Re: [PATCH v2] x86/smpboot: Don't unconditionally call memguard_guard_stack() in cpu_smpboot_alloc()

[Jan Beulich <jbeulich@suse.com>]

On 14.10.2020 20:47, Andrew Cooper wrote:
> cpu_smpboot_alloc() is designed to be idempotent with respect to partially
> initialised state.  This occurs for S3 and CPU parking, where enough state to
> handle NMIs/#MCs needs to remain valid for the entire lifetime of Xen, even
> when we otherwise want to offline the CPU.
> 
> For simplicity between various configuration, Xen always uses shadow stack
> mappings (Read-only + Dirty) for the guard page, irrespective of whether
> CET-SS is enabled.
> 
> Unfortunately, the CET-SS changes in memguard_guard_stack() broke idempotency
> by first writing out the supervisor shadow stack tokens with plain writes,
> then changing the mapping to being read-only.
> 
> This ordering is strictly necessary to configure the BSP, which cannot have
> the supervisor tokens be written with WRSS.
> 
> Instead of calling memguard_guard_stack() unconditionally, call it only when
> actually allocating a new stack.  Xenheap allocates are guaranteed to be
> writeable, and the net result is idempotency WRT configuring stack_base[].
> 
> Fixes: 91d26ed304f ("x86/shstk: Create shadow stacks")
> Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Roger Pau Monné <roger.pau@citrix.com>
> CC: Wei Liu <wl@xen.org>
> 
> This can more easily be demonstrated with CPU hotplug than S3, and the absence
> of bug reports goes to show how rarely hotplug is used.
> 
> v2:
>  * Don't break S3/CPU parking in combination with CET-SS.  v1 would, for S3,
>    turn the BSP shadow stack into regular mappings, and #DF as soon as the TLB
>    shootdown completes.

The code change looks correct to me, but since I don't understand
this part I'm afraid I may be overlooking something. I understand
the "turn the BSP shadow stack into regular mappings" relates to
cpu_smpboot_free()'s call to memguard_unguard_stack(), but I
didn't think we come through cpu_smpboot_free() for the BSP upon
entering or leaving S3.

Jan