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Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
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        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
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On Mon, Mar 9, 2015 at 10:45 AM, Andy Lutomirski <luto@amacapital.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