Subject: Re: [PATCH net-next 00/15] tools: ynl: stop using libmnl

From: Donald Hunter <[email-masked]>
Date: Tue, 27 Feb 2024 10:49:42 +0000

Jakub Kicinski <[email-masked]> writes:

> On Mon, 26 Feb 2024 09:04:12 +0000 Donald Hunter wrote:
>> > On Fri, 23 Feb 2024 16:26:33 +0000 Donald Hunter wrote:  
>> >> Is the absence of buffer bounds checking intentional, i.e. relying on libasan?  
>> >
>> > In ynl.c or the generated code?  
>> 
>> I'm looking at ynl_attr_nest_start() and ynl_attr_put*() in ynl-priv.h
>> and there's no checks for buffer overrun. It is admittedly a big
>> buffer, with rx following tx, but still.
>
> You're right. But this series isn't making it worse, AFAIU.
> We weren't checking before, we aren't checking now.

Agreed, libmnl had the same issue.

> I don't want to have to add another arg to all put() calls.
> How about we sash the max len on nlmsg_pid?

Seems reasonable. Minor comments below.

> Something like:
>
> diff --git a/tools/net/ynl/lib/ynl-priv.h b/tools/net/ynl/lib/ynl-priv.h
> index 6361318e5c4c..d4ffe18b00f9 100644
> --- a/tools/net/ynl/lib/ynl-priv.h
> +++ b/tools/net/ynl/lib/ynl-priv.h
> @@ -135,6 +135,8 @@ int ynl_error_parse(struct ynl_parse_arg *yarg, const char *msg);
>  
>  /* Netlink message handling helpers */
>  
> +#define YNL_MSG_OVERFLOW	1
> +
>  static inline struct nlmsghdr *ynl_nlmsg_put_header(void *buf)
>  {
>  	struct nlmsghdr *nlh = buf;
> @@ -239,11 +241,26 @@ ynl_attr_first(const void *start, size_t len, size_t skip)
>  	return ynl_attr_if_good(start + len, attr);
>  }
>  
> +static inline bool
> +__ynl_attr_put_overflow(struct nlmsghdr *nlh, size_t size)
> +{
> +	bool o;
> +
> +	/* We stash buffer length on nlmsg_pid. */
> +	o = nlh->nlmsg_len + NLA_HDRLEN + NLMSG_ALIGN(size) > nlh->nlmsg_pid;

The comment confused me here. How about "We compare against stashed buffer
length in nlmsg_pid".

> +	if (o)
> +		nlh->nlmsg_pid = YNL_MSG_OVERFLOW;

It took me a moment to realise that this behaves like a very short
buffer length for subsequent calls to __ynl_attr_put_overflow(). Is it
worth extending the comment in ynl_msg_start() to say "buffer length or
overflow status"?

> +	return o;
> +}
> +
>  static inline struct nlattr *
>  ynl_attr_nest_start(struct nlmsghdr *nlh, unsigned int attr_type)
>  {
>  	struct nlattr *attr;
>  
> +	if (__ynl_attr_put_overflow(nlh, 0))
> +		return ynl_nlmsg_end_addr(nlh) - NLA_HDRLEN;

Is the idea here to return a struct nlattr * that is safe to use?
Shouldn't we zero the values in the buffer first?

