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

[Szabolcs Nagy]

The 10/22/2020 09:18, Lennart Poettering wrote:
> On Mi, 21.10.20 22:44, Jeremy Linton (jeremy.linton at arm.com) wrote:
> 
> > Hi,
> >
> > There is a problem with glibc+systemd on BTI enabled systems. Systemd
> > has a service flag "MemoryDenyWriteExecute" which uses seccomp to deny
> > PROT_EXEC changes. Glibc enables BTI only on segments which are marked as
> > being BTI compatible by calling mprotect PROT_EXEC|PROT_BTI. That call is
> > caught by the seccomp filter, resulting in service failures.
> >
> > So, at the moment one has to pick either denying PROT_EXEC changes, or BTI.
> > This is obviously not desirable.
> >
> > 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)