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Date: Tue, 17 Mar 2015 16:09:04 +0100
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Ingo Molnar <mingo@kernel.org>
CC: Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>, Oleg Nesterov <oleg@redhat.com>,
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        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2 v2] x86/asm/entry/64: Enable interrupts *after* we
 fetch PER_CPU_VAR(old_rsp)
References: <1426600344-8254-1-git-send-email-dvlasenk@redhat.com> <20150317143614.GA18462@gmail.com>
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On 03/17/2015 03:36 PM, Ingo Molnar wrote:
> 
> * Denys Vlasenko <dvlasenk@redhat.com> wrote:
> 
>> Without this change, it is still not possible to get rid of
>> PER_CPU_VAR(old_rsp) usage in switch_to: if preemption happens
>> while we did not fetch PER_CPU_VAR(old_rsp) and stored it in pt_regs->sp,
>> PER_CPU_VAR(old_rsp) gets corrupted by other task's user sp.
> 
> Well, wouldn't this be a much clearer explanation:
> 
> "We want to use PER_CPU_VAR(old_rsp) as a simple temporary register, 
>  to shuffle user-space RSP into (and from) when we set up the system 
>  call stack frame. At that point we cannot shuffle values into general 
>  purpose registers, because we have not saved them yet.
> 
>  To be able to do this shuffling into a memory location, we must be 
>  atomic and must not be preempted while we do the shuffling, otherwise 
>  the 'temporary' register gets overwritten by some other task's 
>  temporary register contents ..."
> 
> Agreed?


Sounds good to me.