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Date:   Thu, 29 Nov 2018 11:10:50 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        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@ni.com, jeyu@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
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On Thu, Nov 29, 2018 at 08:59:31AM -0800, Andy Lutomirski wrote:
> 
> 
> > On Nov 29, 2018, at 8:49 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > On Thu, Nov 29, 2018 at 10:33:42AM -0600, Josh Poimboeuf wrote:
> >>> can't we 'fix' that again? The alternative is moving that IRET-frame and
> >>> fixing everything up, which is going to be fragile, ugly and such
> >>> things more.
> > 
> >> This seems to work...
> > 
> > That's almost too easy... nice!
> 
> It is indeed too easy: you’re putting pt_regs in the wrong place for
> int3 from user mode, which is probably a root hole if you arrange for
> a ptraced process to do int3 and try to write to whatever register
> aliases CS.
> 
> If you make it conditional on CPL, do it for 32-bit as well, add
> comments convince yourself that there isn’t a better solution

I could do that - but why subject 32-bit to it?  I was going to make it
conditional on CONFIG_HAVE_STATIC_CALL_INLINE which is 64-bit only.

> (like pointing IP at a stub that retpolines to the target by reading
> the function pointer, a la the unoptimizable version), then okay, I
> guess, with only a small amount of grumbling.

I tried that in v2, but Peter pointed out it's racy:

  https://lkml.kernel.org/r/20181126160217.GR2113@hirez.programming.kicks-ass.net

-- 
Josh