coredump: fix error handling for replace_fd()

commit 95c5f43 upstream.

The replace_fd() helper returns the file descriptor number on success
and a negative error code on failure. The current error handling in
umh_pipe_setup() only works because the file descriptor that is replaced
is zero but that's pretty volatile. Explicitly check for a negative
error code.

Link: https://lore.kernel.org/20250414-work-coredump-v2-2-685bf231f828@kernel.org
Tested-by: Luca Boccassi <luca.boccassi@gmail.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: brauner

fs/coredump.c

@@ -507,18 +507,23 @@ static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
 {
 	struct file *files[2];
 	struct coredump_params *cp = (struct coredump_params *)info->data;
-	int err = create_pipe_files(files, 0);
+	int err;
+
+	err = create_pipe_files(files, 0);
 	if (err)
 		return err;
 
 	cp->file = files[1];
 
 	err = replace_fd(0, files[0], 0);
 	fput(files[0]);
+	if (err < 0)
+		return err;
+
 	/* and disallow core files too */
 	current->signal->rlim[RLIMIT_CORE] = (struct rlimit){1, 1};
 
-	return err;
+	return 0;
 }
 
 void do_coredump(const kernel_siginfo_t *siginfo)