linux-kernel - Re: [patch] i386: remove IOPL check on task switch

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Message-ID: <454CF388.509@vmware.com>
Date:	Sat, 04 Nov 2006 12:09:44 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Chuck Ebbert <76306.1226@...puserve.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...l.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [patch] i386: remove IOPL check on task switch

Linus Torvalds wrote:
> On Sat, 4 Nov 2006, Zachary Amsden wrote:
>   
>> Ok, checking shows Linus put it back to stop NT leakage.  This is correct, but
>> unlikely.  Would be nice to avoid it unless absolutely necessary.  Perhaps xor
>> eflags old and new and only set_system_eflags() if non-ALU bits have changed.
>>     
>
> Not just NT. AC also leaked, and caused crashes in other programs (Wine) 
> that didn't expect AC to be set and did unaligned accesses.

Yes, AC, NT, IOPL, ID are bad to leak.  DF / TF / RF are impossible to 
leak by privilege contract.  SF, ZF, PF, OF, CF can be clobbered.

VM / VIF / VIP are dealt with in separate switch paths (although I have 
witnessed a VIF leak once from a userspace process that managed to get 
VIF set).  These can't even be set with popf, and require iret to fix.

But 99% of the time, only SF / ZF / PF / OF / CF will be different, so 
you can avoid the popf.

Zach
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