linux-kernel - Re: What exactly do 32-bit x86 exceptions push on the stack in the CS slot?

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Date:   Sun, 20 Nov 2016 20:54:12 -0800
From:   hpa@...or.com
To:     Andy Lutomirski <luto@...nel.org>, tedheadster@...il.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        George Spelvin <linux@...izon.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: What exactly do 32-bit x86 exceptions push on the stack in the CS slot?

On November 19, 2016 5:52:57 PM PST, Andy Lutomirski <luto@...nel.org> wrote:
>This is a question for the old-timers here, since I can't find
>anything resembling an answer in the SDM.
>
>Suppose an exception happens (#UD in this case, but I assume it
>doesn't really matter).  We're not in long mode, and the IDT is set up
>to deliver to a normal 32-bit kernel code segment.  We're running in
>that very same code segment when the exception hits, so no CPL change
>occurs and the TSS doesn't particularly matter.
>
>The CPU will push EFLAGS, CS, and RIP.  Here's the question: what
>happens to the high word of CS on the stack?
>
>The SDM appears to say nothing at all about this.  Modern systems
>(e.g. my laptop running in 32-bit legacy mode under KVM) appear to
>zero-extend CS.  But Matthew's 486DX appears to put garbage in the
>high bits (or maybe just leave whatever was already on the stack in
>place).
>
>Do any of you happen to know what's going on and when the behavior
>changed?  I'd like to know just how big of a problem this is.  Because
>if lots of CPUs work like Matthew's, we have lots of subtle bugs on
>them.
>
>--Andy

I believe i686+ writes zero, older CPUs leave unchanged.
-- 
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