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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 24 Jan 2024 12:47:34 -0800
Message-ID: <CAHk-=wiUwRG7LuR=z5sbkFVGQh+7qVB6_1NM0Ny9SVNL1Un4Sw@mail.gmail.com>
Subject: Re: [PATCH] exec: Check __FMODE_EXEC instead of in_execve for LSMs
To: Kees Cook <keescook@chromium.org>
Cc: Jann Horn <jannh@google.com>, Josh Triplett <josh@joshtriplett.org>, 
	Kevin Locke <kevin@kevinlocke.name>, John Johansen <john.johansen@canonical.com>, 
	Paul Moore <paul@paul-moore.com>, James Morris <jmorris@namei.org>, 
	"Serge E. Hallyn" <serge@hallyn.com>, Kentaro Takeda <takedakn@nttdata.co.jp>, 
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>, 
	Alexander Viro <viro@zeniv.linux.org.uk>, Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>, 
	Eric Biederman <ebiederm@xmission.com>, Andrew Morton <akpm@linux-foundation.org>, 
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, 
	apparmor@lists.ubuntu.com, linux-security-module@vger.kernel.org, 
	linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
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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.

Well, we could just remove the __FMODE_EXEC from uselib.

It's kind of wrong anyway.

Unlike a real execve(), where the target executable actually takes
control and you can't actually control it (except with ptrace, of
course), 'uselib()' really is just a wrapper around a special mmap.

And you can see it in the "acc_mode" flags: uselib already requires
MAY_READ for that reason. So you cannot uselib() a non-readable file,
unlike execve().

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

In fact, I don't think __FMODE_EXEC really ever did anything for the
uselib() case, so removing it *really* shouldn't matter, and only fix
the new AppArmor / Tomoyo use.

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.

And at least the nfs stuff was about "don't require read permissions
for exec", which was already wrong for the uselib() case as per above.

So I think the simple oneliner is literally just

  --- a/fs/exec.c
  +++ b/fs/exec.c
  @@ -128,7 +128,7 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
        struct filename *tmp = getname(library);
        int error = PTR_ERR(tmp);
        static const struct open_flags uselib_flags = {
  -             .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
  +             .open_flag = O_LARGEFILE | O_RDONLY,
                .acc_mode = MAY_READ | MAY_EXEC,
                .intent = LOOKUP_OPEN,
                .lookup_flags = LOOKUP_FOLLOW,

but I obviously have nothing that uses uselib(). I don't see how it
really *could* break anything, though, exactly because of that

                .acc_mode = MAY_READ | MAY_EXEC,

that means that the *regular* permission checks already require the
file to be readable. Never mind any LSM checks that might be confused.

           Linus