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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 10 Jan 2019 16:56:02 -0800
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Subject: Re: [PATCH v3 0/6] Static calls
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Nadav Amit <namit@vmware.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
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On Thu, Jan 10, 2019 at 3:02 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Thu, Jan 10, 2019 at 12:52 PM Josh Poimboeuf <jpoimboe@redhat.com> 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