Merge branch 'improve-msg_control-kernel-vs-user-pointer-handling'

Christoph Hellwig says:

====================
improve msg_control kernel vs user pointer handling

this series replace the msg_control in the kernel msghdr structure
with an anonymous union and separate fields for kernel vs user
pointers.  In addition to helping a bit with type safety and reducing
sparse warnings, this also allows to remove the set_fs() in
kernel_recvmsg, helping with an eventual entire removal of set_fs().
====================

Signed-off-by: David S. Miller <davem@davemloft.net>

5 files changed, 104 insertions, 84 deletions

diff --git a/include/linux/socket.h b/include/linux/socket.h
index 54338fac45cb..04d2bc97f497 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -50,7 +50,17 @@ struct msghdr {
 	void		*msg_name;	/* ptr to socket address structure */
 	int		msg_namelen;	/* size of socket address structure */
 	struct iov_iter	msg_iter;	/* data */
-	void		*msg_control;	/* ancillary data */
+
+	/*
+	 * Ancillary data. msg_control_user is the user buffer used for the
+	 * recv* side when msg_control_is_user is set, msg_control is the kernel
+	 * buffer used for all other cases.
+	 */
+	union {
+		void		*msg_control;
+		void __user	*msg_control_user;
+	};
+	bool		msg_control_is_user : 1;
 	__kernel_size_t	msg_controllen;	/* ancillary data buffer length */
 	unsigned int	msg_flags;	/* flags on received message */
 	struct kiocb	*msg_iocb;	/* ptr to iocb for async requests */
@@ -94,7 +104,10 @@ struct cmsghdr {
 
 #define CMSG_ALIGN(len) ( ((len)+sizeof(long)-1) & ~(sizeof(long)-1) )
 
-#define CMSG_DATA(cmsg)	((void *)((char *)(cmsg) + sizeof(struct cmsghdr)))
+#define CMSG_DATA(cmsg) \
+	((void *)(cmsg) + sizeof(struct cmsghdr))
+#define CMSG_USER_DATA(cmsg) \
+	((void __user *)(cmsg) + sizeof(struct cmsghdr))
 #define CMSG_SPACE(len) (sizeof(struct cmsghdr) + CMSG_ALIGN(len))
 #define CMSG_LEN(len) (sizeof(struct cmsghdr) + (len))
 
diff --git a/net/compat.c b/net/compat.c
index 4bed96e84d9a..69fc6d1e4e6e 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -56,7 +56,8 @@ int __get_compat_msghdr(struct msghdr *kmsg,
 	if (kmsg->msg_namelen > sizeof(struct sockaddr_storage))
 		kmsg->msg_namelen = sizeof(struct sockaddr_storage);
 
-	kmsg->msg_control = compat_ptr(msg.msg_control);
+	kmsg->msg_control_is_user = true;
+	kmsg->msg_control_user = compat_ptr(msg.msg_control);
 	kmsg->msg_controllen = msg.msg_controllen;
 
 	if (save_addr)
@@ -121,7 +122,7 @@ int get_compat_msghdr(struct msghdr *kmsg,
 	((ucmlen) >= sizeof(struct compat_cmsghdr) && \
 	 (ucmlen) <= (unsigned long) \
 	 ((mhdr)->msg_controllen - \
-	  ((char *)(ucmsg) - (char *)(mhdr)->msg_control)))
+	  ((char __user *)(ucmsg) - (char __user *)(mhdr)->msg_control_user)))
 
 static inline struct compat_cmsghdr __user *cmsg_compat_nxthdr(struct msghdr *msg,
 		struct compat_cmsghdr __user *cmsg, int cmsg_len)
diff --git a/net/core/scm.c b/net/core/scm.c
index dc6fed1f221c..a75cd637a71f 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -212,16 +212,12 @@ EXPORT_SYMBOL(__scm_send);
 
 int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
 {
-	struct cmsghdr __user *cm
-		= (__force struct cmsghdr __user *)msg->msg_control;
-	struct cmsghdr cmhdr;
 	int cmlen = CMSG_LEN(len);
-	int err;
 
-	if (MSG_CMSG_COMPAT & msg->msg_flags)
+	if (msg->msg_flags & MSG_CMSG_COMPAT)
 		return put_cmsg_compat(msg, level, type, len, data);
 
