Kees Cook wrote this patch, which is in Andrew Morton's tree, but I
think you're talking about O_MAYEXEC, not this patch specifically.

On 11/08/2020 21:36, Eric W. Biederman wrote:

Mickaël Salaün [off-list ref] writes:

quoted

From: Kees Cook <redacted>

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:

A big question arises, that I think someone already asked.

Al Viro and Jann Horn expressed such concerns for O_MAYEXEC:
https://lore.kernel.org/lkml/0cc94c91-afd3-27cd-b831-8ea16ca8ca93@digikod.net/ (local)

Why perform this test in may_open directly instead of moving
it into inode_permission.  That way the code can be shared with
faccessat, and any other code path that wants it?

This patch is just a refactoring.

About O_MAYEXEC, path-based LSM, IMA and IPE need to work on a struct
file, whereas inode_permission() only gives a struct inode. However,
faccessat2(2) (with extended flags) seems to be the perfect candidate if
we want to be able to check file descriptors.

That would look to provide a more maintainable kernel.

Why would it be more maintainable?

Eric

quoted

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: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Kees Cook <redacted>
Link: https://lore.kernel.org/r/20200605160013.3954297-4-keescook@chromium.org (local)
---
 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 bdc6a6eb5dce..4eea20c27b01 100644
--- a/fs/exec.c
+++ b/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);

@@ -897,10 +895,8 @@ 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 a559ad943970..ddc9b25540fe 100644
--- a/fs/namei.c
+++ b/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);