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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.10.2020 16:02, Andrew Cooper wrote:
> On 15/10/2020 09:50, Jan Beulich wrote:
>> 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@xxxxxxxxxxxxxxxxxxxxxx>
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> ---
>>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>> CC: Wei Liu <wl@xxxxxxx>
>>>
>>> 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.
> 
> The v1 really did fix Marek's repro of the problem.
> 
> The only possible way this can occur is if, somewhere, there is a call
> to cpu_smpboot_free() for CPU0 with remove=0 on the S3 path

I didn't think it was the BSP's stack that got written to, but the
first AP's before letting it run.

Jan