linux-kernel - Re: [PATCH] proc: Track /proc/$pid/attr/ opener mm

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Message-ID: <202106151449.816D7DA682@keescook>
Date:   Tue, 15 Jun 2021 14:50:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     youling 257 <youling257@...il.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Andrea Righi <andrea.righi@...onical.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>, regressions@...ts.linux.dev,
        LSM List <linux-security-module@...r.kernel.org>,
        Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        SElinux list <selinux@...r.kernel.org>
Subject: Re: [PATCH] proc: Track /proc/$pid/attr/ opener mm_struct

On Tue, Jun 15, 2021 at 11:19:04AM -0700, Linus Torvalds wrote:
> On Mon, Jun 14, 2021 at 6:55 PM youling 257 <youling257@...il.com> wrote:
> >
> > if try to find problem on userspace, i used linux 5.13rc6 on old
> > android 7 cm14.1, not aosp android 11.
> > http://git.osdn.net/view?p=android-x86/system-core.git;a=blob;f=init/service.cpp;h=a5334f447fc2fc34453d2f6a37523bedccadc690;hb=refs/heads/cm-14.1-x86#l457
> >
> >  457         if (!seclabel_.empty()) {
> >  458             if (setexeccon(seclabel_.c_str()) < 0) {
> >  459                 ERROR("cannot setexeccon('%s'): %s\n",
> >  460                       seclabel_.c_str(), strerror(errno));
> >  461                 _exit(127);
> >  462             }
> >  463         }
> 
> I have no idea where the cm14.1 libraries are. Does anybody know where
> the matching source code for setexeccon() would be?
> 
> For me - obviously not on cm14.1 - all "setexeccon()" does is
> 
>    n = openat(AT_FDCWD, "/proc/thread-self/attr/exec", O_RDWR|O_CLOEXEC)
>    write(n, string, len)
>    close(n)
> 
> and if that fails, it would seem to indicate that proc_mem_open()
> failed. Which would be mm_access() failing. But I don't see how that
> can be the case, because mm_access() explicitly allows "mm ==
> current->mm" (which the above clearly should be).

Yeah, that was what I saw too.

> youling, can you double-check with the current -git tree? But as far
> as I can tell, my minimal patch is exactly the same as Kees' patch
> (just smaller and simpler).

FWIW, for that patch:

Acked-by: Kees Cook <keescook@...omium.org>
Cc: stable@...r.kernel.org

> 
> Kees, do you see anything?

No, I haven't been able to reproduce the failure. :(

-- 
Kees Cook