linux-kernel - Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT

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Date:	Mon, 9 Mar 2015 10:59:35 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

On Mon, Mar 9, 2015 at 10:45 AM, Andy Lutomirski <luto@...capital.net> wrote:
>
> If sp0 is set to the very top of the stack, then an NMI immediately
> after sysenter will have OLDSS off the top of the stack, and reading
> it can crash.  This is why 32-bit kernels have a (buggy!) 8 byte
> offset in sp0.

So I think that for sysenter, we *should* have that 8-byte buffer.

Not in general for sp0, but for MSR_IA32_SYSENTER_ESP (which is sp1, afaik).

Just make the rule be that you can never ever have a kernel stack
frame that doesn't contain room for ss/sp at the top.

We have various code that looks at and touches "pt_regs" anyway, and
accesses things out for debugging/oopsing/tracing etc. Let's not make
the rule be that you cannot look at regs->ss without checking various
random other fields first.

                         Linus
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