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

[Topi Miettinen]

On 26.10.2020 18.24, Dave Martin wrote:
> 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?

Looks good to me.

-Topi