Re: [PATCH v4 1/2] sev/i386: Introduce sev_add_kernel_loader_hashes for measured linux boot

[Tom Lendacky]

On 10/19/21 1:18 AM, Dov Murik wrote:
> 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.

Right, just not in a release, yet.

> 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).

Correct, scenario A4.

> > 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?

That sounds like it might be the best approach if there are no security 
concerns. I agree with printing a message, either informational or warning 
is ok by me.

Lets see if anyone else has some thoughts/ideas.

Thanks,
Tom

> Other ideas?
>
>
> -Dov