Re: [OOPS] repeatable 2.4.8-ac7, 2.4.7-ac6 just run xdos

Andi Kleen
2001-08-22T12:11:05Z

On Wed, Aug 22, 2001 at 07:57:59AM -0400, Brian Gerst wrote:
> Yes. What happened here is that %ds and %es were not being updated
> atomically. Under normal operation, this would just leave %es with
> USER_DS, which is sufficiently equivalent to KERNEL_DS to not cause a
> fault. Coming out of vm86 mode however forces the data segment
> registers to null after saving the real mode values on the stack. If an
> interrupt happened between setting %ds and %es (what are the odds?) then
> that assumption would fail and leave %es null, causing the next string
> instruction to go boom. The same fix should be applied to entry.S as
> well.

No that's not the problem. interrupt gates come in with interrupts off,
so there are no other interrupts that could race here. The syscall entry
always updates %ds/%es unconditionally and %ds first, so there is no
race.

It was much simpler. It assumed that __KERNEL_DS could not be loaded
from user space because of the segment register priviledge checking; and
that was obviously not true from vm86 mode.

-Andi