Re: [PATCH v5 06/10] hw/tpm: Add support for VM migration with TPM CRB chunking

[Arun Menon <armenon@redhat.com>]

On Wed, Apr 29, 2026 at 07:14:38PM -0400, Stefan Berger wrote:
> 
> 
> On 4/22/26 6:30 AM, Arun Menon wrote:
> > From: Arun Menon <armenon@redhat.com>
> > 
> > - Add subsection in VMState for TPM CRB with the newly introduced
> >    command and response buffer GByteArrays, along with a needed callback,
> >    so that newer QEMU only sends the buffers if it is necessary.
> > - Implement a migration blocker to prevent migration of the VM if the
> >    user manually enables chunking capability, cap-chunk, but the machine
> >    type does not support it, using a new hw_compat property called
> >    allow_chunk_migration.
> > - Add a post_load_errp hook so that during a migration, the buffers are
> >    validated before destination VM is started.
> > 
> > Signed-off-by: Arun Menon <armenon@redhat.com>
> > ---
> >   hw/core/machine.c |  1 +
> >   hw/tpm/tpm_crb.c  | 71 +++++++++++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 72 insertions(+)
> > 
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index 6d27cf69a2..b590af0125 100644
> > --- a/hw/core/machine.c
> > +++ b/hw/core/machine.c
> > @@ -40,6 +40,7 @@
> > 
> >   GlobalProperty hw_compat_11_0[] = {
> >       { "tpm-crb", "cap-chunk", "off"},
> > +    { "tpm-crb", "x-allow-chunk-migration", "off"},
> >   };
> >   const size_t hw_compat_11_0_len = G_N_ELEMENTS(hw_compat_11_0);
> > 
> > diff --git a/hw/tpm/tpm_crb.c b/hw/tpm/tpm_crb.c
> > index 29370d6f49..23e6948aee 100644
> > --- a/hw/tpm/tpm_crb.c
> > +++ b/hw/tpm/tpm_crb.c
> > @@ -24,6 +24,7 @@
> >   #include "hw/pci/pci_ids.h"
> >   #include "hw/acpi/tpm.h"
> >   #include "migration/vmstate.h"
> > +#include "migration/blocker.h"
> >   #include "system/tpm_backend.h"
> >   #include "system/tpm_util.h"
> >   #include "system/reset.h"
> > @@ -51,6 +52,8 @@ struct CRBState {
> >       TPMPPI ppi;
> > 
> >       bool cap_chunk;
> > +    bool allow_chunk_migration;
> > +    Error *migration_blocker;
> >   };
> >   typedef struct CRBState CRBState;
> > 
> > @@ -353,12 +356,63 @@ static int tpm_crb_pre_save(void *opaque)
> >       return 0;
> >   }
> > 
> > +static bool tpm_crb_chunk_needed(void *opaque)
> > +{
> > +    CRBState *s = opaque;
> > +
> > +    if (!s->allow_chunk_migration) {
> > +        return false;
> > +    }
> > +
> > +    return ((s->command_buffer && s->command_buffer->len > 0) ||
> > +            (s->response_buffer && s->response_buffer->len > 0));
> > +}
> > +
> > +static bool tpm_crb_chunk_post_load(void *opaque, int version_id, Error **errp)
> > +{
> > +    CRBState *s = opaque;
> > +
> > +    if (!s->response_buffer || !s->command_buffer) {
> > +        error_setg(errp, "tpm-crb: Internal buffers are not allocated");
> 
> Could this happen that this state type is resumed but the buffers are not
> allocated?

Yes, this is not required. Its just a defense. The tpm_realize()
function should allocate the command and response buffers.

> 
> > +        return false;
> > +    }
> > +    if (s->response_offset > s->response_buffer->len) {
> > +        error_setg(errp, "tpm-crb: Invalid response "
> > +                   "offset %" PRIu32 " in migration stream",
> > +                   s->response_offset);
> > +        return false;
> > +    }
> 
> The check is correct but can this particular case occur other than through
> crafted input?

Not that I can think of. The check is indeed added to address malicious
migration stream / crafted input.

