[CIFS] Prevent OOPs when mounting with remote prefixpath.

Fixes OOPs with message 'kernel BUG at fs/cifs/cifs_dfs_ref.c:274!'.
Checks if the prefixpath in an accesible while we are still in cifs_mount
and fails with reporting a error if we can't access the prefixpath

Should fix Samba bugs 6086 and 5861 and kernel bug 12192
Signed-off-by: Igor Mammedov <niallain@gmail.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/cifsproto.h

@@ -42,6 +42,7 @@ extern void _FreeXid(unsigned int);
 #define GetXid() (int)_GetXid(); cFYI(1,("CIFS VFS: in %s as Xid: %d with uid: %d",__func__, xid,current_fsuid()));
 #define FreeXid(curr_xid) {_FreeXid(curr_xid); cFYI(1,("CIFS VFS: leaving %s (xid = %d) rc = %d",__func__,curr_xid,(int)rc));}
 extern char *build_path_from_dentry(struct dentry *);
+extern char *cifs_build_path_to_root(struct cifs_sb_info *cifs_sb);
 extern char *build_wildcard_path_from_dentry(struct dentry *direntry);
 /* extern void renew_parental_timestamps(struct dentry *direntry);*/
 extern int SendReceive(const unsigned int /* xid */ , struct cifsSesInfo *,

fs/cifs/connect.c

@@ -2180,6 +2180,33 @@ static void setup_cifs_sb(struct smb_vol *pvolume_info,
 			   "mount option supported"));
 }
 
+static int
+is_path_accessible(int xid, struct cifsTconInfo *tcon,
+		   struct cifs_sb_info *cifs_sb, const char *full_path)
+{
+	int rc;
+	__u64 inode_num;
+	FILE_ALL_INFO *pfile_info;
+
+	rc = CIFSGetSrvInodeNumber(xid, tcon, full_path, &inode_num,
+				   cifs_sb->local_nls,
+				   cifs_sb->mnt_cifs_flags &
+						CIFS_MOUNT_MAP_SPECIAL_CHR);
+	if (rc != -EOPNOTSUPP)
+		return rc;
+
+	pfile_info = kmalloc(sizeof(FILE_ALL_INFO), GFP_KERNEL);
+	if (pfile_info == NULL)
+		return -ENOMEM;
+
+	rc = CIFSSMBQPathInfo(xid, tcon, full_path, pfile_info,
+			      0 /* not legacy */, cifs_sb->local_nls,
+			      cifs_sb->mnt_cifs_flags &
+				CIFS_MOUNT_MAP_SPECIAL_CHR);
+	kfree(pfile_info);
+	return rc;
+}
+
 int
 cifs_mount(struct super_block *sb, struct cifs_sb_info *cifs_sb,
 	   char *mount_data, const char *devname)
@@ -2190,6 +2217,7 @@ cifs_mount(struct super_block *sb, struct cifs_sb_info *cifs_sb,
 	struct cifsSesInfo *pSesInfo = NULL;
 	struct cifsTconInfo *tcon = NULL;
 	struct TCP_Server_Info *srvTcp = NULL;
+	char   *full_path;
 
 	xid = GetXid();
 
@@ -2426,6 +2454,23 @@ cifs_mount(struct super_block *sb, struct cifs_sb_info *cifs_sb,
 		cifs_sb->rsize = min(cifs_sb->rsize,
 			       (tcon->ses->server->maxBuf - MAX_CIFS_HDR_SIZE));
 
+	if (!rc && cifs_sb->prepathlen) {
+		/* build_path_to_root works only when we have a valid tcon */
+		full_path = cifs_build_path_to_root(cifs_sb);
+		if (full_path == NULL) {
+			rc = -ENOMEM;
+			goto mount_fail_check;
+		}
+		rc = is_path_accessible(xid, tcon, cifs_sb, full_path);
+		if (rc) {
+			cERROR(1, ("Path %s in not accessible: %d",
+						full_path, rc));
+			kfree(full_path);
+			goto mount_fail_check;
+		}
+		kfree(full_path);
+	}
+
 	/* volume_info->password is freed above when existing session found
 	(in which case it is not needed anymore) but when new sesion is created
 	the password ptr is put in the new session structure (in which case the

fs/cifs/inode.c

@@ -621,7 +621,7 @@ static const struct inode_operations cifs_ipc_inode_ops = {
 	.lookup = cifs_lookup,
 };
 
-static char *build_path_to_root(struct cifs_sb_info *cifs_sb)
+char *cifs_build_path_to_root(struct cifs_sb_info *cifs_sb)
 {
 	int pplen = cifs_sb->prepathlen;
 	int dfsplen;
@@ -678,7 +678,7 @@ struct inode *cifs_iget(struct super_block *sb, unsigned long ino)
 		return inode;
 
 	cifs_sb = CIFS_SB(inode->i_sb);
-	full_path = build_path_to_root(cifs_sb);
+	full_path = cifs_build_path_to_root(cifs_sb);
 	if (full_path == NULL)
 		return ERR_PTR(-ENOMEM);