linux-kernel - Re: [PATCH 14/43] x86/asm/64: Return to userspace from the trampoline stack

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Date:   Sun, 26 Nov 2017 15:18:47 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 14/43] x86/asm/64: Return to userspace from the
 trampoline stack


* Borislav Petkov <bp@...en8.de> wrote:

> On Fri, Nov 24, 2017 at 06:23:42PM +0100, Ingo Molnar wrote:
> > From: Andy Lutomirski <luto@...nel.org>
> > 
> > By itself, this is useless.  It gives us the ability to run some final
> > code before exit that cannnot run on the kernel stack.  This could
> > include a CR3 switch a la KAISER or some kernel stack erasing, for
> > example.  (Or even weird things like *changing* which kernel stack
> > gets used as an ASLR-strengthening mechanism.)
> > 
> > The SYSRET32 path is not covered yet.  It could be in the future or
> > we could just ignore it and force the slow path if needed.
> > 
> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Borislav Petkov <bpetkov@...e.de>
> > Cc: Brian Gerst <brgerst@...il.com>
> > Cc: Dave Hansen <dave.hansen@...el.com>
> > Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Link: https://lkml.kernel.org/r/d350017000eed20922c3b2711a2d9229dc809256.1511497875.git.luto@kernel.org
> > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> > ---
> >  arch/x86/entry/entry_64.S | 55 +++++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 51 insertions(+), 4 deletions(-)
> 
> Nice commenting, future generations will appreciate it!
> 
> :-)
> 
> Reviewed-by: Borislav Petkov <bp@...e.de>

Heh, indeed - if by 'future generations' you mean 'the guys who merge it,
a few months down the line'. ;-)

Thanks,

	Ingo