exec: move path_noexec() check earlier

The path_noexec() check, like the regular file check, was happening too
late, letting LSMs see impossible execve()s.  Check it earlier as well in
may_open() and collect the redundant fs/exec.c path_noexec() test under
the same robustness comment as the S_ISREG() check.

My notes on the call path, and related arguments, checks, etc:

do_open_execat()
    struct open_flags open_exec_flags = {
        .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
        .acc_mode = MAY_EXEC,
        ...
    do_filp_open(dfd, filename, open_flags)
        path_openat(nameidata, open_flags, flags)
            file = alloc_empty_file(open_flags, current_cred());
            do_open(nameidata, file, open_flags)
                may_open(path, acc_mode, open_flag)
                    /* new location of MAY_EXEC vs path_noexec() test */
                    inode_permission(inode, MAY_OPEN | acc_mode)
                        security_inode_permission(inode, acc_mode)
                vfs_open(path, file)
                    do_dentry_open(file, path->dentry->d_inode, open)
                        security_file_open(f)
                        open()
    /* old location of path_noexec() test */
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Link: http://lkml.kernel.org/r/20200605160013.3954297-4-keescook@chromium.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -147,10 +147,8 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
 	 * and check again at the very end too.
 	 */
 	error = -EACCES;
-	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
-		goto exit;
-
-	if (path_noexec(&file->f_path))
+	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
+			 path_noexec(&file->f_path)))
 		goto exit;
 
 	fsnotify_open(file);
@@ -919,10 +917,8 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
 	 * and check again at the very end too.
 	 */
 	err = -EACCES;
-	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
-		goto exit;
-
-	if (path_noexec(&file->f_path))
+	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
+			 path_noexec(&file->f_path)))
 		goto exit;
 
 	err = deny_write_access(file);

fs/namei.c

@@ -2863,6 +2863,10 @@ static int may_open(const struct path *path, int acc_mode, int flag)
 			return -EACCES;
 		flag &= ~O_TRUNC;
 		break;
+	case S_IFREG:
+		if ((acc_mode & MAY_EXEC) && path_noexec(path))
+			return -EACCES;
+		break;
 	}
 
 	error = inode_permission(inode, MAY_OPEN | acc_mode);