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

Lennart Poettering
2020-10-22T07:26:56Z

On Mi, 21.10.20 22:44, Jeremy Linton (jeremy.linton@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.

Lennart

--
Lennart Poettering, Berlin