Re: [Qemu-devel] [PATCH v6 wave 2 1/3] hw/isa/lpc_ich9: add SMI feature negotiation via fw_cfg

[Igor Mammedov]

On Thu, 12 Jan 2017 19:24:44 +0100
Laszlo Ersek <address@hidden> wrote:

> Introduce the following fw_cfg files:
> 
> - "etc/smi/supported-features": a little endian uint64_t feature bitmap,
>   presenting the features known by the host to the guest. Read-only for
>   the guest.
> 
>   The content of this file will be determined via bit-granularity ICH9-LPC
>   device properties, to be introduced later. For now, the bitmask is left
>   zeroed. The bits will be set from machine type compat properties and on
>   the QEMU command line, hence this file is not migrated.
> 
> - "etc/smi/requested-features": a little endian uint64_t feature bitmap,
>   representing the features the guest would like to request. Read-write
>   for the guest.
> 
>   The guest can freely (re)write this file, it has no direct consequence.
>   Initial value is zero. A nonzero value causes the SMI-related fw_cfg
>   files and fields that are under guest influence to be migrated.
> 
> - "etc/smi/features-ok": contains a uint8_t value, and it is read-only for
>   the guest. When the guest selects the associated fw_cfg key, the guest
>   features are validated against the host features. In case of error, the
>   negotiation doesn't proceed, and the "features-ok" file remains zero. In
>   case of success, the "features-ok" file becomes (uint8_t)1, and the
>   negotiated features are locked down internally (to which no further
>   changes are possible until reset).
> 
>   The initial value is zero.  A nonzero value causes the SMI-related
>   fw_cfg files and fields that are under guest influence to be migrated.
I'm still not quite sure if we need all this negotiation thingy with
all complexity it brings in when looking from cpu hotplug pov.

Paolo mentioned following security implications:
 1: OS could trigger broadcast SMI and try to hijack SMI handler for not
    yet relocated default SMI base of hotplugged CPU.
    That [c|sh]ould be handled by firmware protecting default SMI base.
 2: Even if firmware protected default SMI base, OS still could
    cause undefined behavior by sending broadcast SMI in case if more than
    1 CPU has been hotplugged, which would make unconfigured CPUs
    use the same SMI base simultaneously.
    Paolo suggested that real HW avoids the issue by making hotplugged CPUs
    "parked" until firmware unparks it from its SMI handler.
    So that's adds one more runtime state to migrate and qemu-guest ABI knob
    to maintain even if we ignore that there is no such terms as '(un)parked' 
in SDM.

How about considering an alternative simpler approach:
 * QEMU provides only "etc/smi/supported-features" file with SMI broadcast
   (no need to migrate)
 * firmware takes care of #1 by protecting default SMI base and using
   broadcast SMI if "etc/smi/supported-features" advertises it.
 * and QEMU could deal with #2 issue by just crashing guest as it tries
   to invoke undefined behavior (i.e. check that there is only 1 CPU with
   default SMI base and crash if there are more).
With this approach there is not need to negotiate nor migrate extra state
and inventing an unSPECed unpark knob for CPU hotplug could be avoided
(i.e. less qemu-guest ABI to maintain).


