[systemd-devel] BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures

[Florian Weimer]

* Topi Miettinen:

> Allowing mprotect(PROT_EXEC|PROT_BTI) would mean that all you need to
> circumvent MDWX is to add PROT_BTI flag. I'd suggest getting the flags 
> right at mmap() time or failing that, reverting the PROT_BTI for
> legacy programs later.
>
> Could the kernel tell the loader of the BTI situation with auxiliary
> vectors? Then it would be easy for the loader to always use the best 
> mmap() flags without ever needing to mprotect().

I think what we want is a mprotect2 call with a flags argument (separate
from protection flags) that tells the kernel that the request *removes*
protection flags and should fail otherwise.  seccomp could easily filter
that then.

But like the other proposals, the migration story isn't great.  You
would need kernel and seccomp/systemd etc. updates before glibc starts
working, even if glibc has a fallback from mprotect2 to mprotect
(because the latter would be blocked).

Thanks,
Florian
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