-	if (cm==NULL || msg->msg_controllen < sizeof(*cm)) {
+	if (!msg->msg_control || msg->msg_controllen < sizeof(struct cmsghdr)) {
 		msg->msg_flags |= MSG_CTRUNC;
 		return 0; /* XXX: return error? check spec. */
 	}
@@ -229,23 +225,30 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
 		msg->msg_flags |= MSG_CTRUNC;
 		cmlen = msg->msg_controllen;
 	}
-	cmhdr.cmsg_level = level;
-	cmhdr.cmsg_type = type;
-	cmhdr.cmsg_len = cmlen;
-
-	err = -EFAULT;
-	if (copy_to_user(cm, &cmhdr, sizeof cmhdr))
-		goto out;
-	if (copy_to_user(CMSG_DATA(cm), data, cmlen - sizeof(struct cmsghdr)))
-		goto out;
-	cmlen = CMSG_SPACE(len);
-	if (msg->msg_controllen < cmlen)
-		cmlen = msg->msg_controllen;
+
+	if (msg->msg_control_is_user) {
+		struct cmsghdr __user *cm = msg->msg_control_user;
+		struct cmsghdr cmhdr;
+
+		cmhdr.cmsg_level = level;
+		cmhdr.cmsg_type = type;
+		cmhdr.cmsg_len = cmlen;
+		if (copy_to_user(cm, &cmhdr, sizeof cmhdr) ||
+		    copy_to_user(CMSG_USER_DATA(cm), data, cmlen - sizeof(*cm)))
+			return -EFAULT;
+	} else {
+		struct cmsghdr *cm = msg->msg_control;
+
+		cm->cmsg_level = level;
+		cm->cmsg_type = type;
+		cm->cmsg_len = cmlen;
+		memcpy(CMSG_DATA(cm), data, cmlen - sizeof(*cm));
+	}
+
+	cmlen = min(CMSG_SPACE(len), msg->msg_controllen);
 	msg->msg_control += cmlen;
 	msg->msg_controllen -= cmlen;
-	err = 0;
-out:
-	return err;
+	return 0;
 }
 EXPORT_SYMBOL(put_cmsg);
 
@@ -277,78 +280,90 @@ 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;
+	/* no use for FD passing from kernel space callers */
+	if (WARN_ON_ONCE(!msg->msg_control_is_user))
+		return;
 
-	for (i=0, cmfptr=(__force int __user *)CMSG_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);
 }
diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index aa3fd61818c4..8206047d70b6 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -1492,7 +1492,8 @@ static int do_ip_getsockopt(struct sock *sk, int level, int optname,
 		if (sk->sk_type != SOCK_STREAM)
 			return -ENOPROTOOPT;
 
-		msg.msg_control = (__force void *) optval;
+		msg.msg_control_is_user = true;
+		msg.msg_control_user = optval;
 		msg.msg_controllen = len;
 		msg.msg_flags = flags;
 
diff --git a/net/socket.c b/net/socket.c
index 2dd739fba866..1c9a7260a41d 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -924,14 +924,9 @@ EXPORT_SYMBOL(sock_recvmsg);
 int kernel_recvmsg(struct socket *sock, struct msghdr *msg,
 		   struct kvec *vec, size_t num, size_t size, int flags)
 {
-	mm_segment_t oldfs = get_fs();
-	int result;
-
+	msg->msg_control_is_user = false;
 	iov_iter_kvec(&msg->msg_iter, READ, vec, num, size);
-	set_fs(KERNEL_DS);
-	result = sock_recvmsg(sock, msg, flags);
-	set_fs(oldfs);
-	return result;
+	return sock_recvmsg(sock, msg, flags);
 }
 EXPORT_SYMBOL(kernel_recvmsg);
 
@@ -2239,7 +2234,8 @@ int __copy_msghdr_from_user(struct msghdr *kmsg,
 	if (copy_from_user(&msg, umsg, sizeof(*umsg)))
 		return -EFAULT;
 
-	kmsg->msg_control = (void __force *)msg.msg_control;
+	kmsg->msg_control_is_user = true;
+	kmsg->msg_control_user = msg.msg_control;
 	kmsg->msg_controllen = msg.msg_controllen;
 	kmsg->msg_flags = msg.msg_flags;
 
@@ -2331,16 +2327,10 @@ static int ____sys_sendmsg(struct socket *sock, struct msghdr *msg_sys,
 				goto out;
 		}
 		err = -EFAULT;
-		/*
-		 * Careful! Before this, msg_sys->msg_control contains a user pointer.
-		 * Afterwards, it will be a kernel pointer. Thus the compiler-assisted
-		 * checking falls down on this.
-		 */
-		if (copy_from_user(ctl_buf,
-				   (void __user __force *)msg_sys->msg_control,
-				   ctl_len))
+		if (copy_from_user(ctl_buf, msg_sys->msg_control_user, ctl_len))
 			goto out_freectl;
 		msg_sys->msg_control = ctl_buf;
+		msg_sys->msg_control_is_user = false;
 	}
 	msg_sys->msg_flags = flags;