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

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Date:   Sat, 19 Nov 2016 18:15:00 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Brian Gerst <brgerst@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>, tedheadster@...il.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.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 Sat, Nov 19, 2016 at 6:11 PM, Brian Gerst <brgerst@...il.com> wrote:
> On Sat, Nov 19, 2016 at 8:52 PM, 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
>
> This came up a while back, and we was determined that we can't assume
> zero-extension in 32-bit mode because older processors only do a
> 16-bit write even on a 32-bit push.  So all segments have to be
> treated as 16-bit values, or we have to explicitly zero-extend them.
>
> All 64-bit capable processors do zero-extend segments, even in 32-bit mode.

This almost makes me want to change the definition of pt_regs on
32-bit rather than fixing all the entry code.

>
> --
> Brian Gerst



-- 
Andy Lutomirski
AMA Capital Management, LLC