linux-kernel - Re: [PATCH 1/5] x86/mm/kaiser: Alternative ESPFIX

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Message-ID: <7ba63732-8824-bb2b-d66b-bddd823de8e8@linux.intel.com>
Date:   Mon, 27 Nov 2017 14:41:24 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org
Cc:     Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>, daniel.gruss@...k.tugraz.at,
        hughd@...gle.com, keescook@...gle.com, linux-mm@...ck.org,
        michael.schwarz@...k.tugraz.at, moritz.lipp@...k.tugraz.at,
        richard.fellner@...dent.tugraz.at
Subject: Re: [PATCH 1/5] x86/mm/kaiser: Alternative ESPFIX

On 11/27/2017 02:31 PM, Peter Zijlstra wrote:
> Change the asm to do the CR3 switcheroo so we can remove the magic
> mappings.
> 
> Since RDI is unused after SWAPGS we can use it as a scratch reg for
> SWITCH_TO_KERNEL. And once we've computed the new RSP (in RAX) we no
> longer need RDI and can again use it as scratch reg for
> SWITCH_TO_USER.

This definitely looks like the right thing.  Either I missed something
obvious before, or Andy's entry rework made this much more obviously
correct to do the simple thing here.