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

From: Ido Schimmel <[email-masked]>
Date: Wed, 13 May 2020 12:58:11 +0300

On Wed, May 13, 2020 at 11:49:08AM +0200, Christoph Hellwig wrote:
> On Wed, May 13, 2020 at 12:29:18PM +0300, Ido Schimmel wrote:
> > 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.
> 
> To start we can try reverting just this commit, which requires a
> little manual work.  Patch below:

Thanks for the quick reply. With the below patch ssh is working again.

> 
> ---
> From fe4f53219b42aeded3c1464dbe2bbc9365f6a853 Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <[email-masked]>
> Date: Wed, 13 May 2020 11:48:33 +0200
> Subject: Revert "net/scm: cleanup scm_detach_fds"
> 
> This reverts commit 2618d530dd8b7ac0fdcb83f4c95b88f7b0d37ce6.
> ---
>  net/core/scm.c | 94 +++++++++++++++++++++++---------------------------
>  1 file changed, 43 insertions(+), 51 deletions(-)
> 
> diff --git a/net/core/scm.c b/net/core/scm.c
> index a75cd637a71ff..2d9aa5682bed2 100644
> --- a/net/core/scm.c
> +++ b/net/core/scm.c
> @@ -280,53 +280,18 @@ 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 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 fdmax = 0;
> +	int fdnum = scm->fp->count;
> +	struct file **fp = scm->fp->fp;
> +	int __user *cmfptr;
>  	int err = 0, i;
>  
> -	if (msg->msg_flags & MSG_CMSG_COMPAT) {
> +	if (MSG_CMSG_COMPAT & msg->msg_flags) {
>  		scm_detach_fds_compat(msg, scm);
>  		return;
>  	}
> @@ -335,35 +300,62 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
>  	if (WARN_ON_ONCE(!msg->msg_control_is_user))
>  		return;
>  
> -	for (i = 0; i < fdmax; i++) {
> -		err = __scm_install_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
> +	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]);
>  		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 < scm->fp->count || (scm->fp->count && fdmax <= 0))
> +	if (i < fdnum || (fdnum && 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
>