Re: [PATCH] 9p/net: put a lower bound on msize

[Dominique Martinet]

Dominique Martinet wrote on Mon, Nov 05, 2018:
> From: Dominique Martinet <dominique.martinet@xxxxxx>
> 
> If the requested msize is too small (either from command line argument
> or from the server version reply), we won't get any work done.
> If it's *really* too small, nothing will work, and this got caught by
> syzbot recently (on a new kmem_cache_create_usercopy() call)
> 
> Just set a minimum msize to 4k in both code paths, until someone
> complains they have a use-case for a smaller msize.
> 
> We need to check in both mount option and server reply individually
> because the msize for the first version request would be unchecked
> with just a global check on clnt->msize.
> 
> Reported-by: syzbot+0c1d61e4db7db94102ca@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Dominique Martinet <dominique.martinet@xxxxxx>
> Cc: Eric Van Hensbergen <ericvh@xxxxxxxxx>
> Cc: Latchesar Ionkov <lucho@xxxxxxxxxx>
> ---
>  net/9p/client.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/net/9p/client.c b/net/9p/client.c
> index 2c9a17b9b46b..b1163ebdc622 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -181,6 +181,12 @@ static int parse_opts(char *opts, struct p9_client *clnt)
>  				ret = r;
>  				continue;
>  			}
> +			if (r < 4096) {

This should read if (option < 4096), amending what I've put in for -next
last week :(

Reviews wanted!


> +				p9_debug(P9_DEBUG_ERROR,
> +					 "msize should be at least 4k\n");
> +				ret = -EINVAL;
> +				continue;
> +			}
>  			clnt->msize = option;
>  			break;
>  		case Opt_trans:
> @@ -983,10 +989,18 @@ static int p9_client_version(struct p9_client *c)
>  	else if (!strncmp(version, "9P2000", 6))
>  		c->proto_version = p9_proto_legacy;
>  	else {
> +		p9_debug(P9_DEBUG_ERROR,
> +			 "server returned an unknown version: %s\n", version);
>  		err = -EREMOTEIO;
>  		goto error;
>  	}
>  
> +	if (msize < 4096) {
> +		p9_debug(P9_DEBUG_ERROR,
> +			 "server returned a msize < 4096: %d\n", msize);
> +		err = -EREMOTEIO;
> +		goto error;
> +	}
>  	if (msize < c->msize)
>  		c->msize = msize;
>  
> @@ -1043,6 +1057,13 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
>  	if (clnt->msize > clnt->trans_mod->maxsize)
>  		clnt->msize = clnt->trans_mod->maxsize;
>  
> +	if (clnt->msize < 4096) {
> +		p9_debug(P9_DEBUG_ERROR,
> +			 "Please specify a msize of at least 4k\n");
> +		err = -EINVAL;
> +		goto free_client;
> +	}
> +
>  	err = p9_client_version(clnt);
>  	if (err)
>  		goto close_trans;

-- 
Dominique