[RFC] [PATCH] net: socket: Fix the wrong returns for recvmsg and sendmsg

[Junling Zheng <zhengjunling@huawei.com>]

Hi, Greg:

We found that after v3.10.73, recvmsg might return -EFAULT while -EINVAL
was expected.

We tested it through the recvmsg01 testcase come from LTP testsuit. It set
msg->msg_namelen to -1 and the recvmsg syscall returned errno 14, which is
unexpected (errno 22 is expected):

recvmsg01    4  TFAIL  :  invalid socket length ; returned -1 (expected -1),
errno 14 (expected 22)

Linux mainline has no this bug for commit 08adb7dab fixes it accidentally.
However, it is too large and complex to be backported to LTS 3.10.

So, I made the following patch to fix the above problem for LTS 3.10.

Cheers,

Junling

============

Commit 281c9c36 (net: compat: Update get_compat_msghdr() to match
copy_msghdr_from_user() behaviour) made get_compat_msghdr() return
error if msg_sys->msg_namelen was negative, which changed the behaviors
of recvmsg and sendmsg syscall in a lib32 system:

Before commit 281c9c36, get_compat_msghdr() wouldn't fail and it would
return -EINVAL in move_addr_to_user() or somewhere if msg_sys->msg_namelen
was invalid and then syscall returned -EINVAL, which is correct.

And now, when msg_sys->msg_namelen is negative, get_compat_msghdr() will
fail and wants to return -EINVAL, however, the outer syscall will return
-EFAULT directly, which is unexpected.

This patch gets the return value of get_compat_msghdr() as well as
copy_msghdr_from_user(), then returns this expected value if
get_compat_msghdr() fails.

Fixes: 281c9c36 (net: compat: Update get_compat_msghdr() to match copy_msghdr_from_user() behaviour)
Signed-off-by: Junling Zheng <zhengjunling@huawei.com>
Signed-off-by: Hanbing Xu <xuhanbing@huawei.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: David Miller <davem@davemloft.net>
---
 net/socket.c | 24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

diff --git a/net/socket.c b/net/socket.c
index fc90b4f..53b6e41 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1988,14 +1988,12 @@ static int ___sys_sendmsg(struct socket *sock, struct msghdr __user *msg,
 	int err, ctl_len, total_len;
 
 	err = -EFAULT;
-	if (MSG_CMSG_COMPAT & flags) {
-		if (get_compat_msghdr(msg_sys, msg_compat))
-			return -EFAULT;
-	} else {
+	if (MSG_CMSG_COMPAT & flags)
+		err = get_compat_msghdr(msg_sys, msg_compat);
+	else
 		err = copy_msghdr_from_user(msg_sys, msg);
-		if (err)
-			return err;
-	}
+	if (err)
+		return err;
 
 	if (msg_sys->msg_iovlen > UIO_FASTIOV) {
 		err = -EMSGSIZE;
@@ -2200,14 +2198,12 @@ static int ___sys_recvmsg(struct socket *sock, struct msghdr __user *msg,
 	struct sockaddr __user *uaddr;
 	int __user *uaddr_len;
 
-	if (MSG_CMSG_COMPAT & flags) {
-		if (get_compat_msghdr(msg_sys, msg_compat))
-			return -EFAULT;
-	} else {
+	if (MSG_CMSG_COMPAT & flags)
+		err = get_compat_msghdr(msg_sys, msg_compat);
+	else
 		err = copy_msghdr_from_user(msg_sys, msg);
-		if (err)
-			return err;
-	}
+	if (err)
+		return err;
 
 	if (msg_sys->msg_iovlen > UIO_FASTIOV) {
 		err = -EMSGSIZE;
-- 
1.8.3.4