[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 <[e-mail]>
Acked-by: Jeff Layton <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/cifsproto.h

···
 #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

···
 			   "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)
···
 	struct cifsSesInfo *pSesInfo = NULL;
 	struct cifsTconInfo *tcon = NULL;
 	struct TCP_Server_Info *srvTcp = NULL;
+	char   *full_path;
 
 	xid = GetXid();
 
···
 	if (!(tcon->ses->capabilities & CAP_LARGE_READ_X))
 		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

fs/cifs/inode.c

···
 	.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;
···
 		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);