BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures

[Dave Martin]

On Mon, Oct 26, 2020 at 05:45:42PM +0100, Florian Weimer via Libc-alpha wrote:
> * Dave Martin via Libc-alpha:
> 
> > Would it now help to add something like:
> >
> > int mchangeprot(void *addr, size_t len, int old_flags, int new_flags)
> > {
> > 	int ret = -EINVAL;
> > 	mmap_write_lock(current->mm);
> > 	if (all vmas in [addr .. addr + len) have
> > 			their mprotect flags set to old_flags) {
> >
> > 		ret = mprotect(addr, len, new_flags);
> > 	}
> > 	
> > 	mmap_write_unlock(current->mm);
> > 	return ret;
> > }
> 
> I suggested something similar as well.  Ideally, the interface would
> subsume pkey_mprotect, though, and have a separate flags argument from
> the protection flags.  But then we run into argument list length limits.
>
> Thanks,
> Florian

I suppose.  Assuming that a syscall filter can inspect memory, we might
be able to bundle arguments into a struct if necessary.

[...]

Cheers
---Dave