> +
>  	attr = ynl_nlmsg_end_addr(nlh);
>  	attr->nla_type = attr_type | NLA_F_NESTED;
>  	nlh->nlmsg_len += NLMSG_ALIGN(sizeof(struct nlattr));
> @@ -263,6 +280,9 @@ ynl_attr_put(struct nlmsghdr *nlh, unsigned int attr_type,
>  {
>  	struct nlattr *attr;
>  
> +	if (__ynl_attr_put_overflow(nlh, size))
> +		return;
> +
>  	attr = ynl_nlmsg_end_addr(nlh);
>  	attr->nla_type = attr_type;
>  	attr->nla_len = NLA_HDRLEN + size;
> @@ -276,14 +296,17 @@ static inline void
>  ynl_attr_put_str(struct nlmsghdr *nlh, unsigned int attr_type, const char *str)
>  {
>  	struct nlattr *attr;
> -	const char *end;
> +	size_t len;
> +
> +	len = strlen(str);
> +	if (__ynl_attr_put_overflow(nlh, len))
> +		return;
>  
>  	attr = ynl_nlmsg_end_addr(nlh);
>  	attr->nla_type = attr_type;
>  
> -	end = stpcpy(ynl_attr_data(attr), str);
> -	attr->nla_len =
> -		NLA_HDRLEN + NLA_ALIGN(end - (char *)ynl_attr_data(attr));
> +	strcpy(ynl_attr_data(attr), str);
> +	attr->nla_len = NLA_HDRLEN + NLA_ALIGN(len);
>  
>  	nlh->nlmsg_len += NLMSG_ALIGN(attr->nla_len);
>  }
> diff --git a/tools/net/ynl/lib/ynl.c b/tools/net/ynl/lib/ynl.c
> index 86729119e1ef..c2ba72f68028 100644
> --- a/tools/net/ynl/lib/ynl.c
> +++ b/tools/net/ynl/lib/ynl.c
> @@ -404,9 +404,33 @@ struct nlmsghdr *ynl_msg_start(struct ynl_sock *ys, __u32 id, __u16 flags)
>  	nlh->nlmsg_flags = flags;
>  	nlh->nlmsg_seq = ++ys->seq;
>  
> +	/* This is a local YNL hack for length checking, we put the buffer
> +	 * length in nlmsg_pid, since messages sent to the kernel always use
> +	 * PID 0. Message needs to be terminated with ynl_msg_end().
> +	 */
> +	nlh->nlmsg_pid = YNL_SOCKET_BUFFER_SIZE;
> +
>  	return nlh;
>  }
>  
> +static int ynl_msg_end(struct ynl_sock *ys, struct nlmsghdr *nlh)
> +{
> +	/* We stash buffer length on nlmsg_pid */
> +	if (nlh->nlmsg_pid == 0) {
> +		yerr(ys, YNL_ERROR_INPUT_INVALID,
> +		     "Unknwon input buffer lenght");

Typo: lenght -> length

> +		return -EINVAL;
> +	}
> +	if (nlh->nlmsg_pid == YNL_MSG_OVERFLOW) {
> +		yerr(ys, YNL_ERROR_INPUT_TOO_BIG,
> +		     "Constructred message longer than internal buffer");
> +		return -EMSGSIZE;
> +	}
> +
> +	nlh->nlmsg_pid = 0;
> +	return 0;
> +}
> +
>  struct nlmsghdr *
>  ynl_gemsg_start(struct ynl_sock *ys, __u32 id, __u16 flags,
>  		__u8 cmd, __u8 version)
> @@ -606,6 +630,10 @@ static int ynl_sock_read_family(struct ynl_sock *ys, const char *family_name)
>  	nlh = ynl_gemsg_start_req(ys, GENL_ID_CTRL, CTRL_CMD_GETFAMILY, 1);
>  	ynl_attr_put_str(nlh, CTRL_ATTR_FAMILY_NAME, family_name);
>  
> +	err = ynl_msg_end(ys, nlh);
> +	if (err < 0)
> +		return err;
> +
>  	err = send(ys->socket, nlh, nlh->nlmsg_len, 0);
>  	if (err < 0) {
>  		perr(ys, "failed to request socket family info");
> @@ -867,6 +895,10 @@ int ynl_exec(struct ynl_sock *ys, struct nlmsghdr *req_nlh,
>  {
>  	int err;
>  
> +	err = ynl_msg_end(ys, req_nlh);
> +	if (err < 0)
> +		return err;
> +
>  	err = send(ys->socket, req_nlh, req_nlh->nlmsg_len, 0);
>  	if (err < 0)
>  		return err;
> @@ -920,6 +952,10 @@ int ynl_exec_dump(struct ynl_sock *ys, struct nlmsghdr *req_nlh,
>  {
>  	int err;
>  
> +	err = ynl_msg_end(ys, req_nlh);
> +	if (err < 0)
> +		return err;
> +
>  	err = send(ys->socket, req_nlh, req_nlh->nlmsg_len, 0);
>  	if (err < 0)
>  		return err;
> diff --git a/tools/net/ynl/lib/ynl.h b/tools/net/ynl/lib/ynl.h
> index dbeeef8ce91a..9842e85a8c57 100644
> --- a/tools/net/ynl/lib/ynl.h
> +++ b/tools/net/ynl/lib/ynl.h
> @@ -20,6 +20,8 @@ enum ynl_error_code {
>  	YNL_ERROR_ATTR_INVALID,
>  	YNL_ERROR_UNKNOWN_NTF,
>  	YNL_ERROR_INV_RESP,
> +	YNL_ERROR_INPUT_INVALID,
> +	YNL_ERROR_INPUT_TOO_BIG,
>  };
>  
>  /**