Re: [PATCH] mm: Add additional consistency check

[Michal Hocko <mhocko@kernel.org>]

On Tue 11-04-17 07:14:01, Kees Cook wrote:
> On Tue, Apr 11, 2017 at 6:46 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > On Mon 10-04-17 21:58:22, Kees Cook wrote:
> >> On Tue, Apr 4, 2017 at 1:13 PM, Michal Hocko <mhocko@kernel.org> wrote:
> >> > On Tue 04-04-17 14:58:06, Cristopher Lameter wrote:
> >> >> On Tue, 4 Apr 2017, Michal Hocko wrote:
> >> >>
> >> >> > On Tue 04-04-17 14:13:06, Cristopher Lameter wrote:
> >> >> > > On Tue, 4 Apr 2017, Michal Hocko wrote:
> >> >> > >
> >> >> > > > Yes, but we do not have to blow the kernel, right? Why cannot we simply
> >> >> > > > leak that memory?
> >> >> > >
> >> >> > > Because it is a serious bug to attempt to free a non slab object using
> >> >> > > slab operations. This is often the result of memory corruption, coding
> >> >> > > errs etc. The system needs to stop right there.
> >> >> >
> >> >> > Why when an alternative is a memory leak?
> >> >>
> >> >> Because the slab allocators fail also in case you free an object multiple
> >> >> times etc etc. Continuation is supported by enabling a special resiliency
> >> >> feature via the kernel command line. The alternative is selectable but not
> >> >> the default.
> >> >
> >> > I disagree! We should try to continue as long as we _know_ that the
> >> > internal state of the allocator is still consistent and a further
> >> > operation will not spread the corruption even more. This is clearly not
> >> > the case for an invalid pointer to kfree.
> >> >
> >> > I can see why checking for an early allocator corruption is not always
> >> > feasible and you can only detect after-the-fact but this is not the case
> >> > here and putting your system down just because some buggy code is trying
> >> > to free something it hasn't allocated is not really useful. I completely
> >> > agree with Linus that we overuse BUG way too much and this is just
> >> > another example of it.
> >>
> >> Instead of the proposed BUG here, what's the correct "safe" return value?
> >
> > I would assume that _you_ as the one who proposes the change would take
> > some time to read and understand the code and know this answer. This is
> > how we do changes to the kernel: have an objective, understand the code
> > and generate the patch.
> >
> > I am really sad that this particular patch has shown that you didn't
> > bother to consider the later part and blindly applied something that you
> > haven't thought through properly. Please try harder next time.
> 
> Our objectives are different: I want the kernel to immediately stop
> when corruption is detected. Since others are interested in making it
> survivable, I was hoping to get a hint about what such an improvement
> would look like.

I do not think sprinkling BUG_ONs will help that objective. And BUG_ON
under IRQ disable is likely not helping an error survivable...

> Instead this condescending attitude, can you instead
> provide constructive help that will get our users closer to the safe
> kernel operation we're all interested in?

I would do something like...
---
diff --git a/mm/slab.c b/mm/slab.c
index bd63450a9b16..87c99a5e9e18 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -393,10 +393,15 @@ static inline void set_store_user_dirty(struct kmem_cache *cachep) {}
 static int slab_max_order = SLAB_MAX_ORDER_LO;
 static bool slab_max_order_set __initdata;
 
+static inline struct kmem_cache *page_to_cache(struct page *page)
+{
+	return page->slab_cache;
+}
+
 static inline struct kmem_cache *virt_to_cache(const void *obj)
 {
 	struct page *page = virt_to_head_page(obj);
-	return page->slab_cache;
+	return page_to_cache(page);
 }
 
 static inline void *index_to_obj(struct kmem_cache *cache, struct page *page,
@@ -3813,14 +3818,18 @@ void kfree(const void *objp)
 {
 	struct kmem_cache *c;
 	unsigned long flags;
+	struct page *page;
 
 	trace_kfree(_RET_IP_, objp);
 
 	if (unlikely(ZERO_OR_NULL_PTR(objp)))
 		return;
+	page = virt_to_head_page(obj);
+	if (CHECK_DATA_CORRUPTION(!PageSlab(page)))
+		return;
 	local_irq_save(flags);
 	kfree_debugcheck(objp);
-	c = virt_to_cache(objp);
+	c = page_to_cache(page);
 	debug_check_no_locks_freed(objp, c->object_size);
 
 	debug_check_no_obj_freed(objp, c->object_size);

-- 
Michal Hocko
SUSE Labs