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

[Kees Cook <keescook@chromium.org>]

On Wed, Jan 24, 2024 at 10:40:49PM +0100, Jann Horn wrote:
> On Wed, Jan 24, 2024 at 10:32 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Wed, Jan 24, 2024 at 12:47:34PM -0800, Linus Torvalds wrote:
> > > On Wed, 24 Jan 2024 at 12:15, Kees Cook <keescook@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;
> 
> Call path from here is:
> 
> sys_uselib -> load_elf_library -> elf_load -> elf_map -> vm_mmap ->
> vm_mmap_pgoff
> 
> Call path for normal mmap is:
> 
> sys_mmap_pgoff -> ksys_mmap_pgoff -> vm_mmap_pgoff
> 
> So I think the call paths converge before any real security checks
> happen, and the check you're suggesting should be superfluous. (There
> is some weird audit call in ksys_mmap_pgoff() but that's just to
> record the FD number, so I guess that doesn't matter.)

Yeah, I was just noticing this. I was over thinking. :) It does look
like all that is needed is to remove __FMODE_EXEC.

-- 
Kees Cook