)
kernel bug.

>> Removing the executable bit is attractive because of a combination of
>> two bugs: a binutils wart which until recently always set the entry
>> point address in the ELF header to zero, and the kernel not checking for
>> a zero entry point (maybe in combination with an absent program
>> interpreter) and failing the execve with ELIBEXEC, instead of doing the
>> execve and then faulting at virtual address zero. Removing the
>> executable bit is currently the only way to avoid these confusing
>> crashes, so I understand the temptation.
>
> Interesting. Can you please point to the bug report and the fix? I
> don't see any ELIBEXEC in the kernel.

The kernel hasn't been fixed yet. I do think this should be fixed, so
that distributions can bring back the executable bit.

Thanks,
Florian