[PATCH] Fix 32bit sendmsg() flaw

When we copy 32bit ->msg_control contents to kernel, we walk the same
userland data twice without sanity checks on the second pass.

Second version of this patch: the original broke with 64-bit arches
running 32-bit-compat-mode executables doing sendmsg() syscalls with
unaligned CMSG data areas

Another thing is that we use kmalloc() to allocate and sock_kfree_s()
to free afterwards; less serious, but also needs fixing.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Chris Wright <chrisw@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/net/compat.h

@@ -33,7 +33,7 @@ extern asmlinkage long compat_sys_sendmsg(int,struct compat_msghdr __user *,unsi
 extern asmlinkage long compat_sys_recvmsg(int,struct compat_msghdr __user *,unsigned);
 extern asmlinkage long compat_sys_getsockopt(int, int, int, char __user *, int __user *);
 extern int put_cmsg_compat(struct msghdr*, int, int, int, void *);
-extern int cmsghdr_from_user_compat_to_kern(struct msghdr *, unsigned char *,
+extern int cmsghdr_from_user_compat_to_kern(struct msghdr *, struct sock *, unsigned char *,
 		int);
 
 #endif /* NET_COMPAT_H */

net/compat.c

@@ -135,13 +135,14 @@ static inline struct compat_cmsghdr __user *cmsg_compat_nxthdr(struct msghdr *ms
  * thus placement) of cmsg headers and length are different for
  * 32-bit apps.  -DaveM
  */
-int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg,
+int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg, struct sock *sk,
 			       unsigned char *stackbuf, int stackbuf_size)
 {
 	struct compat_cmsghdr __user *ucmsg;
 	struct cmsghdr *kcmsg, *kcmsg_base;
 	compat_size_t ucmlen;
 	__kernel_size_t kcmlen, tmp;
+	int err = -EFAULT;
 
 	kcmlen = 0;
 	kcmsg_base = kcmsg = (struct cmsghdr *)stackbuf;
@@ -156,6 +157,7 @@ int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg,
 
 		tmp = ((ucmlen - CMSG_COMPAT_ALIGN(sizeof(*ucmsg))) +
 		       CMSG_ALIGN(sizeof(struct cmsghdr)));
+		tmp = CMSG_ALIGN(tmp);
 		kcmlen += tmp;
 		ucmsg = cmsg_compat_nxthdr(kmsg, ucmsg, ucmlen);
 	}
@@ -167,30 +169,34 @@ int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg,
 	 * until we have successfully copied over all of the data
 	 * from the user.
 	 */
-	if(kcmlen > stackbuf_size)
-		kcmsg_base = kcmsg = kmalloc(kcmlen, GFP_KERNEL);
-	if(kcmsg == NULL)
+	if (kcmlen > stackbuf_size)
+		kcmsg_base = kcmsg = sock_kmalloc(sk, kcmlen, GFP_KERNEL);
+	if (kcmsg == NULL)
 		return -ENOBUFS;
 
 	/* Now copy them over neatly. */
 	memset(kcmsg, 0, kcmlen);
 	ucmsg = CMSG_COMPAT_FIRSTHDR(kmsg);
 	while(ucmsg != NULL) {
-		__get_user(ucmlen, &ucmsg->cmsg_len);
+		if (__get_user(ucmlen, &ucmsg->cmsg_len))
+			goto Efault;
+		if (!CMSG_COMPAT_OK(ucmlen, ucmsg, kmsg))
+			goto Einval;
 		tmp = ((ucmlen - CMSG_COMPAT_ALIGN(sizeof(*ucmsg))) +
 		       CMSG_ALIGN(sizeof(struct cmsghdr)));
+		if ((char *)kcmsg_base + kcmlen - (char *)kcmsg < CMSG_ALIGN(tmp))
+			goto Einval;
 		kcmsg->cmsg_len = tmp;
-		__get_user(kcmsg->cmsg_level, &ucmsg->cmsg_level);
-		__get_user(kcmsg->cmsg_type, &ucmsg->cmsg_type);
-
-		/* Copy over the data. */
-		if(copy_from_user(CMSG_DATA(kcmsg),
-				  CMSG_COMPAT_DATA(ucmsg),
-				  (ucmlen - CMSG_COMPAT_ALIGN(sizeof(*ucmsg)))))
-			goto out_free_efault;
+		tmp = CMSG_ALIGN(tmp);
+		if (__get_user(kcmsg->cmsg_level, &ucmsg->cmsg_level) ||
+		    __get_user(kcmsg->cmsg_type, &ucmsg->cmsg_type) ||
+		    copy_from_user(CMSG_DATA(kcmsg),
+				   CMSG_COMPAT_DATA(ucmsg),
+				   (ucmlen - CMSG_COMPAT_ALIGN(sizeof(*ucmsg)))))
+			goto Efault;
 
 		/* Advance. */
-		kcmsg = (struct cmsghdr *)((char *)kcmsg + CMSG_ALIGN(tmp));
+		kcmsg = (struct cmsghdr *)((char *)kcmsg + tmp);
 		ucmsg = cmsg_compat_nxthdr(kmsg, ucmsg, ucmlen);
 	}
 
@@ -199,10 +205,12 @@ int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg,
 	kmsg->msg_controllen = kcmlen;
 	return 0;
 
-out_free_efault:
-	if(kcmsg_base != (struct cmsghdr *)stackbuf)
-		kfree(kcmsg_base);
-	return -EFAULT;
+Einval:
+	err = -EINVAL;
+Efault:
+	if (kcmsg_base != (struct cmsghdr *)stackbuf)
+		sock_kfree_s(sk, kcmsg_base, kcmlen);
+	return err;
 }
 
 int put_cmsg_compat(struct msghdr *kmsg, int level, int type, int len, void *data)

net/socket.c

@@ -1745,10 +1745,11 @@ asmlinkage long sys_sendmsg(int fd, struct msghdr __user *msg, unsigned flags)
 		goto out_freeiov;
 	ctl_len = msg_sys.msg_controllen; 
 	if ((MSG_CMSG_COMPAT & flags) && ctl_len) {
-		err = cmsghdr_from_user_compat_to_kern(&msg_sys, ctl, sizeof(ctl));
+		err = cmsghdr_from_user_compat_to_kern(&msg_sys, sock->sk, ctl, sizeof(ctl));
 		if (err)
 			goto out_freeiov;
 		ctl_buf = msg_sys.msg_control;
+		ctl_len = msg_sys.msg_controllen;
 	} else if (ctl_len) {
 		if (ctl_len > sizeof(ctl))
 		{