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

[Greg Kroah-Hartman]

On Mon, Dec 31, 2018 at 05:38:51PM +0800, joeyli wrote:
> 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.

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 :)

> 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.  

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.

> > 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.

Neither do I :)

so all should be fine, don't allow crazy executables with odd
permissions to be run by any user and you should be fine.  That's always
been the case, right?

thanks,

greg k-h