Re: [PATCH] usercopy: Skip HIGHMEM page checking

[Matthew Wilcox]

On Mon, Sep 16, 2019 at 08:05:00PM -0700, Kees Cook wrote:
> On Mon, Sep 16, 2019 at 05:32:09PM -0700, Matthew Wilcox wrote:
> > On Mon, Sep 16, 2019 at 02:32:56PM -0700, Kees Cook wrote:
> > > When running on a system with >512MB RAM with a 32-bit kernel built with:
> > > 
> > > 	CONFIG_DEBUG_VIRTUAL=y
> > > 	CONFIG_HIGHMEM=y
> > > 	CONFIG_HARDENED_USERCOPY=y
> > > 
> > > all execve()s will fail due to argv copying into kmap()ed pages, and on
> > > usercopy checking the calls ultimately of virt_to_page() will be looking
> > > for "bad" kmap (highmem) pointers due to CONFIG_DEBUG_VIRTUAL=y:
> > 
> > I don't understand why you want to skip the check.  We must not cross a
> > page boundary of a kmapped page.
> 
> That requires a new test which hasn't existed before. First I need to
> fix the bug, and then we can add a new test and get that into -next,
> etc.

I suppose that depends where your baseline is.  From the perspective
of "before Kees added this feature", your point of view makes sense.
>From the perspective of "what's been shipping for the last six months",
this is a case which has simply not happened before now (or we'd've seen
a bug report).

I don't think you need to change anything for check_page_span() to do
the right thing.  The rodata/data/bss checks will all fall through.
If the copy has the correct bounds, the 'wholly within one base page'
check will pass and it'll return.  If the copy does span a page,
the virt_to_head_page(end) call will return something bogus, then the
PageReserved and CMA test will cause the usercopy_abort() test to fail.

So I think your first patch is the right patch.