On Mon, Jul 24, 2017 at 2:17 PM, Alexander Popov <alex.popov@xxxxxxxxx> wrote: > From 86f4f1f6deb76849e00c761fa30eeb479f789c35 Mon Sep 17 00:00:00 2001 > From: Alexander Popov <alex.popov@xxxxxxxxx> > Date: Mon, 24 Jul 2017 23:16:28 +0300 > Subject: [PATCH 2/2] mm/slub.c: add a naive detection of double free or > corruption > > On 06.07.2017 03:27, Kees Cook wrote: >> This SLUB free list pointer obfuscation code is modified from Brad >> Spengler/PaX Team's code in the last public patch of grsecurity/PaX based >> on my understanding of the code. Changes or omissions from the original >> code are mine and don't reflect the original grsecurity/PaX code. >> >> This adds a per-cache random value to SLUB caches that is XORed with >> their freelist pointer address and value. This adds nearly zero overhead >> and frustrates the very common heap overflow exploitation method of >> overwriting freelist pointers. A recent example of the attack is written >> up here: http://cyseclabs.com/blog/cve-2016-6187-heap-off-by-one-exploit >> >> This is based on patches by Daniel Micay, and refactored to minimize the >> use of #ifdef. > > Hello! > > This is an addition to the SLAB_FREELIST_HARDENED feature. I'm sending it > according the discussion here: > http://www.openwall.com/lists/kernel-hardening/2017/07/17/9 > > -- >8 -- > > Add an assertion similar to "fasttop" check in GNU C Library allocator > as a part of SLAB_FREELIST_HARDENED feature. An object added to a singly > linked freelist should not point to itself. That helps to detect some > double free errors (e.g. CVE-2017-2636) without slub_debug and KASAN. > > Signed-off-by: Alexander Popov <alex.popov@xxxxxxxxx> > --- > mm/slub.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/mm/slub.c b/mm/slub.c > index c92d636..f39d06e 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -290,6 +290,10 @@ static inline void set_freepointer(struct kmem_cache *s, > void *object, void *fp) > { > unsigned long freeptr_addr = (unsigned long)object + s->offset; > > +#ifdef CONFIG_SLAB_FREELIST_HARDENED > + BUG_ON(object == fp); /* naive detection of double free or corruption */ > +#endif > + > *(void **)freeptr_addr = freelist_ptr(s, fp, freeptr_addr); What happens if, instead of BUG_ON, we do: if (unlikely(WARN_RATELIMIT(object == fp, "double-free detected")) return; That would ignore adding it back to the list, since it's already there, yes? Or would this make SLUB go crazy? I can't tell from the accounting details around callers to set_freepointer(). I assume it's correct, since it's close to the same effect as BUG (i.e. we don't do the update, but the cache remains visible to the system) -Kees -- Kees Cook Pixel Security -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
