Re: [Xen-devel] [PATCH V5] x86/altp2m: Fix crash with INVALID_ALTP2M EPTP index

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.07.18 at 10:30, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> On 07/02/2018 08:48 AM, Tian, Kevin wrote:
>>> From: Razvan Cojocaru [mailto:rcojocaru@xxxxxxxxxxxxxxx]
>>> Sent: Thursday, June 28, 2018 10:35 PM
>>>
>>> A VM exit handler executed immediately after enabling #VE might
>>> find a stale __vmsave()d EPTP_INDEX, stored by calling
>>> altp2m_vcpu_destroy() when
>>> SECONDARY_EXEC_ENABLE_VIRT_EXCEPTIONS
>>> had been enabled by altp2m_vcpu_update_vmfunc_ve().
>>>
>>> vmx_vmexit_handler() __vmread()s EPTP_INDEX as soon as
>>> SECONDARY_EXEC_ENABLE_VIRT_EXCEPTIONS is set, so if an
>>> application enables altp2m on a domain, succesfully calls
>>> xc_altp2m_set_vcpu_enable_notify(), then disables altp2m and
>>> exits, a second run of said application will likely read the
>>> INVALID_ALTP2M EPTP_INDEX set when disabling altp2m in the first
>>> run, and crash the host with the BUG_ON(idx >= MAX_ALTP2M),
>>> between xc_altp2m_set_vcpu_enable_notify() and
>>> xc_altp2m_set_domain_state(..., false).
>>>
>>> The problem is not restricted to an INVALID_ALTP2M EPTP_INDEX
>>> (which can only sanely happen on altp2m uninit), but applies
>>> to any stale index previously saved - which means that all
>>> altp2m_vcpu_update_vmfunc_ve() calls must also call
>>> altp2m_vcpu_update_p2m() after setting
>>> SECONDARY_EXEC_ENABLE_VIRT_EXCEPTIONS, in order to make sure
>>> that the stored EPTP_INDEX is always valid at
>>> vmx_vmexit_handler() time.
>>>
>>> I don't however fold the two functions into one everywhere,
>>> since in p2m_switch_domain_altp2m_by_id() and
>>> p2m_switch_vcpu_altp2m_by_id() the extra work done by
>>> altp2m_vcpu_update_vmfunc_ve() is unnecessary and has side
>>> effects (such as __vmwrite(VM_FUNCTION_CONTROL, ...)).
>>>
>>> Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
>>>
>> 
>> Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>. 
>> 
>> btw next time please use more descriptive words and less
>> function names in commit message.
> 
> Fair enough, thanks for the review! Is further action required for this
> patch to go in (perhaps George's ack)?

George's ack, indeed.

Jan



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