Re: [RFC] x86_64: A real proposal for iret-less return to kernel

[Andy Lutomirski]

On Wed, May 21, 2014 at 4:51 PM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Thu, May 22, 2014 at 08:30:33AM +0900, Linus Torvalds wrote:
>> If the OS then decides to take down the whole machine, the OS - not
>> the hardware - can choose to do something that will punch through
>> other CPU's NMI blocking (notably, init/reset), but the hardware doing
>> this on its own is just broken if true.
>
> Not that it is any consolation but MCE is not broadcast on AMD.
>
> Regardless, exceptions like MCE cannot be held pending and do pierce the
> NMI handler on both.
>
> Now, if the NMI handler experiences a non-broadcast MCE on the same CPU,
> while running, we're simply going to panic as we're in kernel space
> anyway.
>
> The only problem is if the NMI handler gets interrupted while running
> on a bystander CPU. And I think we could deal with this because the
> bystander would not see an MCE and will return safely. We just need
> to make sure that it returns back to the said NMI handler and not to
> userspace. Unless I'm missing something ...

Under my "always RET unless returning from IST to weird CS or to
specific known-invalid-stack regions" proposal this should work fine.
In the current code it'll also work fine *unless* it hits really early
in the NMI, in which case a second NMI can kill us.

--Andy
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