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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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 have to admit that I can't actually spot where it is.


Either way - it doesn't impact the fix, which attempts to make "the
stack" into a single object.  I experimented with introducing
smpboot_{alloc,free}_stack(), but the result wasn't clean and I
abandoned that approach.

~Andrew