Re: [PATCH v4 1/3] kasan: support backing vmalloc space with real shadow memory

[Andy Lutomirski <luto@kernel.org>]

On Fri, Aug 16, 2019 at 10:08 AM Mark Rutland <mark.rutland@arm.com> wrote:
>
> Hi Christophe,
>
> On Fri, Aug 16, 2019 at 09:47:00AM +0200, Christophe Leroy wrote:
> > Le 15/08/2019 à 02:16, Daniel Axtens a écrit :
> > > Hook into vmalloc and vmap, and dynamically allocate real shadow
> > > memory to back the mappings.
> > >
> > > Most mappings in vmalloc space are small, requiring less than a full
> > > page of shadow space. Allocating a full shadow page per mapping would
> > > therefore be wasteful. Furthermore, to ensure that different mappings
> > > use different shadow pages, mappings would have to be aligned to
> > > KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE.
> > >
> > > Instead, share backing space across multiple mappings. Allocate
> > > a backing page the first time a mapping in vmalloc space uses a
> > > particular page of the shadow region. Keep this page around
> > > regardless of whether the mapping is later freed - in the mean time
> > > the page could have become shared by another vmalloc mapping.
> > >
> > > This can in theory lead to unbounded memory growth, but the vmalloc
> > > allocator is pretty good at reusing addresses, so the practical memory
> > > usage grows at first but then stays fairly stable.
> >
> > I guess people having gigabytes of memory don't mind, but I'm concerned
> > about tiny targets with very little amount of memory. I have boards with as
> > little as 32Mbytes of RAM. The shadow region for the linear space already
> > takes one eighth of the RAM. I'd rather avoid keeping unused shadow pages
> > busy.
>
> I think this depends on how much shadow would be in constant use vs what
> would get left unused. If the amount in constant use is sufficiently
> large (or the residue is sufficiently small), then it may not be
> worthwhile to support KASAN_VMALLOC on such small systems.
>
> > Each page of shadow memory represent 8 pages of real memory. Could we use
> > page_ref to count how many pieces of a shadow page are used so that we can
> > free it when the ref count decreases to 0.
> >
> > > This requires architecture support to actually use: arches must stop
> > > mapping the read-only zero page over portion of the shadow region that
> > > covers the vmalloc space and instead leave it unmapped.
> >
> > Why 'must' ? Couldn't we switch back and forth from the zero page to real
> > page on demand ?
> >
> > If the zero page is not mapped for unused vmalloc space, bad memory accesses
> > will Oops on the shadow memory access instead of Oopsing on the real bad
> > access, making it more difficult to locate and identify the issue.
>
> I agree this isn't nice, though FWIW this can already happen today for
> bad addresses that fall outside of the usual kernel address space. We
> could make the !KASAN_INLINE checks resilient to this by using
> probe_kernel_read() to check the shadow, and treating unmapped shadow as
> poison.

Could we instead modify the page fault handlers to detect this case
and print a useful message?