[PATCH] exec: Check __FMODE_EXEC instead of in_execve for LSMs

[Kees Cook]

On Wed, Jan 24, 2024 at 12:47:34PM -0800, Linus Torvalds wrote:
> On Wed, 24 Jan 2024 at 12:15, Kees Cook <keescook at chromium.org> wrote:
> >
> > Hmpf, and frustratingly Ubuntu (and Debian) still builds with
> > CONFIG_USELIB, even though it was reported[2] to them almost 4 years ago.

For completeness, Fedora hasn't had CONFIG_USELIB for a while now.

> Well, we could just remove the __FMODE_EXEC from uselib.
> 
> It's kind of wrong anyway.

Yeah.

> So I think just removing __FMODE_EXEC would just do the
> RightThing(tm), and changes nothing for any sane situation.

Agreed about these:

- fs/fcntl.c is just doing a bitfield sanity check.

- nfs_open_permission_mask(), as you say, is only checking for
  unreadable case.

- fsnotify would also see uselib() as a read, but afaict,
  that's what it would see for an mmap(), so this should
  be functionally safe.

This one, though, I need some more time to examine:

- AppArmor, TOMOYO, and LandLock will see uselib() as an
  open-for-read, so that might still be a problem? As you
  say, it's more of a mmap() call, but that would mean
  adding something a call like security_mmap_file() into
  uselib()...

The issue isn't an insane "support uselib() under AppArmor" case, but
rather "Can uselib() be used to bypass exec/mmap checks?"

This totally untested patch might give appropriate coverage:

diff --git a/fs/exec.c b/fs/exec.c
index d179abb78a1c..0c9265312c8d 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -143,6 +143,10 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
 	if (IS_ERR(file))
 		goto out;
 
+	error = security_mmap_file(file, PROT_READ | PROT_EXEC, MAP_FIXED | MAP_SHARED);
+	if (error)
+		goto exit;
+
 	/*
 	 * may_open() has already checked for this, so it should be
 	 * impossible to trip now. But we need to be extra cautious

> Of course, as you say, not having CONFIG_USELIB enabled at all is the
> _truly_ sane thing, but the only thing that used the FMODE_EXEC bit
> were landlock and some special-case nfs stuff.

Do we want to attempt deprecation again? This was suggested last time:
https://lore.kernel.org/lkml/20200518130251.zih2s32q2rxhxg6f@wittgenstein/

-Kees

-- 
Kees Cook