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

[John Johansen]

On 1/25/24 08:38, Mickaël Salaün wrote:
> On Wed, Jan 24, 2024 at 01:32:02PM -0800, Kees Cook 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@xxxxxxxxxxxx> 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()...
>
> If user space can emulate uselib() without opening a file with
> __FMODE_EXEC, then there is no security reason to keep __FMODE_EXEC for
> uselib().
agreed

> Removing __FMODE_EXEC from uselib() looks OK for Landlock.  We use
> __FMODE_EXEC to infer if a file is being open for execution i.e., by
> execve(2).

apparmor the hint should be to avoid doing permission work again that we
are doing in exec. That it regressed anything more than performance here
is a bug, that will get fixed.


> If __FMODE_EXEC is removed from uselib(), I think it should also be
> backported to all stable kernels for consistency though.
hrmmm, I am not opposed to it being backported but I don't know that
it should be backported. Consistency is good but its not a serious
bug fix either

> > 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