Linux Virtualization - Jun 2020 - [PATCH] virtio_mem: prevent overflow with subblock size

On Mon, Jun 08, 2020 at 08:58:31AM +0200, David Hildenbrand
wrote:
> On 08.06.20 08:14, Michael S. Tsirkin wrote:
> > If subblock size is large (e.g. 1G) 32 bit math involving it
> > can overflow. Rather than try to catch all instances of that,
> > let's tweak block size to 64 bit.
> 
> I fail to see where we could actually trigger an overflow. The reported
> warning looked like a false positive to me.

So

    const uint64_t size = count * vm->subblock_size;

is it unreasonable for count to be 4K with subblock_size being 1M?
> > 
> > It ripples through UAPI which is an ABI change, but it's not too
late to
> > make it, and it will allow supporting >4Gbyte blocks while might
> > become necessary down the road.
> > 
> 
> This might break cloud-hypervisor, who's already implementing this
> protocol upstream (ccing Hui).
>
https://github.com/cloud-hypervisor/cloud-hypervisor/blob/master/vm-virtio/src/mem.rs
> 
> (blocks in the gigabyte range were never the original intention of
> virtio-mem, but I am not completely opposed to that)

So in that case, can you code up validation in the probe function?

> > Fixes: 5f1f79bbc9e26 ("virtio-mem: Paravirtualized memory
hotplug")
> > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> > ---
> >  drivers/virtio/virtio_mem.c     | 14 +++++++-------
> >  include/uapi/linux/virtio_mem.h |  4 ++--
> >  2 files changed, 9 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
> > index 2f357142ea5e..7b1bece8a331 100644
> > --- a/drivers/virtio/virtio_mem.c
> > +++ b/drivers/virtio/virtio_mem.c
> > @@ -77,7 +77,7 @@ struct virtio_mem {
> >  	uint64_t requested_size;
> >  
> >  	/* The device block size (for communicating with the device). */
> > -	uint32_t device_block_size;
> > +	uint64_t device_block_size;
> >  	/* The translated node id. NUMA_NO_NODE in case not specified. */
> >  	int nid;
> >  	/* Physical start address of the memory region. */
> > @@ -86,7 +86,7 @@ struct virtio_mem {
> >  	uint64_t region_size;
> >  
> >  	/* The subblock size. */
> > -	uint32_t subblock_size;
> > +	uint64_t subblock_size;
> >  	/* The number of subblocks per memory block. */
> >  	uint32_t nb_sb_per_mb;
> >  
> > @@ -1698,9 +1698,9 @@ static int virtio_mem_init(struct virtio_mem
*vm)
> >  	 * - At least the device block size.
> >  	 * In the worst case, a single subblock per memory block.
> >  	 */
> > -	vm->subblock_size = PAGE_SIZE * 1u << max_t(uint32_t,
MAX_ORDER - 1,
> > -						    pageblock_order);
> > -	vm->subblock_size = max_t(uint32_t, vm->device_block_size,
> > +	vm->subblock_size = PAGE_SIZE * 1ul << max_t(uint32_t,
MAX_ORDER - 1,
> > +						     pageblock_order);
> > +	vm->subblock_size = max_t(uint64_t, vm->device_block_size,
> >  				  vm->subblock_size);
> >  	vm->nb_sb_per_mb = memory_block_size_bytes() /
vm->subblock_size;
> >  
> > @@ -1713,8 +1713,8 @@ static int virtio_mem_init(struct virtio_mem
*vm)
> >  
> >  	dev_info(&vm->vdev->dev, "start address:
0x%llx", vm->addr);
> >  	dev_info(&vm->vdev->dev, "region size: 0x%llx",
vm->region_size);
> > -	dev_info(&vm->vdev->dev, "device block size:
0x%x",
> > -		 vm->device_block_size);
> > +	dev_info(&vm->vdev->dev, "device block size:
0x%llx",
> > +		 (unsigned long long)vm->device_block_size);
> >  	dev_info(&vm->vdev->dev, "memory block size:
0x%lx",
> >  		 memory_block_size_bytes());
> >  	dev_info(&vm->vdev->dev, "subblock size: 0x%x",
> > diff --git a/include/uapi/linux/virtio_mem.h
b/include/uapi/linux/virtio_mem.h
> > index a455c488a995..a9ffe041843c 100644
> > --- a/include/uapi/linux/virtio_mem.h
> > +++ b/include/uapi/linux/virtio_mem.h
> > @@ -185,10 +185,10 @@ struct virtio_mem_resp {
> >  
> >  struct virtio_mem_config {
> >  	/* Block size and alignment. Cannot change. */
> > -	__u32 block_size;
> > +	__u64 block_size;
> >  	/* Valid with VIRTIO_MEM_F_ACPI_PXM. Cannot change. */
> >  	__u16 node_id;
> > -	__u16 padding;
> > +	__u8 padding[6];
> >  	/* Start address of the memory region. Cannot change. */
> >  	__u64 addr;
> >  	/* Region size (maximum). Cannot change. */
> > 
> 
> 
> -- 
> Thanks,
> 
> David / dhildenb

