On Tue, Jun 05, 2018 at 04:07:15PM +0400, Ilya Matveychikov wrote: > > On Jun 5, 2018, at 3:53 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > On Tue, Jun 05, 2018 at 03:35:55PM +0400, Ilya Matveychikov wrote: > >>> On Jun 5, 2018, at 3:26 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > >>>>> On Jun 5, 2018, at 6:00 AM, Ilya Matveychikov <matvejchikov@xxxxxxxxx> wrote: > >>>>> Early check for mount permissions prevents possible allocation of 3 > >>>>> pages from kmalloc() pool by unpriveledged user which can be used for > >>>>> spraying the kernel heap. > >>> > >>> I'm sorry, but there are arseloads of unpriveleged syscalls that do the same, > >>> starting with read() from procfs files. So what the hell does it buy? > >> > >> Means that if all do the same shit no reason to fix it? Sounds weird... > > > > Fix *what*? You do realize that there's no permission checks to stop e.g. > > stat(2) from copying the pathname in, right? With user-supplied contents, > > even... > > > > If you depend upon preventing kmalloc'ed temporary allocations filled > > with user-supplied data, you are screwed, plain and simple. It really can't > > be prevented, in a lot of ways that are much less exotic than mount(2). > > Most of syscall arguments are copied in, before we get any permission > > checks. It does happen and it will happen - examining them while they are > > still in userland is a nightmare in a lot of respects, starting with > > security. > > I agree that it’s impossible to completely avoid this kind of allocations > and examining data in user-land will be the bigger problem than copying > arguments to the kernel. But aside of that what’s wrong with the idea of > having the permission check before doing any kind of work? Isn't there some sysctl knob or config option to sanitize freed memory? I doubt that using kzfree everywhere unconditionally would be welcome, also would not scale as there are too many of them. This IMHO leaves only the build-time option for those willing to pay the performance hit. > BTW, sys_umount() has this check in the right place - before doing anything. > So, why not to have the same logic for mount/umount? What if the check is not equivalent to the one done later? may_mount needs namespace, it will be available at umount time but not necessarily during mount due to the security hooks.
