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SubjectRE: [PATCH 5/5] Drivers: hv: check VMBus messages lengths
Michael Kelley <> writes:

> From: Vitaly Kuznetsov <> Sent: Wednesday, April 1, 2020 3:38 AM
>> VMBus message handlers (channel_message_table) receive a pointer to
>> 'struct vmbus_channel_message_header' and cast it to a structure of their
>> choice, which is sometimes longer than the header. We, however, don't check
>> that the message is long enough so in case hypervisor screws up we'll be
>> accessing memory beyond what was allocated for temporary buffer.
>> Previously, we used to always allocate and copy 256 bytes from message page
>> to temporary buffer but this is hardly better: in case the message is
>> shorter than we expect we'll be trying to consume garbage as some real
>> data and no memory guarding technique will be able to identify an issue.
>> Introduce 'min_payload_len' to 'struct vmbus_channel_message_table_entry'
>> and check against it in vmbus_on_msg_dpc(). Note, we can't require the
>> exact length as new hypervisor versions may add extra fields to messages,
>> we only check that the message is not shorter than we expect.
> This assumes that the current structure definitions don't already
> include extra fields that were added in newer versions of Hyper-V. If they did,
> the minimum length test could fail on older versions of Hyper-V. But I
> looked through the structure definitions and don't see any indication that
> such extra fields have been added, so this should be OK.

Yes, my understanding as well. When we decide to extend some of these
structures for newer VMbus version we'll have a choice: keep the require
length minimal or implement a more somplex check (e.g. add a 'length
check' function pointer to vmbus_channel_message_table_entry() -- or
pass 'length' to all message handlers and handle it ther). But as we
currently have no such cases this will definitely look over-engineered
so I decided to go the easiest way.

>> Signed-off-by: Vitaly Kuznetsov <>
>> ---
>> drivers/hv/channel_mgmt.c | 54 ++++++++++++++++++++++-----------------
>> drivers/hv/hyperv_vmbus.h | 1 +
>> drivers/hv/vmbus_drv.c | 6 +++++
>> 3 files changed, 37 insertions(+), 24 deletions(-)
> Reviewed-by: Michael Kelley <>



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