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

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Message-ID: <202106141503.B3144DFE@keescook>
Date:   Mon, 14 Jun 2021 15:50:50 -0700
From:   Kees Cook <keescook@...omium.org>
To:     youling 257 <youling257@...il.com>
Cc:     torvalds@...ux-foundation.org, christian.brauner@...ntu.com,
        andrea.righi@...onical.com, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, regressions@...ts.linux.dev,
        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 02:46:19AM +0800, youling 257 wrote:
> I test this patch cause "init: cannot setexeccon(u:r:ueventd:s0)
> operation not permitted.
> init ctrl_write_limited.

Thanks for testing!

This appears to come from here:
https://github.com/aosp-mirror/platform_system_core/blob/master/init/service.cpp#L242


In setexeccon(), I see (pid=0, attr="exec"):

        fd = openattr(pid, attr, O_RDWR | O_CLOEXEC);
...
                        ret = write(fd, context2, strlen(context2) + 1);
...
        close(fd);


and openattr() is doing:
...
                rc = asprintf(&path, "/proc/thread-self/attr/%s", attr);
                if (rc < 0)
                        return -1;
                fd = open(path, flags | O_CLOEXEC);
...

I'm not sure how the above could fail. (mm_access() always allows
introspection...)

The only way I can understand the check failing is if a process did:

open, exec, write

But setexeccon() is not doing anything between the open and the write...

I will keep looking...

-Kees

-- 
Kees Cook