On Mon, Jan 11, 2016 at 12:10 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > On Sun, Jan 10, 2016 at 10:51:52PM +0300, Konstantin Khlebnikov wrote: >> On Sun, Jan 10, 2016 at 10:30 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: >> > On Sun, Jan 10, 2016 at 06:48:32PM +0300, Konstantin Khlebnikov wrote: >> >> I think this should be done in mmap/mprotect. Code in sys_mmap is trivial. >> >> >> >> In sys_mprotect you can check file_needs_remove_privs() and VM_SHARED >> >> under mmap_sem, then if needed grab reference to struct file from vma and >> >> clear suid after unlocking mmap_sem. >> > >> > Which vma? mprotect(2) can cover more than one mapping... You'd have to >> > play interesting games to collect the set of affected struct file; it >> > _might_ be doable (e.g. by using task_work_add() to have the damn thing >> > trigger on the way to userland), but it would require some care to avoid >> > hitting the same file more than once - it might, after all, be mmapped >> > in more than one process, so racing mprotect() would need to be taken >> > into account. Hell knows - might be doable, but I'm not sure it'll be >> > any prettier. >> >> Ok, I didn't thought about that. mprotect don't have to be atomic for whole >> range -- we could drop mmap_sem, clear suid from one file and restart it >> for next vma and so on. > > Won't be fun. Even aside of the user-visible behaviour changes, you'll have > a lot of new corner cases, starting with the fact that you can't hold onto > vma - virtual address is the best you can do and vma you find after regaining > mmap_sem might start at lower address than one where you are restarting; > getting the splitting-related logics right will be interesting, to put it > mildly. I don't see any problems here -- in this case mprotect virtually turns into series of indendent mprotect calls. Yes, we have to find vma again. Not a big deal. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
