From: Dov Murik <dovmurik@linux.ibm.com>
To: Tom Lendacky <thomas.lendacky@amd.com>, qemu-devel@nongnu.org
Cc: "Dov Murik" <dovmurik@linux.ibm.com>,
"Ashish Kalra" <ashish.kalra@amd.com>,
"Daniel P. Berrangé" <berrange@redhat.com>,
"Eduardo Habkost" <ehabkost@redhat.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
"Connor Kuehl" <ckuehl@redhat.com>,
"Laszlo Ersek" <lersek@redhat.com>,
"James Bottomley" <jejb@linux.ibm.com>,
"Richard Henderson" <richard.henderson@linaro.org>,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
"Hubertus Franke" <frankeh@us.ibm.com>,
"Tobin Feldman-Fitzthum" <tobin@linux.ibm.com>,
"Jim Cadden" <jcadden@ibm.com>,
"Brijesh Singh" <brijesh.singh@amd.com>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Philippe Mathieu-Daudé" <philmd@redhat.com>
Subject: Re: [PATCH v4 1/2] sev/i386: Introduce sev_add_kernel_loader_hashes for measured linux boot
Date: Tue, 19 Oct 2021 09:18:04 +0300 [thread overview]
Message-ID: <9474f5d8-68c5-606b-a123-419556afe5d4@linux.ibm.com> (raw)
In-Reply-To: <3b9d10d9-5d9c-da52-f18c-cd93c1931706@amd.com>
On 18/10/2021 21:02, Tom Lendacky wrote:
> On 9/30/21 12:49 AM, Dov Murik wrote:
>
> ...
>
>> +/*
>> + * Add the hashes of the linux kernel/initrd/cmdline to an encrypted
>> guest page
>> + * which is included in SEV's initial memory measurement.
>> + */
>> +bool sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error
>> **errp)
>> +{
>> + uint8_t *data;
>> + SevHashTableDescriptor *area;
>> + SevHashTable *ht;
>> + uint8_t cmdline_hash[HASH_SIZE];
>> + uint8_t initrd_hash[HASH_SIZE];
>> + uint8_t kernel_hash[HASH_SIZE];
>> + uint8_t *hashp;
>> + size_t hash_len = HASH_SIZE;
>> + int aligned_len;
>> +
>> + if (!pc_system_ovmf_table_find(SEV_HASH_TABLE_RV_GUID, &data,
>> NULL)) {
>> + error_setg(errp, "SEV: kernel specified but OVMF has no hash
>> table guid");
>> + return false;
>> + }
>
> This breaks backwards compatibility with an older OVMF image. Any older
> OVMF image with SEV support that doesn't have the hash table GUID will
> now fail to boot using -kernel/-initrd/-append, where it used to be able
> to boot before.
>
Thanks Tom for noticing this.
Just so we're on the same page: this patch is already merged.
We're dealing with a scenario of launching a guest with SEV enabled and
with -kernel. The behaviours are:
A. With current QEMU:
A1. New AmdSev OVMF build: OVMF will verify the hashes and boot correctly.
A2. New Generic OvmfPkgX64 build: No verification but will boot correctly.
A3. Old AmdSev OVMF build: QEMU aborts the launch because there's no
hash table GUID.
A4. Old Generic OvmfPkgX64 build: QEMU aborts the launch because there's
no hash table GUID.
B. With older QEMU (before this patch was merged):
B1. New AmdSev OVMF build: OVMF will try to verify the hashes but they
are not populated; boot aborted.
B2. New Generic OvmfPkgX64 build: No verification but will boot correctly.
B3. Old AmdSev OVMF build: OVMF aborts the launch because -kernel is not
supported at all.
B4. Old Generic OvmfPkgX64 build: No verification but will boot correctly.
So the problem you are raising is scenario A4 (as opposed to previous
behaviour B4).
> Is that anything we need to be concerned about?
>
Possible solutions:
1. Do nothing. For users that encounter this: tell them to upgrade OVMF.
2. Modify the code: remove the line: error_setg(errp, "SEV: kernel
specified but OVMF has no hash table guid")
I think that option 2 will not degrade security *if* the Guest Owner
verifies the measurement (which is mandatory anyway; otherwise the
untrusted host can replace OVMF with a "malicious" version that doesn't
verify the hashes). Skipping silently might make debugging a bit harder.
Maybe we can print a warning and return, and then the guest launch will
continue?
Other ideas?
-Dov
next prev parent reply other threads:[~2021-10-19 6:19 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-30 5:49 [PATCH v4 0/2] x86/sev: Measured Linux SEV guest with kernel/initrd/cmdline Dov Murik
2021-09-30 5:49 ` [PATCH v4 1/2] sev/i386: Introduce sev_add_kernel_loader_hashes for measured linux boot Dov Murik
2021-09-30 8:32 ` Daniel P. Berrangé
2021-09-30 10:13 ` Dov Murik
2021-10-18 18:02 ` Tom Lendacky
2021-10-19 6:18 ` Dov Murik [this message]
2021-10-20 15:26 ` Tom Lendacky
2021-10-27 19:43 ` Brijesh Singh
2021-10-28 8:41 ` Dov Murik
2021-11-01 10:28 ` Dov Murik
2021-09-30 5:49 ` [PATCH v4 2/2] x86/sev: generate SEV kernel loader hashes in x86_load_linux Dov Murik
2021-10-04 8:03 ` [PATCH v4 0/2] x86/sev: Measured Linux SEV guest with kernel/initrd/cmdline Paolo Bonzini
2021-10-04 17:23 ` Dov Murik
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