Re: [PATCH v7 45/72] x86/entry/64: Add entry code for #VC handler

[Joerg Roedel]

Hello Lai,

On Sun, Jan 24, 2021 at 10:11:14PM +0800, Lai Jiangshan wrote:
> > +
> > +       /*
> > +        * No need to switch back to the IST stack. The current stack is either
> > +        * identical to the stack in the IRET frame or the VC fall-back stack,
> > +        * so it is definitly mapped even with PTI enabled.
> > +        */
> > +       jmp     paranoid_exit
> > +
> >
> 
> Hello
> 
> I know we don't enable PTI on AMD, but the above comment doesn't align to the
> next code.
> 
> We assume PTI is enabled as the comments said "even with PTI enabled".
> 
> When #VC happens after entry_SYSCALL_64 but before it switches to the
> kernel CR3.  vc_switch_off_ist() will switch the stack to the kernel stack
> and paranoid_exit can't work when it switches to user CR3 on the kernel stack.
> 
> The comment above lost information that the current stack is possible to be
> the kernel stack which is mapped not user CR3.
> 
> Maybe I missed something.

You are right, the scenario above would cause problems for the current
#VC entry code. With SEV-ES an #VC exception can't happen in the early
syscall entry code, so I think its the best to update the comment
reflecting this.

In the future this might change and then the #VC entry code needs to
take care of this case too. Thanks for pointing it out.

Regards,

	Joerg