> 
> The C-language fields backing the "supported-features" and
> "requested-features" files are uint8_t arrays. This is because they carry
> guest-side representation (our choice is little endian), while
> VMSTATE_UINT64() assumes / implies host-side endianness for any uint64_t
> fields. If we migrate a guest between hosts with different endiannesses
> (which is possible with TCG), then the host-side value is preserved, and
> the host-side representation is translated. This would be visible to the
> guest through fw_cfg, unless we used plain byte arrays. So we do.
> 
> Cc: "Michael S. Tsirkin" <address@hidden>
> Cc: Gerd Hoffmann <address@hidden>
> Cc: Igor Mammedov <address@hidden>
> Cc: Paolo Bonzini <address@hidden>
> Signed-off-by: Laszlo Ersek <address@hidden>
> Reviewed-by: Michael S. Tsirkin <address@hidden>
> ---
> 
> Notes:
>     v6:
>     - no changes, pick up Michael's R-b
>     
>     v5:
>     - rename the "etc/smi/host-features" fw_cfg file to
>       "etc/smi/supported-features" [Igor]
>     
>     - rename the "etc/smi/guest-features" fw_cfg file to
>       "etc/smi/requested-features" [Igor]
>     
>     - suffix the names of the "ICH9LPCState.smi_host_features" and
>       "ICH9LPCState.smi_guest_features" array fields with "_le" for
>       representing their guest-visible encoding [Igor]
>     
>     - Replace the "smi_host_features" parameter of ich9_lpc_pm_init() --
>       which was meant in v4 to be set by  board code -- with a new
>       "ICH9LPCState.smi_host_features" field, of type uint64_t.
>       Bit-granularity ICH9-LPC device properties will be carved out of this
>       field. [Igor]
>     
>     - Given the "ICH9LPCState.smi_host_features" uint64_t field, we can now
>       use that directly for feature validation in
>       smi_features_ok_callback(). Converting the (otherwise guest-read-only)
>       "ICH9LPCState.smi_host_features_le" array back to CPU endianness just
>       for this is no longer necessary.
> 
>  include/hw/i386/ich9.h | 10 +++++++
>  hw/isa/lpc_ich9.c      | 79 
> ++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 89 insertions(+)
> 
> diff --git a/include/hw/i386/ich9.h b/include/hw/i386/ich9.h
> index 5fd7e97d2347..da1118727146 100644
> --- a/include/hw/i386/ich9.h
> +++ b/include/hw/i386/ich9.h
> @@ -64,6 +64,16 @@ typedef struct ICH9LPCState {
>      uint8_t rst_cnt;
>      MemoryRegion rst_cnt_mem;
>  
> +    /* SMI feature negotiation via fw_cfg */
> +    uint64_t smi_host_features;       /* guest-invisible, host endian */
> +    uint8_t smi_host_features_le[8];  /* guest-visible, read-only, little
> +                                       * endian uint64_t */
> +    uint8_t smi_guest_features_le[8]; /* guest-visible, read-write, little
> +                                       * endian uint64_t */
> +    uint8_t smi_features_ok;          /* guest-visible, read-only; selecting 
> it
> +                                       * triggers feature lockdown */
> +    uint64_t smi_negotiated_features; /* guest-invisible, host endian */
> +
>      /* isa bus */
>      ISABus *isa_bus;
>      MemoryRegion rcrb_mem; /* root complex register block */
> diff --git a/hw/isa/lpc_ich9.c b/hw/isa/lpc_ich9.c
> index 10d1ee8b9310..376b7801a42c 100644
> --- a/hw/isa/lpc_ich9.c
> +++ b/hw/isa/lpc_ich9.c
> @@ -48,6 +48,8 @@
>  #include "exec/address-spaces.h"
>  #include "sysemu/sysemu.h"
>  #include "qom/cpu.h"
> +#include "hw/nvram/fw_cfg.h"
> +#include "qemu/cutils.h"
>  
>  
> /*****************************************************************************/
>  /* ICH9 LPC PCI to ISA bridge */
> @@ -360,13 +362,62 @@ static void ich9_set_sci(void *opaque, int irq_num, int 
> level)
>      }
>  }
>  
> +static void smi_features_ok_callback(void *opaque)
> +{
> +    ICH9LPCState *lpc = opaque;
