Re: [PATCH v13 20/22] x86/kexec(): Reset TDX private memory on platforms with TDX erratum

[Edgecombe, Rick P]

On Fri, 2023-09-15 at 11:42 +0000, Huang, Kai wrote:
> On Thu, 2023-09-14 at 21:36 +0000, Edgecombe, Rick P wrote:
> > On Sat, 2023-08-26 at 00:14 +1200, Kai Huang wrote:
> > > diff --git a/arch/x86/kernel/machine_kexec_64.c
> > > b/arch/x86/kernel/machine_kexec_64.c
> > > index 1a3e2c05a8a5..03d9689ef808 100644
> > > --- a/arch/x86/kernel/machine_kexec_64.c
> > > +++ b/arch/x86/kernel/machine_kexec_64.c
> > > @@ -28,6 +28,7 @@
> > >  #include <asm/setup.h>
> > >  #include <asm/set_memory.h>
> > >  #include <asm/cpu.h>
> > > +#include <asm/tdx.h>
> > >  
> > >  #ifdef CONFIG_ACPI
> > >  /*
> > > @@ -301,6 +302,14 @@ void machine_kexec(struct kimage *image)
> > >         void *control_page;
> > >         int save_ftrace_enabled;
> > >  
> > > +       /*
> > > +        * For platforms with TDX "partial write machine check"
> > > erratum,
> > > +        * all TDX private pages need to be converted back to
> > > normal
> > > +        * before booting to the new kernel, otherwise the new
> > > kernel
> > > +        * may get unexpected machine check.
> > > +        */
> > > +       tdx_reset_memory();
> > > +
> > >  #ifdef CONFIG_KEXEC_JUMP
> > >         if (image->preserve_context)
> > >                 save_processor_state();
> > 
> > Without a ton of knowledge on TDX arch stuff, I'm mostly looked at
> > the
> > kexec flow with respect to anything that might be tinkering with
> > the
> > PAMT. Everything there looked good to me.
> > 
> > But I'm wondering if you want to skip the tdx_reset_memory() in the
> > KEXEC_JUMP/preserve_context case. Somehow (I'm not clear on all the
> > details), kexec can be configured to have the new kernel jump back
> > to
> > the old kernel and resume execution as if nothing happened. Then I
> > think you would want to keep the TDX data around. Does that make
> > any
> > sense?
> > 
> 
> Good point.  Thanks!
> 
> Based on my understanding, it should be OK to skip tdx_reset_memory()
> (or better
> to) when preserve_context is on.  The second kernel shouldn't touch
> first
> kernel's memory anyway otherwise it may corrupt the first kernel
> state (if it
> does this maliciously or accidentally, then the first kernel isn't
> guaranteed to
> work anyway).  

I think it may read the memory, is it ok?

> 
> In fact, if we do tdx_reset_memory() when preserve_memory is on, we
> will need to
> do additional things to mark TDX as dead otherwise after jumping back
> other
> kernel code will still believe TDX is alive and continue to use TDX.
> 
> I'll do this if I don't hear objection from other people.  
> 
> Something like below?
> 
> diff --git a/arch/x86/kernel/machine_kexec_64.c
> b/arch/x86/kernel/machine_kexec_64.c
> index 03d9689ef808..73ed01360408 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -307,12 +307,18 @@ void machine_kexec(struct kimage *image)
>          * all TDX private pages need to be converted back to normal
>          * before booting to the new kernel, otherwise the new kernel
>          * may get unexpected machine check.
> +        *
> +        * But skip this when preserve_context is on.  The second
> kernel
> +        * shouldn't touch the first kernel's memory anyway. 
> Skipping
> +        * this also avoids killing TDX in the first kernel, which
> would
> +        * require more complicated handling.
>          */
> -       tdx_reset_memory();
> -
>  #ifdef CONFIG_KEXEC_JUMP
>         if (image->preserve_context)
>                 save_processor_state();
> +       else
> +#else
> +       tdx_reset_memory();
>  #endif
> 
> 

Not the most beautiful ifdeffery, I'd just duplicate the
tdx_reset_memory() call. But not a strong opinion.