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

[Kees Cook <keescook@chromium.org>]

On Wed, Jan 24, 2024 at 08:58:55PM +0100, Jann Horn wrote:
> On Wed, Jan 24, 2024 at 8:22 PM Kees Cook <keescook@chromium.org> wrote:
> > After commit 978ffcbf00d8 ("execve: open the executable file before
> > doing anything else"), current->in_execve was no longer in sync with the
> > open(). This broke AppArmor and TOMOYO which depend on this flag to
> > distinguish "open" operations from being "exec" operations.
> >
> > Instead of moving around in_execve, switch to using __FMODE_EXEC, which
> > is where the "is this an exec?" intent is stored. Note that TOMOYO still
> > uses in_execve around cred handling.
> 
> I think this is wrong. When CONFIG_USELIB is enabled, the uselib()
> syscall will open a file with __FMODE_EXEC but without going through
> execve(). From what I can tell, there are no bprm hooks on this path.

Hrm, that's true.

We've been trying to remove uselib for at least 10 years[1]. :(

> I don't know if it _matters_ much, given that it'll only let you
> read/execute stuff from files with valid ELF headers, but still.

Hmpf, and frustratingly Ubuntu (and Debian) still builds with
CONFIG_USELIB, even though it was reported[2] to them almost 4 years ago.

-Kees

[1] https://lore.kernel.org/lkml/20140221181103.GA5773@jtriplet-mobl1/
[2] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1879454

-- 
Kees Cook