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

[Zachary Amsden <zach@vmware.com>]

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