Re: [patch V3 21/32] x86/exceptions: Split debug IST stack

[Sean Christopherson]

On Sun, Apr 14, 2019 at 05:59:57PM +0200, Thomas Gleixner wrote:
> The debug IST stack is actually two separate debug stacks to handle #DB
> recursion. This is required because the CPU starts always at top of stack
> on exception entry, which means on #DB recursion the second #DB would
> overwrite the stack of the first.
> 
> The low level entry code therefore adjusts the top of stack on entry so a
> secondary #DB starts from a different stack page. But the stack pages are
> adjacent without a guard page between them.
> 
> Split the debug stack into 3 stacks which are separated by guard pages. The
> 3rd stack is never mapped into the cpu_entry_area and is only there to
> catch triple #DB nesting:
> 
>       --- top of DB_stack	<- Initial stack
>       --- end of DB_stack
>       	  guard page
> 
>       --- top of DB1_stack	<- Top of stack after entering first #DB
>       --- end of DB1_stack
>       	  guard page
> 
>       --- top of DB2_stack	<- Top of stack after entering second #DB
>       --- end of DB2_stack	   
>       	  guard page
> 
> If DB2 would not act as the final guard hole, a second #DB would point the
> top of #DB stack to the stack below #DB1 which would be valid and not catch
> the not so desired triple nesting.
> 
> The backing store does not allocate any memory for DB2 and its guard page
> as it is not going to be mapped into the cpu_entry_area.
> 
>  - Adjust the low level entry code so it adjusts top of #DB with the offset
>    between the stacks instead of exception stack size.
> 
>  - Make the dumpstack code aware of the new stacks.
> 
>  - Adjust the in_debug_stack() implementation and move it into the NMI code
>    where it belongs. As this is NMI hotpath code, it just checks the full
>    area between top of DB_stack and bottom of DB1_stack without checking
>    for the guard page. That's correct because the NMI cannot hit a
>    stackpointer pointing to the guard page between DB and DB1 stack.  Even
>    if it would, then the NMI operation still is unaffected, but the resume
>    of the debug exception on the topmost DB stack will crash by touching
>    the guard page.
> 
> Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---

One nit below on the docs, otherwise:

Reviewed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

> V2 -> V3: Fix off by one in in_debug_stack()
> ---
>  Documentation/x86/kernel-stacks       |    7 ++++++-
>  arch/x86/entry/entry_64.S             |    8 ++++----
>  arch/x86/include/asm/cpu_entry_area.h |   14 ++++++++++----
>  arch/x86/include/asm/debugreg.h       |    2 --
>  arch/x86/include/asm/page_64_types.h  |    3 ---
>  arch/x86/kernel/asm-offsets_64.c      |    2 ++
>  arch/x86/kernel/cpu/common.c          |   11 -----------
>  arch/x86/kernel/dumpstack_64.c        |   12 ++++++++----
>  arch/x86/kernel/nmi.c                 |   20 +++++++++++++++++++-
>  arch/x86/mm/cpu_entry_area.c          |    4 +++-
>  10 files changed, 52 insertions(+), 31 deletions(-)
> 
> --- a/Documentation/x86/kernel-stacks
> +++ b/Documentation/x86/kernel-stacks
> @@ -76,7 +76,7 @@ The currently assigned IST stacks are :-
>    middle of switching stacks.  Using IST for NMI events avoids making
>    assumptions about the previous state of the kernel stack.
>  
> -* ESTACK_DB.  DEBUG_STKSZ
> +* ESTACK_DB.  EXCEPTION_STKSZ (PAGE_SIZE).
>  
>    Used for hardware debug interrupts (interrupt 1) and for software
>    debug interrupts (INT3).
> @@ -86,6 +86,11 @@ The currently assigned IST stacks are :-
>    avoids making assumptions about the previous state of the kernel
>    stack.
>  
> +  To handle nested #DB correctly there exist two instances of DB stacks. On
> +  #DB entry the IST stackpointer for #DB is switched to the second instance
> +  so a nested #DB starts from a clean stack. The nested #DB switches to

Pretty sure the "to" at the end is unwanted.

> +  the IST stackpointer to a guard hole to catch triple nesting.
> +
>  * ESTACK_MCE.  EXCEPTION_STKSZ (PAGE_SIZE).
>  
>    Used for interrupt 18 - Machine Check Exception (#MC).