vmdk: Fix overflow if l1_size is 0x20000000

Richard Jones caught this bug with afl fuzzer.

In fact, that's the only possible value to overflow (extent->l1_size =
0x20000000) l1_size:

l1_size = extent->l1_size * sizeof(long) => 0x80000000;

g_try_malloc returns NULL because l1_size is interpreted as negative
during type casting from 'int' to 'gsize', which yields a enormous
value. Hence, by coincidence, we get a "not too bad" behavior:

qemu-img: Could not open '/tmp/afl6.img': Could not open
'/tmp/afl6.img': Cannot allocate memory

Values larger than 0x20000000 will be refused by the validation in
vmdk_add_extent.

Values smaller than 0x20000000 will not overflow l1_size.

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
---
 block/vmdk.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

On Tue, May 05, 2015 at 05:28:13PM +0800, Fam Zheng wrote:
> Richard Jones caught this bug with afl fuzzer.
> 
> In fact, that's the only possible value to overflow (extent->l1_size =
> 0x20000000) l1_size:
> 
> l1_size = extent->l1_size * sizeof(long) => 0x80000000;
> 
> g_try_malloc returns NULL because l1_size is interpreted as negative
> during type casting from 'int' to 'gsize', which yields a enormous
> value. Hence, by coincidence, we get a "not too bad" behavior:
> 
> qemu-img: Could not open '/tmp/afl6.img': Could not open
> '/tmp/afl6.img': Cannot allocate memory
> 
> Values larger than 0x20000000 will be refused by the validation in
> vmdk_add_extent.
> 
> Values smaller than 0x20000000 will not overflow l1_size.
> 
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Signed-off-by: Fam Zheng <famz@redhat.com>

ACK, and:

Tested-by: Richard W.M. Jones <rjones@redhat.com>

Rich.

>  block/vmdk.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/block/vmdk.c b/block/vmdk.c
> index 1c5e2ef..e095156 100644
> --- a/block/vmdk.c
> +++ b/block/vmdk.c
> @@ -451,7 +451,8 @@ static int vmdk_init_tables(BlockDriverState *bs, VmdkExtent *extent,
>                              Error **errp)
>  {
>      int ret;
> -    int l1_size, i;
> +    size_t l1_size;
> +    int i;
>  
>      /* read the L1 table */
>      l1_size = extent->l1_size * sizeof(uint32_t);
> -- 
> 1.9.3

On 05.05.2015 11:28, Fam Zheng wrote:
> Richard Jones caught this bug with afl fuzzer.
>
> In fact, that's the only possible value to overflow (extent->l1_size =
> 0x20000000) l1_size:
>
> l1_size = extent->l1_size * sizeof(long) => 0x80000000;
>
> g_try_malloc returns NULL because l1_size is interpreted as negative
> during type casting from 'int' to 'gsize', which yields a enormous
> value. Hence, by coincidence, we get a "not too bad" behavior:
>
> qemu-img: Could not open '/tmp/afl6.img': Could not open
> '/tmp/afl6.img': Cannot allocate memory
>
> Values larger than 0x20000000 will be refused by the validation in
> vmdk_add_extent.
>
> Values smaller than 0x20000000 will not overflow l1_size.
>
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>   block/vmdk.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)

Okay, so because size_t is unsigned, this will not overflow on systems 
with sizeof(size_t) == 4 either.

Reviewed-by: Max Reitz <mreitz@redhat.com>

Am 06.05.2015 um 19:50 hat Max Reitz geschrieben:
> On 05.05.2015 11:28, Fam Zheng wrote:
> >Richard Jones caught this bug with afl fuzzer.
> >
> >In fact, that's the only possible value to overflow (extent->l1_size =
> >0x20000000) l1_size:
> >
> >l1_size = extent->l1_size * sizeof(long) => 0x80000000;
> >
> >g_try_malloc returns NULL because l1_size is interpreted as negative
> >during type casting from 'int' to 'gsize', which yields a enormous
> >value. Hence, by coincidence, we get a "not too bad" behavior:
> >
> >qemu-img: Could not open '/tmp/afl6.img': Could not open
> >'/tmp/afl6.img': Cannot allocate memory
> >
> >Values larger than 0x20000000 will be refused by the validation in
> >vmdk_add_extent.
> >
> >Values smaller than 0x20000000 will not overflow l1_size.
> >
> >Reported-by: Richard W.M. Jones <rjones@redhat.com>
> >Signed-off-by: Fam Zheng <famz@redhat.com>
> >---
> >  block/vmdk.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Okay, so because size_t is unsigned, this will not overflow on
> systems with sizeof(size_t) == 4 either.
> 
> Reviewed-by: Max Reitz <mreitz@redhat.com>

Thanks, applied to the block branch. I think this one is for stable,
too.

Kevin