Subject: Re: [PATCH 2/3] net/scm: cleanup scm_detach_fds

From: Ido Schimmel <[email-masked]>
Date: Wed, 13 May 2020 12:29:18 +0300

On Mon, May 11, 2020 at 01:59:12PM +0200, Christoph Hellwig wrote:
> Factor out two helpes to keep the code tidy.
> 
> Signed-off-by: Christoph Hellwig <[email-masked]>

Christoph,

After installing net-next (fb9f2e92864f) on a Fedora 32 machine I cannot
ssh to it. Bisected it to this commit [1].

When trying to connect I see these error messages in journal:

sshd[1029]: error: mm_receive_fd: no message header
sshd[1029]: fatal: mm_pty_allocate: receive fds failed
sshd[1029]: fatal: mm_request_receive_expect: buffer error: incomplete message
sshd[1018]: fatal: mm_request_receive: read: Connection reset by peer

Please let me know if more info is required. I can easily test a patch
if you need me to try something.

Thanks

[1]
git bisect start
# bad: [fb9f2e92864f51d25e790947cca2ac4426a12f9c] net: dsa: tag_sja1105: appease sparse checks for ethertype accessors
git bisect bad fb9f2e92864f51d25e790947cca2ac4426a12f9c
# good: [3242956bd610af40e884b530b6c6f76f5bf85f3b] Merge branch 'net-dsa-Constify-two-tagger-ops'
git bisect good 3242956bd610af40e884b530b6c6f76f5bf85f3b
# bad: [1b0cde4091877cd7fe4b29f67645cc391b86c9ca] sfc: siena_check_caps() can be static
git bisect bad 1b0cde4091877cd7fe4b29f67645cc391b86c9ca
# bad: [cba155d591aa28689332bc568632d2f868690be1] ionic: add support for more xcvr types
git bisect bad cba155d591aa28689332bc568632d2f868690be1
# bad: [97cf0ef9305bba857f04b914fd59e83813f1eae2] Merge branch 'improve-msg_control-kernel-vs-user-pointer-handling'
git bisect bad 97cf0ef9305bba857f04b914fd59e83813f1eae2
# bad: [2618d530dd8b7ac0fdcb83f4c95b88f7b0d37ce6] net/scm: cleanup scm_detach_fds
git bisect bad 2618d530dd8b7ac0fdcb83f4c95b88f7b0d37ce6
# good: [0462b6bdb6445b887b8896f28be92e0d94c92e7b] net: add a CMSG_USER_DATA macro
git bisect good 0462b6bdb6445b887b8896f28be92e0d94c92e7b
# first bad commit: [2618d530dd8b7ac0fdcb83f4c95b88f7b0d37ce6] net/scm: cleanup scm_detach_fds

