Re: [PATCH] binfmt_elf: Fail execution of shared objects with ELIBEXEC (was: Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2))

["Mickaël Salaün" <mic@digikod.net>]

On Mon, Jul 08, 2024 at 06:37:14PM +0200, Florian Weimer wrote:
> * Mickaël Salaün:
> 
> > 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?
> 
> I'm not sure if this was ever reported upstream as an RFE to fail with
> ELIBEXEC.  We have downstream bug report:
> 
>   Prevent executed .so files with e_entry == 0 from attempting to become
>   a process.
>   <https://bugzilla.redhat.com/show_bug.cgi?id=2004942>

Thanks for the info.

> 
> I've put together a patch which seems to work, see below.
> 
> I don't think there's any impact on AT_CHECK with execveat because that
> mode will never get to this point.

Correct, that is not an issue for AT_CHECK use cases.