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

Mickaël Salaün
2024-07-08T08:57:15Z

On Sat, Jul 06, 2024 at 05:32:12PM +0200, Florian Weimer wrote:
> * 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.

Can you please point to the mailing list discussion or the bug report?