Re: [PATCH 3/3] x86/mm, tracing: Fix CR2 corruption

[Peter Zijlstra]

On Wed, Jul 03, 2019 at 12:27:34PM +0200, root wrote:
> Despire the current efforts to read CR2 before tracing happens there
> still exist a number of possible holes:
> 
>   idtentry page_fault             do_page_fault           has_error_code=1
>     call error_entry
>       TRACE_IRQS_OFF
>         call trace_hardirqs_off*
>           #PF // modifies CR2
> 
>       CALL_enter_from_user_mode
>         __context_tracking_exit()
>           trace_user_exit(0)
>             #PF // modifies CR2
> 
>     call do_page_fault
>       address = read_cr2(); /* whoopsie */
> 
> And similar for i386.
> 
> Fix it by pulling the CR2 read into the entry code, before any of that
> stuff gets a chance to run and ruin things.
> 
> Ideally we'll clean up the entry code by moving this tracing and
> context tracking nonsense into C some day, but let's not delay fixing
> this longer.
> 

> @@ -1180,10 +1189,10 @@ idtentry xenint3		do_int3			has_error_co
>  #endif
>  
>  idtentry general_protection	do_general_protection	has_error_code=1
> -idtentry page_fault		do_page_fault		has_error_code=1
> +idtentry page_fault		do_page_fault		has_error_code=1	read_cr2=1
>  
>  #ifdef CONFIG_KVM_GUEST
> -idtentry async_page_fault	do_async_page_fault	has_error_code=1
> +idtentry async_page_fault	do_async_page_fault	has_error_code=1	read_cr2=1
>  #endif

While going over the various idt handlers, I found that we probably also
need read_cr2 on do_double_fault(), otherwise it is susceptible to the
same problem.