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

From: Christoph Hellwig <[email-masked]>
Date: Wed, 13 May 2020 11:49:08 +0200

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:

---
>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