> 
> > +    if (s->response_buffer->len > s->be_buffer_size ||
> > +        s->command_buffer->len > s->be_buffer_size) {
> 
> I suppose this could happen if I was running with a PQC-enable swtpm/libtpms
> and suspended at the right moment that when a chunked command or response
> was in the buffer and now I am trying to resume with a non-PQC-enabled swtpm
> that only has 4kb buffer, while the newer PQC one had 8kb buffer.
> 

Yes. This check is for safety. We do not want to migrate to a VM that
has a tpm backend that does not support big buffers.

> > +        error_setg(errp, "tpm-crb: Buffer sizes exceed backend capacity");
> > +        return false;
> > +    }
> > +    return true;
> > +}
> > +
> > +static const VMStateDescription vmstate_tpm_crb_chunk = {
> > +    .name = "tpm-crb/chunk",
> > +    .version_id = 0,
> > +    .needed = tpm_crb_chunk_needed,
> > +    .post_load_errp = tpm_crb_chunk_post_load,
> > +    .fields = (const VMStateField[]) {
> > +        VMSTATE_GBYTEARRAY(command_buffer, CRBState, 0),
> > +        VMSTATE_GBYTEARRAY(response_buffer, CRBState, 0),
> > +        VMSTATE_UINT32(response_offset, CRBState),
> > +        VMSTATE_END_OF_LIST()
> > +    }
> > +};
> > +
> >   static const VMStateDescription vmstate_tpm_crb = {
> >       .name = "tpm-crb",
> >       .pre_save = tpm_crb_pre_save,
> >       .fields = (const VMStateField[]) {
> >           VMSTATE_UINT32_ARRAY(regs, CRBState, TPM_CRB_R_MAX),
> >           VMSTATE_END_OF_LIST(),
> > +    },
> > +    .subsections = (const VMStateDescription * const []) {
> > +        &vmstate_tpm_crb_chunk,
> > +        NULL,
> >       }
> >   };
> > 
> > @@ -366,6 +420,8 @@ static const Property tpm_crb_properties[] = {
> >       DEFINE_PROP_TPMBE("tpmdev", CRBState, tpmbe),
> >       DEFINE_PROP_BOOL("ppi", CRBState, ppi_enabled, true),
> >       DEFINE_PROP_BOOL("cap-chunk", CRBState, cap_chunk, true),
> > +    DEFINE_PROP_BOOL("x-allow-chunk-migration", CRBState,
> > +                     allow_chunk_migration, true),
> >   };
> > 
> >   static void tpm_crb_reset(void *dev)
> > @@ -422,6 +478,7 @@ static void tpm_crb_reset(void *dev)
> >   static void tpm_crb_realize(DeviceState *dev, Error **errp)
> >   {
> >       CRBState *s = CRB(dev);
> > +    int ret;
> > 
> >       if (!tpm_find()) {
> >           error_setg(errp, "at most one TPM device is permitted");
> > @@ -431,6 +488,15 @@ static void tpm_crb_realize(DeviceState *dev, Error **errp)
> >           error_setg(errp, "'tpmdev' property is required");
> >           return;
> >       }
> > +    if (s->cap_chunk && !s->allow_chunk_migration) {
> > +        error_setg(&s->migration_blocker,
> > +                   "The tpm-crb device does not support chunk migration with "
> > +                   "machine version less than 11.1");
> > +        ret = migrate_add_blocker_normal(&s->migration_blocker, errp);
> > +        if (ret < 0) {
> > +            return;
> > +        }
> > +    }
> > 
> >       memory_region_init_io(&s->mmio, OBJECT(s), &tpm_crb_memory_ops, s,
> >           "tpm-crb-mmio", sizeof(s->regs));
> > @@ -463,6 +529,11 @@ static void tpm_crb_unrealize(DeviceState *dev)
> > 
> >       g_clear_pointer(&s->command_buffer, g_byte_array_unref);
> >       g_clear_pointer(&s->response_buffer, g_byte_array_unref);
> > +
> > +    if (s->migration_blocker) {
> > +        migrate_del_blocker(&s->migration_blocker);
> > +        error_free(s->migration_blocker);
> > +    }
> >   }
> > 
> >   static void tpm_crb_class_init(ObjectClass *klass, const void *data)
> 

Regards,
Arun Menon