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