From: Brijesh Singh <brijesh.singh@amd.com>
To: Venu Busireddy <venu.busireddy@oracle.com>
Cc: brijesh.singh@amd.com, Ashish Kalra <Ashish.Kalra@amd.com>,
pbonzini@redhat.com, tglx@linutronix.de, mingo@redhat.com,
hpa@zytor.com, joro@8bytes.org, bp@suse.de,
thomas.lendacky@amd.com, x86@kernel.org, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org, rientjes@google.com,
srutherford@google.com, luto@kernel.org
Subject: Re: [PATCH v6 01/14] KVM: SVM: Add KVM_SEV SEND_START command
Date: Thu, 2 Apr 2020 15:04:54 -0500 [thread overview]
Message-ID: <f715bf99-0158-4d5f-77f3-b27743db3c59@amd.com> (raw)
In-Reply-To: <20200402194313.GA656773@vbusired-dt>
On 4/2/20 2:43 PM, Venu Busireddy wrote:
> On 2020-04-02 14:17:26 -0500, Brijesh Singh wrote:
>> On 4/2/20 1:57 PM, Venu Busireddy wrote:
>> [snip]...
>>
>>>> The question is, how does a userspace know the session length ? One
>>>> method is you can precalculate a value based on your firmware version
>>>> and have userspace pass that, or another approach is set
>>>> params.session_len = 0 and query it from the FW. The FW spec allow to
>>>> query the length, please see the spec. In the qemu patches I choose
>>>> second approach. This is because session blob can change from one FW
>>>> version to another and I tried to avoid calculating or hardcoding the
>>>> length for a one version of the FW. You can certainly choose the first
>>>> method. We want to ensure that kernel interface works on the both cases.
>>> I like the fact that you have already implemented the functionality to
>>> facilitate the user space to obtain the session length from the firmware
>>> (by setting params.session_len to 0). However, I am trying to address
>>> the case where the user space sets the params.session_len to a size
>>> smaller than the size needed.
>>>
>>> Let me put it differently. Let us say that the session blob needs 128
>>> bytes, but the user space sets params.session_len to 16. That results
>>> in us allocating a buffer of 16 bytes, and set data->session_len to 16.
>>>
>>> What does the firmware do now?
>>>
>>> Does it copy 128 bytes into data->session_address, or, does it copy
>>> 16 bytes?
>>>
>>> If it copies 128 bytes, we most certainly will end up with a kernel crash.
>>>
>>> If it copies 16 bytes, then what does it set in data->session_len? 16,
>>> or 128? If 16, everything is good. If 128, we end up causing memory
>>> access violation for the user space.
>> My interpretation of the spec is, if user provided length is smaller
>> than the FW expected length then FW will reports an error with
>> data->session_len set to the expected length. In other words, it should
>> *not* copy anything into the session buffer in the event of failure.
> That is good, and expected behavior.
>
>> If FW is touching memory beyond what is specified in the session_len then
>> its FW bug and we can't do much from kernel.
> Agreed. But let us assume that the firmware is not touching memory that
> it is not supposed to.
>
>> Am I missing something ?
> I believe you are agreeing that if the session blob needs 128 bytes and
> user space sets params.session_len to 16, the firmware does not copy
> any data to data->session_address, and sets data->session_len to 128.
>
> Now, when we return, won't the user space try to access 128 bytes
> (params.session_len) of data in params.session_uaddr, and crash? Because,
> instead of returning an error that buffer is not large enough, we return
> the call successfully!
Ah, so the main issue is we should not be going to e_free on error. If
session_len is less than the expected len then FW will return an error.
In the case of an error we can skip copying the session_data into
userspace buffer but we still need to pass the session_len and policy
back to the userspace.
+ ret = sev_issue_cmd(kvm, SEV_CMD_SEND_START, data, &argp->error);
+
+ if (ret)
+ goto e_free;
+
If user space gets an error then it can decode it further to get
additional information (e.g buffer too small).
>
> That is why I was suggesting the following, which you seem to have
> missed.
