Re: [PATCH] ksys_mount: check for permissions before resource allocation

[Al Viro]

On Tue, Jun 05, 2018 at 04:07:15PM +0400, Ilya Matveychikov wrote:

> > 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 itâs 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 whatâs wrong with the idea of
> having the permission check before doing any kind of work?

Presenting that as mitigating a vulnerability.  It's neither better nor worse
in that respect than the original.