Messages in this thread | | | Date | Sat, 04 Nov 2006 12:09:44 -0800 | From | Zachary Amsden <> | Subject | Re: [patch] i386: remove IOPL check on task switch |
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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.
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