>
>>> Perhaps, this can be dealt a little differently? Why not always call
>>> sev_issue_cmd(kvm, SEV_CMD_SEND_START, ...) with zeroed out data? Then,
>>> if the user space has set params.session_len to 0, we return with the
>>> needed params.session_len. Otherwise, we check if params.session_len is
>>> large enough, and if not, we return -EINVAL?
> Doesn't the above approach address all scenarios?
>
next prev parent reply other threads:[~2020-04-02 20:04 UTC|newest]
Thread overview: 107+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-30 6:19 [PATCH v6 00/14] Add AMD SEV guest live migration support Ashish Kalra
2020-03-30 6:19 ` [PATCH v6 01/14] KVM: SVM: Add KVM_SEV SEND_START command Ashish Kalra
2020-04-02 6:27 ` Venu Busireddy
2020-04-02 12:59 ` Brijesh Singh
2020-04-02 16:37 ` Venu Busireddy
2020-04-02 18:04 ` Brijesh Singh
2020-04-02 18:57 ` Venu Busireddy
2020-04-02 19:17 ` Brijesh Singh
2020-04-02 19:43 ` Venu Busireddy
2020-04-02 20:04 ` Brijesh Singh [this message]
2020-04-02 20:19 ` Venu Busireddy
2020-04-02 17:51 ` Krish Sadhukhan
2020-04-02 18:38 ` Brijesh Singh
2020-03-30 6:20 ` [PATCH v6 02/14] KVM: SVM: Add KVM_SEND_UPDATE_DATA command Ashish Kalra
2020-04-02 17:55 ` Venu Busireddy
2020-04-02 20:13 ` Krish Sadhukhan
2020-03-30 6:20 ` [PATCH v6 03/14] KVM: SVM: Add KVM_SEV_SEND_FINISH command Ashish Kalra
2020-04-02 18:17 ` Venu Busireddy
2020-04-02 20:15 ` Krish Sadhukhan
2020-03-30 6:21 ` [PATCH v6 04/14] KVM: SVM: Add support for KVM_SEV_RECEIVE_START command Ashish Kalra
2020-04-02 21:35 ` Venu Busireddy
2020-04-02 22:09 ` Krish Sadhukhan
2020-03-30 6:21 ` [PATCH v6 05/14] KVM: SVM: Add KVM_SEV_RECEIVE_UPDATE_DATA command Ashish Kalra
2020-04-02 22:25 ` Krish Sadhukhan
2020-04-02 22:29 ` Venu Busireddy
2020-04-07 0:49 ` Steve Rutherford
2020-03-30 6:21 ` [PATCH v6 06/14] KVM: SVM: Add KVM_SEV_RECEIVE_FINISH command Ashish Kalra
2020-04-02 22:24 ` Venu Busireddy
2020-04-02 22:27 ` Krish Sadhukhan
2020-04-07 0:57 ` Steve Rutherford
2020-03-30 6:21 ` [PATCH v6 07/14] KVM: x86: Add AMD SEV specific Hypercall3 Ashish Kalra
2020-04-02 22:36 ` Venu Busireddy
2020-04-02 23:54 ` Krish Sadhukhan
2020-04-07 1:22 ` Steve Rutherford
2020-03-30 6:22 ` [PATCH v6 08/14] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS hypercall Ashish Kalra
2020-04-03 0:00 ` Venu Busireddy
2020-04-03 1:31 ` Krish Sadhukhan
2020-04-03 1:57 ` Ashish Kalra
2020-04-03 2:58 ` Ashish Kalra
2020-04-06 22:27 ` Krish Sadhukhan
2020-04-07 2:17 ` Steve Rutherford
2020-04-07 5:27 ` Ashish Kalra
2020-04-08 0:01 ` Steve Rutherford
2020-04-08 0:29 ` Brijesh Singh
2020-04-08 0:35 ` Steve Rutherford
2020-04-08 1:17 ` Ashish Kalra
2020-04-08 1:38 ` Steve Rutherford
2020-04-08 2:34 ` Brijesh Singh
2020-04-08 3:18 ` Ashish Kalra
