Re: [PATCH v2 1/3] Drivers: hv: vmbus: Add vmbus_requestor data structure for VMBus hardening

[Andres Beltran]

On Mon, Jun 29, 2020 at 6:20 PM Wei Liu <wei.liu@xxxxxxxxxx> wrote:
>
> On Mon, Jun 29, 2020 at 05:51:05PM -0400, Andres Beltran wrote:
> > On Mon, Jun 29, 2020 at 4:46 PM Wei Liu <wei.liu@xxxxxxxxxx> wrote:
> > >
> > > On Mon, Jun 29, 2020 at 04:02:25PM -0400, Andres Beltran wrote:
> > > > Currently, VMbus drivers use pointers into guest memory as request IDs
> > > > for interactions with Hyper-V. To be more robust in the face of errors
> > > > or malicious behavior from a compromised Hyper-V, avoid exposing
> > > > guest memory addresses to Hyper-V. Also avoid Hyper-V giving back a
> > > > bad request ID that is then treated as the address of a guest data
> > > > structure with no validation. Instead, encapsulate these memory
> > > > addresses and provide small integers as request IDs.
> > > >
> > > > Signed-off-by: Andres Beltran <lkmlabelt@xxxxxxxxx>
> > > > ---
> > > > Changes in v2:
> > > >       - Get rid of "rqstor" variable in __vmbus_open().
> > > >
> > > >  drivers/hv/channel.c   | 146 +++++++++++++++++++++++++++++++++++++++++
> > > >  include/linux/hyperv.h |  21 ++++++
> > > >  2 files changed, 167 insertions(+)
> > > >
> > > > diff --git a/drivers/hv/channel.c b/drivers/hv/channel.c
> > > > index 3ebda7707e46..c89d57d0c2d2 100644
> > > > --- a/drivers/hv/channel.c
> > > > +++ b/drivers/hv/channel.c
> > > > @@ -112,6 +112,70 @@ int vmbus_alloc_ring(struct vmbus_channel *newchannel,
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(vmbus_alloc_ring);
> > > >
> > > > +/**
> > > > + * request_arr_init - Allocates memory for the requestor array. Each slot
> > > > + * keeps track of the next available slot in the array. Initially, each
> > > > + * slot points to the next one (as in a Linked List). The last slot
> > > > + * does not point to anything, so its value is U64_MAX by default.
> > > > + * @size The size of the array
> > > > + */
> > > > +static u64 *request_arr_init(u32 size)
> > > > +{
> > > > +     int i;
> > > > +     u64 *req_arr;
> > > > +
> > > > +     req_arr = kcalloc(size, sizeof(u64), GFP_KERNEL);
> > > > +     if (!req_arr)
> > > > +             return NULL;
> > > > +
> > > > +     for (i = 0; i < size - 1; i++)
> > > > +             req_arr[i] = i + 1;
> > > > +
> > > > +     /* Last slot (no more available slots) */
> > > > +     req_arr[i] = U64_MAX;
> > > > +
> > > > +     return req_arr;
> > > > +}
> > > > +
> > > > +/*
> > > > + * vmbus_alloc_requestor - Initializes @rqstor's fields.
> > > > + * Slot at index 0 is the first free slot.
> > > > + * @size: Size of the requestor array
> > > > + */
> > > > +static int vmbus_alloc_requestor(struct vmbus_requestor *rqstor, u32 size)
> > > > +{
> > > > +     u64 *rqst_arr;
> > > > +     unsigned long *bitmap;
> > > > +
> > > > +     rqst_arr = request_arr_init(size);
> > > > +     if (!rqst_arr)
> > > > +             return -ENOMEM;
> > > > +
> > > > +     bitmap = bitmap_zalloc(size, GFP_KERNEL);
> > > > +     if (!bitmap) {
> > > > +             kfree(rqst_arr);
> > > > +             return -ENOMEM;
> > > > +     }
> > > > +
> > > > +     rqstor->req_arr = rqst_arr;
> > > > +     rqstor->req_bitmap = bitmap;
> > > > +     rqstor->size = size;
> > > > +     rqstor->next_request_id = 0;
> > > > +     spin_lock_init(&rqstor->req_lock);
> > > > +
> > > > +     return 0;
> > > > +}
> > > > +
> > > > +/*
> > > > + * vmbus_free_requestor - Frees memory allocated for @rqstor
> > > > + * @rqstor: Pointer to the requestor struct
> > > > + */
> > > > +static void vmbus_free_requestor(struct vmbus_requestor *rqstor)
> > > > +{
> > > > +     kfree(rqstor->req_arr);
> > > > +     bitmap_free(rqstor->req_bitmap);
> > > > +}
> > > > +
> > > >  static int __vmbus_open(struct vmbus_channel *newchannel,
> > > >                      void *userdata, u32 userdatalen,
> > > >                      void (*onchannelcallback)(void *context), void *context)
> > > > @@ -132,6 +196,12 @@ static int __vmbus_open(struct vmbus_channel *newchannel,
> > > >       if (newchannel->state != CHANNEL_OPEN_STATE)
> > > >               return -EINVAL;
> > > >
> > > > +     /* Create and init requestor */
> > > > +     if (newchannel->rqstor_size) {
> > > > +             if (vmbus_alloc_requestor(&newchannel->requestor, newchannel->rqstor_size))
> > > > +                     return -ENOMEM;
> > > > +     }
> > > > +
> > >
> > > Sorry for not noticing this in the last round: this infrastructure is
> > > initialized conditionally but used unconditionally.
> > >
> > > I can think of two options here:
> > >
> > >   1. Mandate rqstor_size to be non-zero. Always initialize this
> > >      infra.
> > >   2. Modify vmbus_next_request_id and vmbus_request_addr to deal with
> > >      uninitialized state.
> > >
> > > For #2, you can simply check rqstor->size _before_ taking the lock
> > > (because it may be uninitialized, and the assumption is ->size will not
> > > change during the channel's lifetime, hence no lock is needed) and
> > > simply return the same value to the caller.
> > >
> > > Wei.
> >
> > Right. I think option #2 would be preferable in this case, because #1 works
> > if we had a default non-zero size for cases where rqstor_size has not been
> > set to a non-zero value before calling vmbus_alloc_requestor(). For #2, what
> > do you mean by "same value"? I think we would need to return
> > VMBUS_RQST_ERROR if the size is 0, because otherwise we would be
> > returning the same guest memory address which we don't want to expose.
>
> By "same value", I meant reverting back to using guest memory address.
> I thought downgrading gracefully is better than making the driver stop
> working.
>
> If exposing guest address is not acceptable, you can return
> VMBUS_RQST_ERROR -- but at the point you may as well mandate requestor
> infrastructure to be always initialized, right?
>

If the allocation of the requestor fails during runtime, vmbus_open()
fails too and therefore,
the channel and the requestor will not be created. So, the 2 functions
(next_id, requestor_addr)
will never get called, right? The only case in which we hit this edge
case is if a driver is using this
mechanism with a size of 0 (i.e. rqstor_size is not set to a non-zero
value before calling vmbus_open()),
but that would be more like a coding bug. So, I think it would be
better to return VMBUS_RQST_ERROR
as a way to assert that there is a bug in the code. I don't know if
I'm missing something here.

Andres.