Re: [Qemu-devel] [PATCH] qcow2: avoid memcpy(dst, NULL, len)

[Paolo Bonzini]

On 13/09/2016 11:15, Stefan Hajnoczi wrote:
> Section "7.1.4 Use of library functions" in the C99 standard says:
> 
>   If an argument to a function has an invalid value (such as [...]
>   a null pointer [...]) [...] the behavior is undefined.
> 
> Additionally the "searching and sorting" functions are specified as
> requiring valid pointer values as described in 7.1.4.
> 
> This patch fixes the following static analyzer errors:
> 
>   block/qcow2.c:1807:41: runtime error: null pointer passed as argument 2, 
> which is declared to never be null
>   block/qcow2-cluster.c:86:26: runtime error: null pointer passed as argument 
> 2, which is declared to never be null
> 
> Reported-by: Peter Maydell <address@hidden>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  block/qcow2-cluster.c | 4 +++-
>  block/qcow2.c         | 5 ++++-
>  2 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index f941835..ab0dcdc 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -83,7 +83,9 @@ int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t 
> min_size,
>      }
>      memset(new_l1_table, 0, align_offset(new_l1_size2, 512));
>  
> -    memcpy(new_l1_table, s->l1_table, s->l1_size * sizeof(uint64_t));
> +    if (s->l1_table) {
> +        memcpy(new_l1_table, s->l1_table, s->l1_size * sizeof(uint64_t));
> +    }
>  
>      /* write new table (align to cluster) */
>      BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ALLOC_TABLE);
> diff --git a/block/qcow2.c b/block/qcow2.c
> index c079aa8..758a997 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1804,7 +1804,10 @@ static size_t header_ext_add(char *buf, uint32_t 
> magic, const void *s,
>          .magic  = cpu_to_be32(magic),
>          .len    = cpu_to_be32(len),
>      };
> -    memcpy(buf + sizeof(QCowExtension), s, len);
> +
> +    if (s) {

Should they be "if (len)" and "if (s->l1_size)"?

I think a SIGSEGV is the right thing to do if s->l1_table is NULL but
s->l1_size is non-zero, or if s is NULL but len is non-zero.

Paolo

> +        memcpy(buf + sizeof(QCowExtension), s, len);
> +    }
>  
>      return ext_len;
>  }
>