netdev - [PATCH 2/2] pidfd: Replace open-coded partial __scm_install

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Date:   Tue,  9 Jun 2020 21:52:14 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "David S. Miller" <davem@...emloft.net>
Cc:     Kees Cook <keescook@...omium.org>, Christoph Hellwig <hch@....de>,
        Christian Brauner <christian@...uner.io>,
        Sargun Dhillon <sargun@...gun.me>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] pidfd: Replace open-coded partial __scm_install_fd()

The sock counting (sock_update_netprioidx() and sock_update_classid())
was missing from this implementation of fd installation, compared to
SCM_RIGHTS. Use the new scm helper to get the work done, after adjusting
it to return the installed fd and accept a NULL user pointer.

Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
Signed-off-by: Kees Cook <keescook@...omium.org>
---
AFAICT, the following patches are needed for back-porting this to stable:

0462b6bdb644 ("net: add a CMSG_USER_DATA macro")
2618d530dd8b ("net/scm: cleanup scm_detach_fds")
1f466e1f15cf ("net: cleanly handle kernel vs user buffers for ->msg_control")
6e8a4f9dda38 ("net: ignore sock_from_file errors in __scm_install_fd")
---
 kernel/pid.c   | 12 ++----------
 net/compat.c   |  2 +-
 net/core/scm.c | 27 ++++++++++++++++++++-------
 3 files changed, 23 insertions(+), 18 deletions(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index f1496b757162..a7ce4ba898d3 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -42,6 +42,7 @@
 #include <linux/sched/signal.h>
 #include <linux/sched/task.h>
 #include <linux/idr.h>
+#include <net/scm.h>
 
 struct pid init_struct_pid = {
 	.count		= REFCOUNT_INIT(1),
@@ -635,18 +636,9 @@ static int pidfd_getfd(struct pid *pid, int fd)
 	if (IS_ERR(file))
 		return PTR_ERR(file);
 
-	ret = security_file_receive(file);
-	if (ret) {
-		fput(file);
-		return ret;
-	}
-
-	ret = get_unused_fd_flags(O_CLOEXEC);
+	ret = __scm_install_fd(file, NULL, O_CLOEXEC);
 	if (ret < 0)
 		fput(file);
-	else
-		fd_install(ret, file);
-
 	return ret;
 }
 
diff --git a/net/compat.c b/net/compat.c
index 117f1869bf3b..f8575555b6d7 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -299,7 +299,7 @@ void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm)
 
 	for (i = 0; i < fdmax; i++) {
 		err = __scm_install_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
-		if (err)
+		if (err < 0)
 			break;
 	}
 
diff --git a/net/core/scm.c b/net/core/scm.c
index 86d96152646f..e80648fb4da7 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -280,6 +280,14 @@ void put_cmsg_scm_timestamping(struct msghdr *msg, struct scm_timestamping_inter
 }
 EXPORT_SYMBOL(put_cmsg_scm_timestamping);
 
+/**
+ * __scm_install_fd() - Install received file into file descriptor table
+ *
+ * Installs a received file into the file descriptor table, with appropriate
+ * checks and count updates.
+ *
+ * Returns fd installed or -ve on error.
+ */
 int __scm_install_fd(struct file *file, int __user *ufd, int o_flags)
 {
 	struct socket *sock;
@@ -294,20 +302,25 @@ int __scm_install_fd(struct file *file, int __user *ufd, int o_flags)
 	if (new_fd < 0)
 		return new_fd;
 
-	error = put_user(new_fd, ufd);
-	if (error) {
-		put_unused_fd(new_fd);
-		return error;
+	if (ufd) {
+		error = put_user(new_fd, ufd);
+		if (error) {
+			put_unused_fd(new_fd);
+			return error;
+		}
 	}
 
-	/* Bump the usage count and install the file. */
+	/* Bump the usage count and install the file. The resulting value of
+	 * "error" is ignored here since we only need to take action when
+	 * the file is a socket and testing "sock" for NULL is sufficient.
+	 */
 	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 0;
+	return new_fd;
 }
 
 static int scm_max_fds(struct msghdr *msg)
@@ -337,7 +350,7 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
 
 	for (i = 0; i < fdmax; i++) {
 		err = __scm_install_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
-		if (err)
+		if (err < 0)
 			break;
 	}
 
-- 
2.25.1