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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 15/10/2020 16:16, Jan Beulich wrote:
> 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@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.
>> 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.

Oh yes - my analysis was wrong.  The CPU notifier for CPU 1 to come up
runs on CPU 0.

So only the --- text was wrong.  Are you happy with the fix now?

~Andrew