Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

["H. Peter Anvin" <hpa@zytor.com>]

On 04/22/2014 10:19 AM, Linus Torvalds wrote:
> On Tue, Apr 22, 2014 at 10:11 AM, Andrew Lutomirski <amluto@gmail.com> wrote:
>>
>>>
>>> Anyway, if done correctly, this whole espfix should be totally free
>>> for normal processes, since it should only trigger if SS is a LDT
>>> entry (bit #2 set in the segment descriptor). So the normal fast-path
>>> should just have a simple test for that.
>>
>> How?  Doesn't something still need to check whether SS is funny before
>> doing iret?
> 
> Just test bit #2. Don't do anything else if it's clear, because you
> should be done. You don't need to do anything special if it's clear,
> because I don't *think* we have any 16-bit data segments in the GDT on
> x86-64.
> 

And we don't (neither do we on i386, and we depend on that invariance.)

Hence:

 irq_return:
+	/*
+	 * Are we returning to the LDT?  Note: in 64-bit mode
+	 * SS:RSP on the exception stack is always valid.
+	 */
+	testb $4,(SS-RIP)(%rsp)
+	jnz irq_return_ldt
+
+irq_return_iret:
 	INTERRUPT_RETURN
-	_ASM_EXTABLE(irq_return, bad_iret)
+	_ASM_EXTABLE(irq_return_iret, bad_iret)


That is the whole impact of the IRET path.

If using IST for #GP won't cause trouble (ISTs don't nest, so we need to
make sure there is absolutely no way we could end up nested) then the
rest of the fixup code can go away and we kill the common path
exception-handling overhead; the only new overhead is the IST
indirection for #GP, which isn't a performance-critical fault (good
thing, because untangling #GP faults is a major effort.)

	-hpa