Re: [PATCH 2/2] PM / Sleep: Check the file capability when writing wake lock interface

[joeyli <jlee@suse.com>]

Hi Greg,

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 exposed
> > by Andy Lutomirski in CVE-2013-1959.
> 
> 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
>

Sorry for I didn't provide clear explanation.

The privileged means CAP_BLOCK_SUSPEND but not file permission. The file permission
has already relaxed for non-root user. Then the expected behavior is that non-root
process must has CAP_BLOCK_SUSPEND capability for writing wake_lock sysfs.

But, the CAP_BLOCK_SUSPEND restrict can be bypassed:

int main(int argc, char* argv[])
{
        int fd, ret = 0;

        fd = open("/sys/power/wake_lock", O_RDWR);
        if (fd < 0)
                err(1, "open wake_lock");

        if (dup2(fd, 1) != 1)	// overwrite the stdout with wake_lock
                err(1, "dup2");
        sleep(1);
        execl("./string", "string");	//string has capability

        return ret;
}

This program is an unpriviledged process (has no CAP_BLOCK_SUSPEND), it opened
wake_lock sysfs and overwrited stdout. Then it executes the "string" program
that has CAP_BLOCK_SUSPEND. 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.  
 
> And you are checking the namespace of the person trying to do the write
> when the write happens, which is correct here, right?
> 
> If you really want to mess with wake locks in a namespaced environment,
> then put it in a real namespaced environment, which is {HUGE HINT} not
> sysfs.
>

I don't want to mess with wake locks in namespace.
 
> So no, this patch isn't ok...
>

Thanks a lot!
Joey Lee