From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
"Roger Pau Monné" <roger.pau@citrix.com>, "Wei Liu" <wl@xen.org>
Subject: Re: [PATCH v2] x86/smpboot: Don't unconditionally call memguard_guard_stack() in cpu_smpboot_alloc()
Date: Fri, 16 Oct 2020 08:45:40 +0200 [thread overview]
Message-ID: <6f3accdd-1581-cc70-10bf-017b762ea56d@suse.com> (raw)
In-Reply-To: <5df2626b-8755-8cdb-7cbc-74d51b569a0b@citrix.com>
On 15.10.2020 18:38, Andrew Cooper wrote:
> 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?
Indeed I am:
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Jan
prev parent reply other threads:[~2020-10-16 6:45 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-14 18:47 [PATCH v2] x86/smpboot: Don't unconditionally call memguard_guard_stack() in cpu_smpboot_alloc() Andrew Cooper
2020-10-15 8:50 ` Jan Beulich
2020-10-15 14:02 ` Andrew Cooper
2020-10-15 15:16 ` Jan Beulich
2020-10-15 16:38 ` Andrew Cooper
2020-10-16 6:45 ` Jan Beulich [this message]
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