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

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Oct 27, 2020 at 02:15:22PM +0000, Dave P Martin wrote:
> I also wonder whether we actually care whether the pages are marked
> executable or not here; probably the flags can just be independent.  This
> rather depends on whether the how the architecture treats the BTI (a.k.a
> GP) pagetable bit for non-executable pages.  I have a feeling we already
> allow PROT_BTI && !PROT_EXEC through anyway.
> 
> 
> What about a generic-ish set/clear interface that still works by just
> adding a couple of PROT_ flags:
> 
> 	switch (flags & (PROT_SET | PROT_CLEAR)) {
> 	case PROT_SET: prot |= flags; break;
> 	case PROT_CLEAR: prot &= ~flags; break;
> 	case 0: prot = flags; break;
> 
> 	default:
> 		return -EINVAL;
> 	}
> 
> This can't atomically set some flags while clearing some others, but for
> simple stuff it seems sufficient and shouldn't be too invasive on the
> kernel side.
> 
> We will still have to take the mm lock when doing a SET or CLEAR, but
> not for the non-set/clear case.
> 
> 
> Anyway, libc could now do:
> 
> 	mprotect(addr, len, PROT_SET | PROT_BTI);
> 
> with much the same effect as your PROT_BTI_IF_X.
> 
> 
> JITting or breakpoint setting code that wants to change the permissions
> temporarily, without needing to know whether PROT_BTI is set, say:
> 
> 	mprotect(addr, len, PROT_SET | PROT_WRITE);
> 	*addr = BKPT_INSN;
> 	mprotect(addr, len, PROT_CLEAR | PROT_WRITE);

The problem with this approach is that you can't catch
PROT_EXEC|PROT_WRITE mappings via seccomp. So you'd have to limit it to
some harmless PROT_ flags only. I don't like this limitation, nor the
PROT_BTI_IF_X approach.

The only generic solutions I see are to either use a stateful filter in
systemd or pass the old state to the kernel in a cmpxchg style so that
seccomp can check it (I think you suggest this at some point).

The latter requires a new syscall which is not something we can address
as a quick, back-portable fix here. If systemd cannot be changed to use
a stateful filter for w^x detection, my suggestion is to go for the
kernel setting PROT_BTI on the main executable with glibc changed to
tolerate EPERM on mprotect(). I don't mind adding an AT_FLAGS bit if
needed but I don't think it buys us much.

Once the current problem is fixed, we can look at a better solution
longer term as a new syscall.

-- 
Catalin