Re: [Qemu-devel] [Qemu-block] [PATCH] vmdk: Fix overflow if l1_size is 0x20000000

[Max Reitz <mreitz@redhat.com>]

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>