[PATCH v2 3/3] exec: Move path_noexec() check earlier

[Kees Cook]

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@xxxxxxxxxxxx>
---
 fs/exec.c  | 12 ++++--------
 fs/namei.c |  4 ++++
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 2b708629dcd6..7ac50a260df3 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -145,10 +145,8 @@ 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);
@@ -871,10 +869,8 @@ 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);
diff --git a/fs/namei.c b/fs/namei.c
index 0a759b68d66e..41e6fed8ce69 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2849,6 +2849,10 @@ 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);
-- 
2.25.1