Re: [Qemu-devel] [PATCH] fw_cfg: fix memory corruption when all fw_cfg slots are used

[Laszlo Ersek]

On 01/08/18 22:50, Marcel Apfelbaum wrote:
> When all the fw_cfg slots are used, a write is made outside the
> bounds of the fw_cfg files array as part of the sort algorithm.
> 
> Fix it by avoiding an unnecessary array element move.
> Fix also an assert while at it.
> 
> Signed-off-by: Marcel Apfelbaum <address@hidden>
> ---
>  hw/nvram/fw_cfg.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> index 753ac0e4ea..4313484b21 100644
> --- a/hw/nvram/fw_cfg.c
> +++ b/hw/nvram/fw_cfg.c
> @@ -784,7 +784,7 @@ void fw_cfg_add_file_callback(FWCfgState *s,  const char 
> *filename,
>       * index and "i - 1" is the one being copied from, thus the
>       * unusual start and end in the for statement.
>       */
> -    for (i = count + 1; i > index; i--) {
> +    for (i = count; i > index; i--) {
>          s->files->f[i] = s->files->f[i - 1];
>          s->files->f[i].select = cpu_to_be16(FW_CFG_FILE_FIRST + i);
>          s->entries[0][FW_CFG_FILE_FIRST + i] =

This hunk looks correct to me. We currently have count elements in the
array, so we cannot normally access the element *at* count. However, we
are extending the array right now, therefore we can assign (store) the
element at count (and then we'll increment count later). But accessing
an element at (count+1) is wrong.

> @@ -833,7 +833,6 @@ void *fw_cfg_modify_file(FWCfgState *s, const char 
> *filename,
>      assert(s->files);
>  
>      index = be32_to_cpu(s->files->count);
> -    assert(index < fw_cfg_file_slots(s));
>  
>      for (i = 0; i < index; i++) {
>          if (strcmp(filename, s->files->f[i].name) == 0) {
> @@ -843,6 +842,9 @@ void *fw_cfg_modify_file(FWCfgState *s, const char 
> *filename,
>              return ptr;
>          }
>      }
> +
> +    assert(index < fw_cfg_file_slots(s));
> +
>      /* add new one */
>      fw_cfg_add_file_callback(s, filename, NULL, NULL, NULL, data, len, true);
>      return NULL;
> 

I think I agree with Marc-André here, when I say, replace the assert
with a comment instead? (About the fact that fw_cfg_add_file_callback()
will assert(), *if* we reach that far.)

Thanks
Laszlo