RE: [patch 00/38] x86/retbleed: Call depth tracking mitigation

[David Laight]

From: Peter Zijlstra
> Sent: 22 July 2022 12:03
> 
> On Thu, Jul 21, 2022 at 10:01:12PM +0000, David Laight wrote:
> 
> > Since: "If the callee is a variadic function, then the number of floating
> > point arguments passed to the function in vector registers must be provided
> > by the caller in the AL register."
> >
> > And that that never happens in the kernel you can use %eax instead
> > of %r10d.
> 
> Except there's the AMD BTC thing and we should (compiler patch seems
> MIA) have an unconditional: 'xor %eax,%eax' in front of every function
> call.

I've just read https://www.amd.com/system/files/documents/technical-guidance-for-mitigating-branch-type-confusion_v7_20220712.pdf

It doesn't seem to suggest clearing registers except as a vague
'might help' before a function return (to limit what the speculated
code can do.

The only advantage I can think of for 'xor ax,ax' is that it is done as
a register rename - and isn't dependant on older instructions.
So it might reduce some pipeline stalls.

I'm guessing that someone might find a 'gadget' that depends on %eax
and it may be possible to find somewhere that leaves an arbitrary
value in it.
It is also about the only register that isn't live!

> (The official mitigation strategy was CALL; LFENCE IIRC, but that's so
> horrible nobody is actually considering that)
> 
> Yes, the suggested sequence ends with rax being zero, but since we start
> the speculation before that result is computed that's not good enough I
> suspect.

The speculated code can't use the 'wrong' %eax value.
The only problem is that reading from -4(%r11) is likely to be a
D$ miss giving plenty of time for the cpu to execute 'crap'.
But I'm not sure a later 'xor ax,ax' helps.
(OTOH this is all horrid and makes my brian hurt.)

AFAICT with BTC you 'just lose'.
I thought it was bad enough that some cpu used the BTB for predicted
conditional jumps - but using it to decide 'this must be a branch
instruction' seems especially broken.

Seems the best thing to do with those cpu is to run an embedded
system with a busybox+buildroot userspace where almost everything
runs as root :-)

	David

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