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

Zachary Amsden
2006-11-04T20:09:45Z

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