crypto: Kernel memory overwrite attempt detected to spans multiple pages

[Eric Biggers]

On Wed, Apr 10, 2019 at 02:57:46PM -0700, Kees Cook wrote:
> On Wed, Apr 10, 2019 at 12:07 PM Eric Biggers <ebiggers at kernel.org> wrote:
> > That didn't answer my question.  My question is what is the purpose of this?  If
> > there was actual buffer overflow when __GFP_COMP isn't specified that would make
> > perfect sense, but AFAICS there isn't.  So why does hardened usercopy consider
> > it broken when __GFP_COMP isn't specified?
> 
> The goal of CONFIG_HARDENED_USERCOPY_PAGESPAN was to detect copies
> across page boundaries in memory allocated by the page allocator.
> There appear to be enough cases of allocations that span pages but do
> not mark them with __GFP_COMP, so this logic hasn't proven useful in
> the real world (which is why no one should use the ..._PAGESPAN config
> in production). I'd like to get the kernel to the point where hardened
> usercopy can correctly do these checks (right now it's mainly only
> useful at checking for overflows in slub and slab), but it'll take
> time/focus for a while. No one has had time yet to track all of these
> down and fix them. (I defer to Laura and Rik on the design of the
> pagespan checks; they did the bulk of the work there.)
> 
> Does that help explain it, or am I still missing your question?
> 
> -- 
> Kees Cook

You've explained *what* it does again, but not *why*.  *Why* do you want
hardened usercopy to detect copies across page boundaries, when there is no
actual buffer overflow?

- Eric