[PATCH 3/3] net: cleanly handle kernel vs user buffers for ->msg_control

Christoph Hellwig
2020-05-11T11:59:30Z

The msg_control field in struct msghdr can either contain a user
pointer when used with the recvmsg system call, or a kernel pointer
when used with sendmsg. To complicate things further kernel_recvmsg
can stuff a kernel pointer in and then use set_fs to make the uaccess
helpers accept it.

Replace it with a union of a kernel pointer msg_control field, and
a user pointer msg_control_user one, and allow kernel_recvmsg operate
on a proper kernel pointer using a bitfield to override the normal
choice of a user pointer for recvmsg.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
include/linux/socket.h | 12 ++++++++++-
net/compat.c | 5 +++--
net/core/scm.c | 49 ++++++++++++++++++++++++------------------
net/ipv4/ip_sockglue.c | 3 ++-
net/socket.c | 22 ++++++-------------
5 files changed, 50 insertions(+), 41 deletions(-)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index 4cc64d611cf49..04d2bc97f497d 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 */
diff --git a/net/compat.c b/net/compat.c
index 4bed96e84d9a6..69fc6d1e4e6e9 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 168b006a52ff9..a75cd637a71ff 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_USER_DATA(cm), data, cmlen - sizeof(*cm)))
- 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);

@@ -328,6 +331,10 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
return;
}

+ /* no use for FD passing from kernel space callers */
+ 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 (err)
diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index aa3fd61818c47..8206047d70b6b 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 2dd739fba866e..1c9a7260a41de 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;

--
2.26.2