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Date: Mon, 26 Mar 2018 10:32:04 -0700
From: Kees Cook <keescook@...omium.org>
To: alex.popov@...ux.com, Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Laura Abbott <labbott@...hat.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>, 
	PaX Team <pageexec@...email.hu>, Brad Spengler <spender@...ecurity.net>, 
	Ingo Molnar <mingo@...nel.org>, Tycho Andersen <tycho@...ho.ws>, Mark Rutland <mark.rutland@....com>, 
	Ard Biesheuvel <ard.biesheuvel@...aro.org>, Borislav Petkov <bp@...en8.de>, 
	Richard Sandiford <richard.sandiford@....com>, Thomas Gleixner <tglx@...utronix.de>, 
	"H . Peter Anvin" <hpa@...or.com>, "Dmitry V . Levin" <ldv@...linux.org>, Emese Revfy <re.emese@...il.com>, 
	Jonathan Corbet <corbet@....net>, Andrey Ryabinin <aryabinin@...tuozzo.com>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, Thomas Garnier <thgarnie@...gle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Alexei Starovoitov <ast@...nel.org>, Josef Bacik <jbacik@...com>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Nicholas Piggin <npiggin@...il.com>, 
	Al Viro <viro@...iv.linux.org.uk>, "David S . Miller" <davem@...emloft.net>, 
	Ding Tianhong <dingtianhong@...wei.com>, David Woodhouse <dwmw@...zon.co.uk>, 
	Josh Poimboeuf <jpoimboe@...hat.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Dominik Brodowski <linux@...inikbrodowski.net>, Juergen Gross <jgross@...e.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Dan Williams <dan.j.williams@...el.com>, 
	Mathias Krause <minipli@...glemail.com>, Vikas Shivappa <vikas.shivappa@...ux.intel.com>, 
	Kyle Huey <me@...ehuey.com>, Dmitry Safonov <dsafonov@...tuozzo.com>, 
	Will Deacon <will.deacon@....com>, Arnd Bergmann <arnd@...db.de>, x86@...nel.org, 
	linux-kernel@...r.kernel.org, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH RFC v9 2/7] x86/entry: Add STACKLEAK erasing the kernel
 stack at the end of syscalls

On Thu, Mar 22, 2018 at 1:56 PM, Alexander Popov <alex.popov@...ux.com> wrote:
> By the way, during my work on STACKLEAK, I've found one case when we get to the
> userspace directly from the thread stack. Please see sysret32_from_system_call
> in entry_64_compat.S. I checked that.
>
> IMO it seems odd, can the adversary use that to bypass PTI?

If it was missing the page table swap, shouldn't this mean that the
missing NX bit would immediately crash userspace?

-Kees

-- 
Kees Cook
Pixel Security

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