Re: [Qemu-devel] [Qemu-block] [PATCH 1/2] block: vpc - prevent overflow if max_table_entries >= 0x40000000

[Jeff Cody]

On Thu, Jun 25, 2015 at 03:28:35PM +0100, Stefan Hajnoczi wrote:
> On Wed, Jun 24, 2015 at 03:54:27PM -0400, Jeff Cody wrote:
> > @@ -269,7 +270,9 @@ static int vpc_open(BlockDriverState *bs, QDict 
> > *options, int flags,
> >              goto fail;
> >          }
> >  
> > -        s->pagetable = qemu_try_blockalign(bs->file, s->max_table_entries 
> > * 4);
> > +        pagetable_size = (size_t) s->max_table_entries * 4;
> > +
> > +        s->pagetable = qemu_try_blockalign(bs->file, pagetable_size);
> 
> On 32-bit hosts size_t is 32-bit so the overflow hasn't been solved.
> 
> Does it make sense to impose a limit on pagetable_size?

Good point.  Yes, it does.

The VHD spec says that the "Max Table Entries" field should be equal
to the disk size / block size.  I don't know if there are images out
there that treat that as ">= disk size / block size" rather than "==",
however.  But if we assume max size of 2TB for a VHD disk, and a
minimal block size of 512 bytes, that would give us a
max_table_entries of 0x100000000, which exceeds 32 bits by itself.

For pagetable_size to fit in a 32-bit, that means to support 2TB on a
32-bit host in the current implementation, the minimum block size is
4096.

We could check during open / create that 
(disk_size / block_size) * 4 < SIZE_MAX, and refuse to open if this is
not true (and also validate max_table_entries to fit in the same).

It should be noted that the default (per spec) block size for VHD is
2MB, so it may not be too limiting in practice for a 32-bit host to
not be able to open all image sizes / block size combinations.