> +    uint64_t guest_features;
> +
> +    if (lpc->smi_features_ok) {
> +        /* negotiation already complete, features locked */
> +        return;
> +    }
> +
> +    memcpy(&guest_features, lpc->smi_guest_features_le, sizeof 
> guest_features);
> +    le64_to_cpus(&guest_features);
> +    if (guest_features & ~lpc->smi_host_features) {
> +        /* guest requests invalid features, leave @features_ok at zero */
> +        return;
> +    }
> +
> +    /* valid feature subset requested, lock it down, report success */
> +    lpc->smi_negotiated_features = guest_features;
> +    lpc->smi_features_ok = 1;
> +}
> +
>  void ich9_lpc_pm_init(PCIDevice *lpc_pci, bool smm_enabled)
>  {
>      ICH9LPCState *lpc = ICH9_LPC_DEVICE(lpc_pci);
>      qemu_irq sci_irq;
> +    FWCfgState *fw_cfg = fw_cfg_find();
>  
>      sci_irq = qemu_allocate_irq(ich9_set_sci, lpc, 0);
>      ich9_pm_init(lpc_pci, &lpc->pm, smm_enabled, sci_irq);
> +
> +    if (lpc->smi_host_features && fw_cfg) {
> +        uint64_t host_features_le;
> +
> +        host_features_le = cpu_to_le64(lpc->smi_host_features);
> +        memcpy(lpc->smi_host_features_le, &host_features_le,
> +               sizeof host_features_le);
> +        fw_cfg_add_file(fw_cfg, "etc/smi/supported-features",
> +                        lpc->smi_host_features_le,
> +                        sizeof lpc->smi_host_features_le);
> +
> +        /* The other two guest-visible fields are cleared on device reset, we
> +         * just link them into fw_cfg here.
> +         */
> +        fw_cfg_add_file_callback(fw_cfg, "etc/smi/requested-features",
> +                                 NULL, NULL,
> +                                 lpc->smi_guest_features_le,
> +                                 sizeof lpc->smi_guest_features_le,
> +                                 false);
> +        fw_cfg_add_file_callback(fw_cfg, "etc/smi/features-ok",
> +                                 smi_features_ok_callback, lpc,
> +                                 &lpc->smi_features_ok,
> +                                 sizeof lpc->smi_features_ok,
> +                                 true);
> +    }
> +
>      ich9_lpc_reset(&lpc->d.qdev);
>  }
>  
> @@ -507,6 +558,10 @@ static void ich9_lpc_reset(DeviceState *qdev)
>  
>      lpc->sci_level = 0;
>      lpc->rst_cnt = 0;
> +
> +    memset(lpc->smi_guest_features_le, 0, sizeof lpc->smi_guest_features_le);
> +    lpc->smi_features_ok = 0;
> +    lpc->smi_negotiated_features = 0;
>  }
>  
>  /* root complex register block is mapped into memory space */
> @@ -668,6 +723,29 @@ static const VMStateDescription vmstate_ich9_rst_cnt = {
>      }
>  };
>  
> +static bool ich9_smi_feat_needed(void *opaque)
> +{
> +    ICH9LPCState *lpc = opaque;
> +
> +    return !buffer_is_zero(lpc->smi_guest_features_le,
> +                           sizeof lpc->smi_guest_features_le) ||
> +           lpc->smi_features_ok;
> +}
> +
> +static const VMStateDescription vmstate_ich9_smi_feat = {
> +    .name = "ICH9LPC/smi_feat",
> +    .version_id = 1,
> +    .minimum_version_id = 1,
> +    .needed = ich9_smi_feat_needed,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_UINT8_ARRAY(smi_guest_features_le, ICH9LPCState,
> +                            sizeof(uint64_t)),
> +        VMSTATE_UINT8(smi_features_ok, ICH9LPCState),
> +        VMSTATE_UINT64(smi_negotiated_features, ICH9LPCState),
> +        VMSTATE_END_OF_LIST()
> +    }
> +};
> +
>  static const VMStateDescription vmstate_ich9_lpc = {
>      .name = "ICH9LPC",
>      .version_id = 1,
> @@ -683,6 +761,7 @@ static const VMStateDescription vmstate_ich9_lpc = {
>      },
>      .subsections = (const VMStateDescription*[]) {
>          &vmstate_ich9_rst_cnt,
> +        &vmstate_ich9_smi_feat,
>          NULL
>      }
>  };