[Lguest] [PATCH] lguest: simplify lguest_iret

[Rusty Russell]

Denys Vlasenko <dvlasenk at redhat.com> writes:
> On 03/23/2015 04:30 AM, Rusty Russell wrote:
>> +		 * They may be about to iret, where they asked us never to
>> +		 * deliver interrupts.  In this case, we can emulate that iret
>> +		 * then immediately deliver the interrupt.  This is bascially
>> +		 * a noop: the iret would pop the interrupt frame and restore
>> +		 * eflags, and then we'd set it up again.  So just restore the
>> +		 * eflags word and jump straight to the handler in this case.
>>  		 */
>> +		if (cpu->regs->eip >= cpu->lg->noirq_start &&
>> +		    (cpu->regs->eip < cpu->lg->noirq_end)) {
>> +			restore_eflags(cpu);
>
> In truth, this is not _exactly_ true for irets to CPL3.
>
> If a new interrupt comes right after iret, then
> a new transition to CPL0 will happen.
>
> This means ss:esp will be loaded from tss.ss0:tss.sp0.
>
> Meaning, that the new iret frame may be in a different place
> than the one which was used by iret.

True.  We could check the to-be-restored-CPL and reset the sp.  Instead,
I've added this comment:

		/*
		 * They may be about to iret, where they asked us never to
		 * deliver interrupts.  In this case, we can emulate that iret
		 * then immediately deliver the interrupt.  This is basically
		 * a noop: the iret would pop the interrupt frame and restore
		 * eflags, and then we'd set it up again.  So just restore the
		 * eflags word and jump straight to the handler in this case.
		 *
		 * Denys Vlasenko points out that this isn't quite right: if
		 * the iret was returning to userspace, then that interrupt
		 * would reset the stack pointer (which the Guest told us
		 * about via LHCALL_SET_STACK).  But unless the Guest is being
		 * *really* weird, that will be the same as the current stack
		 * anyway.
		 */

> There is no good reason for CPL0 code to move iret frame around,
> but who knows. As an example, look what 32-bit Linux kernel does
> with NMI iret frames... it's mind bending.

Fortunately, lguest is allergic to NMIs :)

Thanks!
Rusty.