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.