2020-04-09 16:18 ` Ashish Kalra
2020-04-09 20:41 ` Steve Rutherford
2020-03-30 6:22 ` [PATCH v6 09/14] KVM: x86: Introduce KVM_GET_PAGE_ENC_BITMAP ioctl Ashish Kalra
2020-04-03 18:30 ` Venu Busireddy
2020-04-03 20:18 ` Krish Sadhukhan
2020-04-03 20:47 ` Ashish Kalra
2020-04-06 22:07 ` Krish Sadhukhan
2020-04-03 20:55 ` Venu Busireddy
2020-04-03 21:01 ` Ashish Kalra
2020-03-30 6:22 ` [PATCH v6 10/14] mm: x86: Invoke hypercall when page encryption status is changed Ashish Kalra
2020-04-03 21:07 ` Krish Sadhukhan
2020-04-03 21:30 ` Ashish Kalra
2020-04-03 21:36 ` Venu Busireddy
2020-03-30 6:22 ` [PATCH v6 11/14] KVM: x86: Introduce KVM_SET_PAGE_ENC_BITMAP ioctl Ashish Kalra
2020-04-03 21:10 ` Krish Sadhukhan
2020-04-03 21:46 ` Venu Busireddy
2020-04-08 0:26 ` Steve Rutherford
2020-04-08 1:48 ` Ashish Kalra
2020-04-10 0:06 ` Steve Rutherford
2020-04-10 1:23 ` Ashish Kalra
2020-04-10 18:08 ` Steve Rutherford
2020-03-30 6:23 ` [PATCH v6 12/14] KVM: x86: Introduce KVM_PAGE_ENC_BITMAP_RESET ioctl Ashish Kalra
2020-04-03 21:14 ` Krish Sadhukhan
2020-04-03 21:45 ` Ashish Kalra
2020-04-06 18:52 ` Krish Sadhukhan
2020-04-08 1:25 ` Steve Rutherford
2020-04-08 1:52 ` Ashish Kalra
2020-04-10 0:59 ` Steve Rutherford
2020-04-10 1:34 ` Ashish Kalra
2020-04-10 18:14 ` Steve Rutherford
2020-04-10 20:16 ` Steve Rutherford
2020-04-10 20:18 ` Steve Rutherford
2020-04-10 20:55 ` Kalra, Ashish
2020-04-10 21:42 ` Brijesh Singh
2020-04-10 21:46 ` Sean Christopherson
2020-04-10 21:58 ` Brijesh Singh
2020-04-10 22:02 ` Brijesh Singh
2020-04-11 0:35 ` Ashish Kalra
2020-04-03 22:01 ` Venu Busireddy
2020-03-30 6:23 ` [PATCH v6 13/14] KVM: x86: Introduce new KVM_FEATURE_SEV_LIVE_MIGRATION feature & Custom MSR Ashish Kalra
2020-03-30 15:52 ` Brijesh Singh
2020-03-30 16:42 ` Ashish Kalra
[not found] ` <20200330162730.GA21567@ashkalra_ubuntu_server>
[not found] ` <1de5e95f-4485-f2ff-aba8-aa8b15564796@amd.com>
[not found] ` <20200331171336.GA24050@ashkalra_ubuntu_server>
[not found] ` <20200401070931.GA8562@ashkalra_ubuntu_server>
2020-04-02 23:29 ` Ashish Kalra
2020-04-03 23:46 ` Krish Sadhukhan
2020-03-30 6:23 ` [PATCH v6 14/14] KVM: x86: Add kexec support for SEV Live Migration Ashish Kalra
2020-03-30 16:00 ` Brijesh Singh
2020-03-30 16:45 ` Ashish Kalra
2020-03-31 14:26 ` Brijesh Singh
2020-04-02 23:34 ` Ashish Kalra
2020-04-03 12:57 ` Dave Young
2020-04-04 0:55 ` Krish Sadhukhan
2020-04-04 21:57 ` Ashish Kalra
2020-04-06 18:37 ` Krish Sadhukhan
2020-03-30 17:24 ` [PATCH v6 00/14] Add AMD SEV guest live migration support Venu Busireddy
2020-03-30 18:28 ` Ashish Kalra
2020-03-30 19:13 ` Venu Busireddy
2020-03-30 21:52 ` Ashish Kalra
2020-03-31 14:42 ` Venu Busireddy
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