Re: [Xen-devel] 回复: Re: [PATCH] Choose retpoline only when it is safe to use

[Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>]

On 2018/2/6 18:50, Andrew Cooper wrote:

On 06/02/18 10:29, zhenzhong.duan wrote:

2018年2月6日 17:20于 Andrew Cooper <andrew.cooper3@xxxxxxxxxx>写道：
>
> On 06/02/2018 09:13, Zhenzhong Duan wrote:
> > 在 2018/2/6 16:59, Andrew Cooper 写道:
> >> On 06/02/2018 08:43, Zhenzhong Duan wrote:
> >>> When ( ibrs && thunk == THUNK_DEFAULT && !retpoline_safe() ) is true,
> >>> thunk is set to THUNK_JMP rather than THUNK_RETPOLINE.
> >>>
> >>> When (!ibrs && thunk == THUNK_DEFAULT && !retpoline_safe() ) is true,
> >>> we should do the same.
> >>>
> >>> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
> >> Why?  What improvement is this intended to give?
> > No improvement, I just feel if retpoline isn't safe, THUNK_JMP is
> > better and safer.
> > Above first check is working that way.
>
> If your only two choices are unsafe repoline or plain jumps, then unsafe
> repoline is far far far safer.
>
> Its unsafe properties only kick in on an RSB underflow, and an attacker
> would have to do call-depths analysis of the running binary to identify
> which rets to attempt to poison.
>
Thanks for explaining.
So, for a retpoline safe processor, it just stop using RSB when it's empty to avoid underflow?

The qualification of whether a processor is retpoline-safe or not is whether an RSB underflow causes a BTB lookup (unsafe) or not (safe).

RSB underflows will always occur; you cannot get rid of them, but in most cases (i.e. pre-Skylake) they don't fall back to using a prediction source which can be poisoned by an attacker.

Understood.

Another question:

if (opt_thunk == THUNK_DEFAULT && opt_ibrs == -1 &&
CONFIG_INDIRECT_THUNK && !cpu_has_lfence_dispatch && !retpoline_safe())
results in "thunk = THUNK_JMP" regardless of the value of "boot_cpu_has(X86_FEATURE_IBRSB)"

Your formatting is hard to follow,

Sorry, sent from mobile.

but cpu_has_lfence_dispatch can only be false when virtualised under an SP2-unaware hypervisor on AMD hardware, at which point retpoline_safe() will return true.  Also, AMD don't have IBRS in microcode and their future plans don't appear to involve using that particular CPUID bit.

That does make sense for AMD. But what about Intel hardware which has (!cpu_has_lfence_dispatch && !retpoline_safe() && !boot_cpu_has(X86_FEATURE_IBRSB)), should we  prefer THUNK_RETPOLINE?

-- 
thanks
zduan

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