> ---
>  net/core/scm.c | 94 +++++++++++++++++++++++++++-----------------------
>  1 file changed, 51 insertions(+), 43 deletions(-)
> 
> diff --git a/net/core/scm.c b/net/core/scm.c
> index abfdc85a64c1b..168b006a52ff9 100644
> --- a/net/core/scm.c
> +++ b/net/core/scm.c
> @@ -277,78 +277,86 @@ void put_cmsg_scm_timestamping(struct msghdr *msg, struct scm_timestamping_inter
>  }
>  EXPORT_SYMBOL(put_cmsg_scm_timestamping);
>  
> +static int __scm_install_fd(struct file *file, int __user *ufd, int o_flags)
> +{
> +	struct socket *sock;
> +	int new_fd;
> +	int error;
> +
> +	error = security_file_receive(file);
> +	if (error)
> +		return error;
> +
> +	new_fd = get_unused_fd_flags(o_flags);
> +	if (new_fd < 0)
> +		return new_fd;
> +
> +	error = put_user(new_fd, ufd);
> +	if (error) {
> +		put_unused_fd(new_fd);
> +		return error;
> +	}
> +
> +	/* Bump the usage count and install the file. */
> +	sock = sock_from_file(file, &error);
> +	if (sock) {
> +		sock_update_netprioidx(&sock->sk->sk_cgrp_data);
> +		sock_update_classid(&sock->sk->sk_cgrp_data);
> +	}
> +	fd_install(new_fd, get_file(file));
> +	return error;
> +}
> +
> +static int scm_max_fds(struct msghdr *msg)
> +{
> +	if (msg->msg_controllen <= sizeof(struct cmsghdr))
> +		return 0;
> +	return (msg->msg_controllen - sizeof(struct cmsghdr)) / sizeof(int);
> +}
> +
>  void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
>  {
>  	struct cmsghdr __user *cm
>  		= (__force struct cmsghdr __user*)msg->msg_control;
> -
> -	int fdmax = 0;
> -	int fdnum = scm->fp->count;
> -	struct file **fp = scm->fp->fp;
> -	int __user *cmfptr;
> +	int o_flags = (msg->msg_flags & MSG_CMSG_CLOEXEC) ? O_CLOEXEC : 0;
> +	int fdmax = min_t(int, scm_max_fds(msg), scm->fp->count);
> +	int __user *cmsg_data = CMSG_USER_DATA(cm);
>  	int err = 0, i;
>  
> -	if (MSG_CMSG_COMPAT & msg->msg_flags) {
> +	if (msg->msg_flags & MSG_CMSG_COMPAT) {
>  		scm_detach_fds_compat(msg, scm);
>  		return;
>  	}
>  
> -	if (msg->msg_controllen > sizeof(struct cmsghdr))
> -		fdmax = ((msg->msg_controllen - sizeof(struct cmsghdr))
> -			 / sizeof(int));
> -
> -	if (fdnum < fdmax)
> -		fdmax = fdnum;
> -
> -	for (i=0, cmfptr =(int __user *)CMSG_USER_DATA(cm); i<fdmax;
> -	     i++, cmfptr++)
> -	{
> -		struct socket *sock;
> -		int new_fd;
> -		err = security_file_receive(fp[i]);
> +	for (i = 0; i < fdmax; i++) {
> +		err = __scm_install_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
>  		if (err)
>  			break;
> -		err = get_unused_fd_flags(MSG_CMSG_CLOEXEC & msg->msg_flags
> -					  ? O_CLOEXEC : 0);
> -		if (err < 0)
> -			break;
> -		new_fd = err;
> -		err = put_user(new_fd, cmfptr);
> -		if (err) {
> -			put_unused_fd(new_fd);
> -			break;
> -		}
> -		/* Bump the usage count and install the file. */
> -		sock = sock_from_file(fp[i], &err);
> -		if (sock) {
> -			sock_update_netprioidx(&sock->sk->sk_cgrp_data);
> -			sock_update_classid(&sock->sk->sk_cgrp_data);
> -		}
> -		fd_install(new_fd, get_file(fp[i]));
>  	}
>  
> -	if (i > 0)
> -	{
> -		int cmlen = CMSG_LEN(i*sizeof(int));
> +	if (i > 0)  {
> +		int cmlen = CMSG_LEN(i * sizeof(int));
> +
>  		err = put_user(SOL_SOCKET, &cm->cmsg_level);
>  		if (!err)
>  			err = put_user(SCM_RIGHTS, &cm->cmsg_type);
>  		if (!err)
>  			err = put_user(cmlen, &cm->cmsg_len);
>  		if (!err) {
> -			cmlen = CMSG_SPACE(i*sizeof(int));
> +			cmlen = CMSG_SPACE(i * sizeof(int));
>  			if (msg->msg_controllen < cmlen)
>  				cmlen = msg->msg_controllen;
>  			msg->msg_control += cmlen;
>  			msg->msg_controllen -= cmlen;
>  		}
>  	}
> -	if (i < fdnum || (fdnum && fdmax <= 0))
> +
> +	if (i < scm->fp->count || (scm->fp->count && fdmax <= 0))
>  		msg->msg_flags |= MSG_CTRUNC;
>  
>  	/*
> -	 * All of the files that fit in the message have had their
> -	 * usage counts incremented, so we just free the list.
> +	 * All of the files that fit in the message have had their usage counts
> +	 * incremented, so we just free the list.
>  	 */
>  	__scm_destroy(scm);
>  }
> -- 
> 2.26.2
>