linux-hardening - Re: [PATCH] exec: Check __FMODE_EXEC instead of in

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Date: Wed, 24 Jan 2024 12:47:34 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Kees Cook <keescook@...omium.org>
Cc: Jann Horn <jannh@...gle.com>, Josh Triplett <josh@...htriplett.org>, 
	Kevin Locke <kevin@...inlocke.name>, John Johansen <john.johansen@...onical.com>, 
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>, 
	"Serge E. Hallyn" <serge@...lyn.com>, Kentaro Takeda <takedakn@...data.co.jp>, 
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Eric Biederman <ebiederm@...ssion.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	apparmor@...ts.ubuntu.com, linux-security-module@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] exec: Check __FMODE_EXEC instead of in_execve for LSMs

On Wed, 24 Jan 2024 at 12:15, Kees Cook <keescook@...omium.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