Subject: Re: [syzbot] KASAN: use-after-free Read in p9_req_put

From: Christian Schoenebeck <[email-masked]>
Date: Thu, 18 Aug 2022 17:12:17 +0200

On Mittwoch, 17. August 2022 07:59:47 CEST [email-masked] wrote:
> syzbot having a fresh look at 9p?
> 
> Well at least that one should be easy enough, the following (untested)
> probably should work around that issue:
> 
> -----
> From 433138e5d36a5b29b46b043c542e14b9dc908460 Mon Sep 17 00:00:00 2001
> From: Dominique Martinet <[email-masked]>
> Date: Wed, 17 Aug 2022 14:49:29 +0900
> Subject: [PATCH] 9p: p9_client_create: use p9_client_destroy on failure
> 
> If trans was connected it's somehow possible to fail with requests in
> flight that could still be accessed after free if we just free the clnt
> on failure.
> Just use p9_client_destroy instead that has proper safeguards.
> 
> Reported-by: [email-masked]
> Signed-off-by: Dominique Martinet <[email-masked]>
> 
> diff --git a/net/9p/client.c b/net/9p/client.c
> index 5bf4dfef0c70..da5d43848600 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -948,7 +948,7 @@ struct p9_client *p9_client_create(const char *dev_name,
> char *options)
> 
>  	err = parse_opts(options, clnt);
>  	if (err < 0)
> -		goto free_client;
> +		goto out;
> 
>  	if (!clnt->trans_mod)
>  		clnt->trans_mod = v9fs_get_default_trans();
> @@ -957,7 +957,7 @@ struct p9_client *p9_client_create(const char *dev_name,
> char *options) err = -EPROTONOSUPPORT;
>  		p9_debug(P9_DEBUG_ERROR,
>  			 "No transport defined or default transport\n");
> -		goto free_client;
> +		goto out;
>  	}
> 
>  	p9_debug(P9_DEBUG_MUX, "clnt %p trans %p msize %d protocol %d\n",
> @@ -965,7 +965,7 @@ struct p9_client *p9_client_create(const char *dev_name,
> char *options)
> 
>  	err = clnt->trans_mod->create(clnt, dev_name, options);
>  	if (err)
> -		goto put_trans;
> +		goto out;
> 
>  	if (clnt->msize > clnt->trans_mod->maxsize) {
>  		clnt->msize = clnt->trans_mod->maxsize;
> @@ -979,12 +979,12 @@ struct p9_client *p9_client_create(const char
> *dev_name, char *options) p9_debug(P9_DEBUG_ERROR,
>  			 "Please specify a msize of at least 4k\n");
>  		err = -EINVAL;
> -		goto close_trans;
> +		goto out;
>  	}
> 
>  	err = p9_client_version(clnt);
>  	if (err)
> -		goto close_trans;
> +		goto out;
> 
>  	/* P9_HDRSZ + 4 is the smallest packet header we can have that is
>  	 * followed by data accessed from userspace by read
> @@ -997,12 +997,8 @@ struct p9_client *p9_client_create(const char
> *dev_name, char *options)
> 
>  	return clnt;
> 
> -close_trans:
> -	clnt->trans_mod->close(clnt);
> -put_trans:
> -	v9fs_put_trans(clnt->trans_mod);
> -free_client:
> -	kfree(clnt);
> +out:
> +	p9_client_destroy(clnt);
>  	return ERR_PTR(err);
>  }
>  EXPORT_SYMBOL(p9_client_create);

Looks like a nice reduction to me!

As p9_client_destroy() is doing a bit more than current code, I would probably 
additionally do s/kmalloc/kzmalloc/ at the start of the function, which would 
add more safety & reduction.

> -----
> 
> I'll test and submit to Linus over the next few weeks.
> 
> I had a quick look at the other new syzbot warnings and:
>  - 'possible deadlock in p9_req_put' is clear enough, we can just drop
> the lock before running through the cancel list and I don't think
> that'll cause any problem as everything has been moved to a local list
> and that lock is abused by trans fd for its local stuff. I'll also send
> that after quick testing.
> ----
> From c46435a4af7c119bd040922886ed2ea3a2a842d7 Mon Sep 17 00:00:00 2001
> From: Dominique Martinet <[email-masked]>
> Date: Wed, 17 Aug 2022 14:58:44 +0900
> Subject: [PATCH] 9p: trans_fd/p9_conn_cancel: drop client lock earlier
> 
> syzbot reported a double-lock here and we no longer need this
> lock after requests have been moved off to local list:
> just drop the lock earlier.
> 
> Reported-by: [email-masked]
> Signed-off-by: Dominique Martinet <[email-masked]>
> 
> diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
> index e758978b44be..60fcc6b30b46 100644
> --- a/net/9p/trans_fd.c
> +++ b/net/9p/trans_fd.c
> @@ -205,6 +205,8 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
>  		list_move(&req->req_list, &cancel_list);
>  	}
> 
> +	spin_unlock(&m->client->lock);
> +
>  	list_for_each_entry_safe(req, rtmp, &cancel_list, req_list) {
>  		p9_debug(P9_DEBUG_ERROR, "call back req %p\n", req);
>  		list_del(&req->req_list);
> @@ -212,7 +214,6 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
>  			req->t_err = err;
>  		p9_client_cb(m->client, req, REQ_STATUS_ERROR);
>  	}
> -	spin_unlock(&m->client->lock);
>  }

Are you sure that would resolve that (other) syzbot report? I just had a 
glimpse at it yet, but I don't see this list iteration portion being involved 
in the backtrace provided by the report, is it?

> 
>  static __poll_t
> ----
> 
>  - but I don't get the two 'inconsistent lock state', the hint says it's
> possibly an interrupt while the lock was held but that doesn't seem to
> be the case from the stack trace (unless we leaked the lock, at which
> point anything goes)
> I'd need to take time to look at it, feel free to beat me to these.
> 
> --
> Dominique