Re: [PATCH v3 0/3] Assorted improvements to usercopy

Kees Cook
2021-12-13T23:47:57Z

On Mon, Dec 13, 2021 at 09:16:46PM +0000, Matthew Wilcox wrote:
> On Mon, Dec 13, 2021 at 12:47:58PM -0800, Kees Cook wrote:
> > On Mon, Dec 13, 2021 at 08:27:42PM +0000, Matthew Wilcox wrote:
> > > On Mon, Dec 13, 2021 at 07:18:57PM +0000, William Kucharski wrote:
> > > > I like these, but a quick question:
> > > >
> > > > Since the usercopy_abort() calls are all because the offset exceeds the page
> > > > size, is there a reason why you don't specifically state that via the detail
> > > > parameter rather than just supply a NULL pointer?
> > >
> > > Hmm ... I'd defer to Kees on this, because I'm not familiar with
> > > usercopy_abort() usage, but the only places which use the detail
> > > parameter today are slab/slub, which use it to pass the name of
> > > the slab. I think the user is supposed to infer that we overran the
> > > end of the page based on the offset & length values.
> >
> > I agree that leaving it NULL is best here. The "detail" is really about
> > adding more information about which thing it was, which for slab makes
> > sense, but most other stuff there isn't really anything to quickly
> > distinguish one from another (i.e. vmap is all vmap).
>
> There _is_ a bit more information in the vmap case (not in the kmap
> or compound page case). You can see it in /proc/vmallocinfo. We
> could pass it in like this?
>
> if (is_vmalloc_addr(ptr)) {
> struct vm_struct *vm = find_vm_area(ptr);
> + char sym[100];
> unsigned long offset;
>
> if (!vm) {
> ..
> + if (vm->caller)
> + snprintf(sym, sizeof(sym), "%pS", vm->caller);
> offset = ptr - vm->addr;
> if (offset + n > vm->size)
> - usercopy_abort("vmalloc", NULL, to_user, offset, n);
> + usercopy_abort("vmalloc", vm->caller ? sym : NULL,
> + to_user, offset, n);

That is interesting, but I think we don't want to do it here; adding
to stack or making an allocation for this (even though it's slow-path)
doesn't seem like a good idea as far as keeping code size down.

-Kees

> return;
>

--
Kees Cook