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

[Lennart Poettering]

On Do, 22.10.20 09:05, Szabolcs Nagy (szabolcs.nagy at arm.com) wrote:

> > > Various changes have been suggested, replacing the mprotect with mmap calls
> > > having PROT_BTI set on the original mapping, re-mmapping the segments,
> > > implying PROT_EXEC on mprotect PROT_BTI calls when VM_EXEC is already set,
> > > and various modification to seccomp to allow particular mprotect cases to
> > > bypass the filters. In each case there seems to be an undesirable attribute
> > > to the solution.
> > >
> > > So, whats the best solution?
> >
> > Did you see Topi's comments on the systemd issue?
> >
> > https://github.com/systemd/systemd/issues/17368#issuecomment-710485532
> >
> > I think I agree with this: it's a bit weird to alter the bits after
> > the fact. Can't glibc set up everything right from the begining? That
> > would keep both concepts working.
>
> that's hard to do and does not work for the main exe currently
> (which is mmaped by the kernel).
>
> (it's hard to do because to know that the elf module requires
> bti the PT_GNU_PROPERTY notes have to be accessed that are
> often in the executable load segment, so either you mmap that
> or have to read that, but the latter has a lot more failure
> modes, so if i have to get the mmap flags right i'd do a mmap
> and then re-mmap if the flags were not right)

Only other option I then see is to neuter one of the two
mechanisms. We could certainly turn off MDWE on arm in systemd, if
people want that. Or make it a build-time choice, so that distros make
the choice: build everything with BTI xor suppport MDWE.

(Might make sense for glibc to gracefully fallback to non-BTI mode if
the mprotect() fails though, to make sure BTI-built binaries work
everywhere.)

I figure your interest in ARM system security is bigger than mine. I
am totally fine to turn off MDWE on ARM if that's what the Linux ARM
folks want. I ave no horse in the race. Just let me know.

[An acceptable compromise might be to allow
mprotect(PROT_EXEC|PROT_BTI) if MDWE is on, but prohibit
mprotect(PROT_EXEC) without PROT_BTI. Then at least you get one of the
two protections, but not both. I mean, MDWE is not perfect anyway on
non-x86-64 already: on 32bit i386 MDWE protection is not complete, due
to ipc() syscall multiplexing being unmatchable with seccomp. I
personally am happy as long as it works fully on x86-64]

Lennart

--
Lennart Poettering, Berlin