On 08.06.20 09:08, Michael S. Tsirkin wrote:
> On Mon, Jun 08, 2020 at 08:58:31AM +0200, David Hildenbrand wrote:
>> On 08.06.20 08:14, Michael S. Tsirkin wrote:
>>> If subblock size is large (e.g. 1G) 32 bit math involving it
>>> can overflow. Rather than try to catch all instances of that,
>>> let's tweak block size to 64 bit.
>>
>> I fail to see where we could actually trigger an overflow. The reported
>> warning looked like a false positive to me.
> 
> 
> So
> 
>     const uint64_t size = count * vm->subblock_size;
> 
> is it unreasonable for count to be 4K with subblock_size being 1M?
virtio_mem_mb_plug_sb() and friends are only called on subblocks
residing within a single Linux memory block. (currently, 128MB .. 2G on
x86-64). A subblock on x86-64 is currently at least 4MB.

So "count * vm->subblock_size" can currently not exceed the Linux
memory
block size (in practice, it is max 128MB).
> 
>>>
>>> It ripples through UAPI which is an ABI change, but it's not
too late to
>>> make it, and it will allow supporting >4Gbyte blocks while might
>>> become necessary down the road.
>>>
>>
>> This might break cloud-hypervisor, who's already implementing this
>> protocol upstream (ccing Hui).
>>
https://github.com/cloud-hypervisor/cloud-hypervisor/blob/master/vm-virtio/src/mem.rs
>>
>> (blocks in the gigabyte range were never the original intention of
>> virtio-mem, but I am not completely opposed to that)
> 
> 
> So in that case, can you code up validation in the probe function?
If we would currently have a "block_size" > Linux memory block
size, we
bail out.

virtio_mem_init():

if (vm->device_block_size > memory_block_size_bytes()) {
	dev_err(&vm->vdev->dev,
		"The block size is not supported (too big).\n");
	return -EINVAL;
}

So what's reported can currently not happen. Having that said, changing
"subblock_size" to be an uint64_t is a good cleanup, especially for
the
future.




-- 
Thanks,

David / dhildenb

On Mon, Jun 08, 2020 at 09:17:45AM +0200, David Hildenbrand
wrote:
> On 08.06.20 09:08, Michael S. Tsirkin wrote:
> > On Mon, Jun 08, 2020 at 08:58:31AM +0200, David Hildenbrand wrote:
> >> On 08.06.20 08:14, Michael S. Tsirkin wrote:
> >>> If subblock size is large (e.g. 1G) 32 bit math involving it
> >>> can overflow. Rather than try to catch all instances of that,
> >>> let's tweak block size to 64 bit.
> >>
> >> I fail to see where we could actually trigger an overflow. The
reported
> >> warning looked like a false positive to me.
> > 
> > 
> > So
> > 
> >     const uint64_t size = count * vm->subblock_size;
> > 
> > is it unreasonable for count to be 4K with subblock_size being 1M?
> 
> virtio_mem_mb_plug_sb() and friends are only called on subblocks
> residing within a single Linux memory block. (currently, 128MB .. 2G on
> x86-64). A subblock on x86-64 is currently at least 4MB.
> 
> So "count * vm->subblock_size" can currently not exceed the
Linux memory
> block size (in practice, it is max 128MB).
> 
> > 
> >>>
> >>> It ripples through UAPI which is an ABI change, but it's
not too late to
> >>> make it, and it will allow supporting >4Gbyte blocks while
might
> >>> become necessary down the road.
> >>>
> >>
> >> This might break cloud-hypervisor, who's already implementing
this
> >> protocol upstream (ccing Hui).
> >>
https://github.com/cloud-hypervisor/cloud-hypervisor/blob/master/vm-virtio/src/mem.rs
> >>
> >> (blocks in the gigabyte range were never the original intention of
> >> virtio-mem, but I am not completely opposed to that)
> > 
> > 
> > So in that case, can you code up validation in the probe function?
> 
> If we would currently have a "block_size" > Linux memory block
size, we
> bail out.
> 
> virtio_mem_init():
> 
> if (vm->device_block_size > memory_block_size_bytes()) {
> 	dev_err(&vm->vdev->dev,
> 		"The block size is not supported (too big).\n");
> 	return -EINVAL;
> }
Sounds good.
> So what's reported can currently not happen. Having that said, changing
> "subblock_size" to be an uint64_t is a good cleanup, especially
for the
> future.
OK, no need to argue about it then. I tweaked the subject as you
suggested and queued it then.
> 
> 
> 
> -- 
> Thanks,
> 
> David / dhildenb