Re: [PATCH v3 03/11] 9pfs: validate count sent by client with T_readdir

[Greg Kurz]

On Mon, 13 Jan 2020 23:22:08 +0100
Christian Schoenebeck <address@hidden> wrote:

> A good 9p client sends T_readdir with "count" parameter that's
> sufficiently smaller than client's initially negotiated msize
> (maximum message size). We perform a check for that though to
> avoid the server to be interrupted with a "Failed to encode
> VirtFS reply type 41" error message by bad clients.
> 
> Signed-off-by: Christian Schoenebeck <address@hidden>
> ---
>  hw/9pfs/9p.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index a5fbe821d4..30da2fedf3 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -2426,6 +2426,7 @@ static void coroutine_fn v9fs_readdir(void *opaque)
>      int32_t count;
>      uint32_t max_count;
>      V9fsPDU *pdu = opaque;
> +    V9fsState *s = pdu->s;
>  
>      retval = pdu_unmarshal(pdu, offset, "dqd", &fid,
>                             &initial_offset, &max_count);
> @@ -2434,6 +2435,13 @@ static void coroutine_fn v9fs_readdir(void *opaque)
>      }
>      trace_v9fs_readdir(pdu->tag, pdu->id, fid, initial_offset, max_count);
>  
> +    if (max_count > s->msize - P9_IOHDRSZ) {

P9_IOHDRSZ relates to Twrite. The Rreaddir message has a smaller header
of size 11:

size[4] Rreaddir tag[2] count[4]

> +        max_count = s->msize - P9_IOHDRSZ;
> +        warn_report_once(
> +            "9p: bad client: T_readdir with count > msize - P9_IOHDRSZ"
> +        );
> +    }
> +
>      fidp = get_fid(pdu, fid);
>      if (fidp == NULL) {
>          retval = -EINVAL;