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

[Dave Martin]

On Wed, Oct 21, 2020 at 10:44:46PM -0500, Jeremy Linton via Libc-alpha 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?

Unrolling this discussion a bit, this problem comes from a few sources:

1) systemd is trying to implement a policy that doesn't fit SECCOMP
syscall filtering very well.

2) The program is trying to do something not expressible through the
syscall interface: really the intent is to set PROT_BTI on the page,
with no intent to set PROT_EXEC on any page that didn't already have it
set.


This limitation of mprotect() was known when I originally added PROT_BTI,
but at that time we weren't aware of a clear use case that would fail.


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;
}


libc would now be able to do

	mchangeprot(addr, len, PROT_EXEC | PROT_READ,
		PROT_EXEC | PROT_READ | PROT_BTI);

while systemd's MDWX filter would reject the call if

	(new_flags & PROT_EXEC) &&
		(!(old_flags & PROT_EXEC) || (new_flags & PROT_WRITE)



This won't magically fix current code, but something along these lines
might be better going forward.


Thoughts?

---Dave