Re: [PATCH v3 0/6] Static calls

[Andy Lutomirski]

On Thu, Jan 10, 2019 at 3:02 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Jan 10, 2019 at 12:52 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >
> > Right, emulating a call instruction from the #BP handler is ugly,
> > because you have to somehow grow the stack to make room for the return
> > address.  Personally I liked the idea of shifting the iret frame by 16
> > bytes in the #DB entry code, but others hated it.
>
> Yeah, I hated it.
>
> But I'm starting to think it's the simplest solution.
>
> So still not loving it, but all the other models have had huge issues too.
>

Putting my maintainer hat on:

I'm okay-ish with shifting the stack by 16 bytes.  If this is done, I
want an assertion in do_int3() or wherever the fixup happens that the
write isn't overlapping pt_regs (which is easy to implement because
that code has the relevant pt_regs pointer).  And I want some code
that explicitly triggers the fixup when a CONFIG_DEBUG_ENTRY=y or
similar kernel is built so that this whole mess actually gets
exercised.  Because the fixup only happens when a
really-quite-improbable race gets hit, and the issues depend on stack
alignment, which is presumably why Josh was able to submit a buggy
series without noticing.

BUT: this is going to be utterly gross whenever anyone tries to
implement shadow stacks for the kernel, and we might need to switch to
a longjmp-like approach if that happens.

--Andy