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Subject: Re: [PATCH 2/2] PM / Sleep: Check the file capability when writing wake lock interface
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20181231123310.GA3038@kroah.com>
Date:   Mon, 31 Dec 2018 08:31:05 -0700
Cc:     Jann Horn <jannh@google.com>, joeyli <jlee@suse.com>,
        Andy Lutomirski <luto@kernel.org>,
        "Lee, Chun-Yi" <joeyli.kernel@gmail.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Pavel Machek <pavel@ucw.cz>, Len Brown <len.brown@intel.com>,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Joe Perches <joe@perches.com>,
        Bart Van Assche <bvanassche@acm.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        linux-pm@vger.kernel.org, Chen Yu <yu.c.chen@intel.com>,
        Giovanni Gherdovich <ggherdovich@suse.cz>
Content-Transfer-Encoding: quoted-printable
Message-Id: <09310D00-114A-4A51-8E98-0B11F9D9541E@amacapital.net>
References: <20181230132856.24095-1-jlee@suse.com> <20181230132856.24095-3-jlee@suse.com> <20181230144835.GB18985@kroah.com> <20181231093851.GN3506@linux-l9pv.suse> <20181231104055.GB27420@kroah.com> <CAG48ez3-YEVsyuk1H6hjGy9Ogm-z46bmdnqy0JbW9YL297FsTw@mail.gmail.com> <20181231123310.GA3038@kroah.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Dec 31, 2018, at 5:33 AM, Greg Kroah-Hartman <gregkh@linuxfoundation.or=
g> wrote:
>=20
>> On Mon, Dec 31, 2018 at 01:02:35PM +0100, Jann Horn wrote:
>> On Mon, Dec 31, 2018 at 11:41 AM Greg Kroah-Hartman
>> <gregkh@linuxfoundation.org> wrote:
>>>=20
>>>> On Mon, Dec 31, 2018 at 05:38:51PM +0800, joeyli wrote:
>>>> Hi Greg,
>>>>=20
>>>>> On Sun, Dec 30, 2018 at 03:48:35PM +0100, Greg Kroah-Hartman wrote:
>>>>>> On Sun, Dec 30, 2018 at 09:28:56PM +0800, Lee, Chun-Yi wrote:
>>>>>> The wake lock/unlock sysfs interfaces check that the writer must has
>>>>>> CAP_BLOCK_SUSPEND capability. But the checking logic can be bypassed
>>>>>> by opening sysfs file within an unprivileged process and then writing=

>>>>>> the file within a privileged process. The tricking way has been expos=
ed
>>>>>> by Andy Lutomirski in CVE-2013-1959.
>>>>>=20
>>>>> Don't you mean "open by privileged and then written by unprivileged?"
>>>>> Or if not, exactly how is this a problem?  You check the capabilities
>>>>> when you do the write and if that is not allowed then, well
>>>>>=20
>>>>=20
>>>> Sorry for I didn't provide clear explanation.
>>>>=20
>>>> The privileged means CAP_BLOCK_SUSPEND but not file permission. The fil=
e permission
>>>> has already relaxed for non-root user. Then the expected behavior is th=
at non-root
>>>> process must has CAP_BLOCK_SUSPEND capability for writing wake_lock sys=
fs.
>>>>=20
>>>> But, the CAP_BLOCK_SUSPEND restrict can be bypassed:
>>>>=20
>>>> int main(int argc, char* argv[])
>>>> {
>>>>        int fd, ret =3D 0;
>>>>=20
>>>>        fd =3D open("/sys/power/wake_lock", O_RDWR);
>>>>        if (fd < 0)
>>>>                err(1, "open wake_lock");
>>>>=20
>>>>        if (dup2(fd, 1) !=3D 1) // overwrite the stdout with wake_lock
>>>>                err(1, "dup2");
>>>>        sleep(1);
>>>>        execl("./string", "string");  //string has capability
>>>>=20
>>>>        return ret;
>>>> }
>>>>=20
>>>> This program is an unpriviledged process (has no CAP_BLOCK_SUSPEND), it=
 opened
>>>> wake_lock sysfs and overwrited stdout. Then it executes the "string" pr=
ogram
>>>> that has CAP_BLOCK_SUSPEND.
>>>=20
>>> That's the problem right there, do not give CAP_BLOCK_SUSPEND rights to
>>> "string".  If any user can run that program, there's nothing the kernel
>>> can do about this, right?  Just don't allow that program on the system :=
)
>>>=20
>>>> The string program writes to stdout, which means that it writes to
>>>> wake_lock. So an unpriviledged opener can trick an priviledged writer
>>>> for writing sysfs.
>>>=20
>>> That sounds like a userspace program that was somehow given incorrect
>>> rights by the admin, and a user is taking advantage of it.  That's not
>>> the kernel's fault.
>>=20
>> Isn't it? Pretty much any setuid program will write to stdout or
>> stderr; even the glibc linker code does so if you set LD_DEBUG.
>> (Normally the output isn't entirely attacker-controlled, but it is in
>> the case of stuff like "procmail", which I think Debian still ships as
>> setuid root.) setuid programs should always be able to safely call
>> read() and write() on caller-provided file descriptors. Also, you're
>> supposed to be able to receive file descriptors over unix domain
>> sockets and then write to them without trusting the sender. Basically,
>> the ->read and ->write VFS handlers should never look at the caller's
>> credentials, only the opener's (with the exception of LSMs, which tend
>> to do weird things to the system's security model).
>=20
> So a root program gets the file handle to the sysfs file and then passes
> it off to a setuid program and the kernel should somehow protect from
> this?

Yes, the kernel should.  If the kernel wants to check caps, it should do it r=
ight.

Calling capable() from a .write handler is wrong, even in sysfs.

>=20
> I think that any sysfs file that is relying on the capable() check
> should just set their permissions properly first, and then it should be
> ok.
>=20

Probably true.=20


> thanks,
>=20
> greg k-h