pipe(2) - race-free error recovery

don't mess with sys_close() if copy_to_user() fails; just postpone
fd_install() until we know it hasn't.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/pipe.c

@@ -1064,9 +1064,8 @@ err_inode:
 	return err;
 }
 
-int do_pipe_flags(int *fd, int flags)
+static int __do_pipe_flags(int *fd, struct file **files, int flags)
 {
-	struct file *files[2];
 	int error;
 	int fdw, fdr;
 
@@ -1088,11 +1087,8 @@ int do_pipe_flags(int *fd, int flags)
 	fdw = error;
 
 	audit_fd_pair(fdr, fdw);
-	fd_install(fdr, files[0]);
-	fd_install(fdw, files[1]);
 	fd[0] = fdr;
 	fd[1] = fdw;
-
 	return 0;
 
  err_fdr:
@@ -1103,21 +1099,38 @@ int do_pipe_flags(int *fd, int flags)
 	return error;
 }
 
+int do_pipe_flags(int *fd, int flags)
+{
+	struct file *files[2];
+	int error = __do_pipe_flags(fd, files, flags);
+	if (!error) {
+		fd_install(fd[0], files[0]);
+		fd_install(fd[1], files[1]);
+	}
+	return error;
+}
+
 /*
  * sys_pipe() is the normal C calling standard for creating
  * a pipe. It's not the way Unix traditionally does this, though.
  */
 SYSCALL_DEFINE2(pipe2, int __user *, fildes, int, flags)
 {
+	struct file *files[2];
 	int fd[2];
 	int error;
 
-	error = do_pipe_flags(fd, flags);
+	error = __do_pipe_flags(fd, files, flags);
 	if (!error) {
-		if (copy_to_user(fildes, fd, sizeof(fd))) {
-			sys_close(fd[0]);
-			sys_close(fd[1]);
+		if (unlikely(copy_to_user(fildes, fd, sizeof(fd)))) {
+			fput(files[0]);
+			fput(files[1]);
+			put_unused_fd(fd[0]);
+			put_unused_fd(fd[1]);
 			error = -EFAULT;
+		} else {
+			fd_install(fd[0], files[0]);
+			fd_install(fd[1], files[1]);
 		}
 	}
 	return error;