Re: [PATCH] x86: refine guest_mode()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Apr 27, 2020 at 10:03:05AM +0200, Jan Beulich wrote:
> The 2nd of the assertions as well as the macro's return value have been
> assuming we're on the primary stack. While for most IST exceptions we
> eventually switch back to the main one, for #DF we intentionally never
> do, and hence a #DF actually triggering on a user mode insn (which then
> is still a Xen bug) would in turn trigger this assertion, rather than
> cleanly logging state.
> 
> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> While we could go further and also assert we're on the correct IST
> stack in an "else" ti the "if()" added, I'm not fully convinced this
> would be generally helpful. I'll be happy to adjust accordingly if
> others think differently; at such a point though I think this should
> then no longer be a macro.
> 
> --- a/xen/include/asm-x86/regs.h
> +++ b/xen/include/asm-x86/regs.h
> @@ -10,9 +10,10 @@
>      /* Frame pointer must point into current CPU stack. */                   
>  \
>      ASSERT(diff < STACK_SIZE);                                               
>  \
>      /* If not a guest frame, it must be a hypervisor frame. */               
>  \
> -    ASSERT((diff == 0) || (r->cs == __HYPERVISOR_CS));                       
>  \
> +    if ( diff < PRIMARY_STACK_SIZE )                                         
>  \
> +        ASSERT(!diff || ((r)->cs == __HYPERVISOR_CS));                       
>  \

Why not use:

ASSERT(diff >= PRIMARY_STACK_SIZE || !diff || ((r)->cs == __HYPERVISOR_CS));

I'm not sure I fully understand this layout, is it possible that you
also need to account for the size of cpu_info?

Roger.