Re: [PATCH v2 0/5] mm/slub: Improve data handling of krealloc() when orig_size is enabled

[Feng Tang]

On Fri, Oct 04, 2024 at 05:52:10PM +0800, Vlastimil Babka wrote:
> On 10/4/24 11:18, Vlastimil Babka wrote:
> > On 10/4/24 08:44, Marco Elver wrote:
> > 
> > I think it's commit d0a38fad51cc7 doing in __do_krealloc()
> > 
> > -               ks = ksize(p);
> > +
> > +               s = virt_to_cache(p);
> > +               orig_size = get_orig_size(s, (void *)p);
> > +               ks = s->object_size;
> > 
> > so for kfence objects we don't get their actual allocation size but the
> > potentially larger bucket size?
> > 
> > I guess we could do:
> > 
> > ks = kfence_ksize(p) ?: s->object_size;
> > 
> > ?
> 
> Hmm this probably is not the whole story, we also have:
> 
> -               memcpy(ret, kasan_reset_tag(p), ks);
> +               if (orig_size)
> +                       memcpy(ret, kasan_reset_tag(p), orig_size);
> 
> orig_size for kfence will be again s->object_size so the memcpy might be a
> (read) buffer overflow from a kfence allocation.
> 
> I think get_orig_size() should perhaps return kfence_ksize(p) for kfence
> allocations, in addition to the change above.
> 
> Or alternatively we don't change get_orig_size() (in a different commit) at
> all, but __do_krealloc() will have an "if is_kfence_address()" that sets
> both orig_size and ks to kfence_ksize(p) appropriately. That might be easier
> to follow.

Thanks for the suggestion!

As there were error report about the NULL slab for big kmalloc object, how
about the following code for 

__do_krealloc(const void *p, size_t new_size, gfp_t flags)
{
	void *ret;
	size_t ks = 0;
	int orig_size = 0;
	struct kmem_cache *s = NULL;

	/* Check for double-free. */
	if (likely(!ZERO_OR_NULL_PTR(p))) {
		if (!kasan_check_byte(p))
			return NULL;

		ks = ksize(p);

		/* Some objects have no orig_size, like big kmalloc case */
		if (is_kfence_address(p)) {
			orig_size = kfence_ksize(p);
		} else if (virt_to_slab(p)) {
			s = virt_to_cache(p);
			orig_size = get_orig_size(s, (void *)p);
		}
	} else {
		goto alloc_new;
	}

	/* If the object doesn't fit, allocate a bigger one */
	if (new_size > ks)
		goto alloc_new;

	/* Zero out spare memory. */
	if (want_init_on_alloc(flags)) {
		kasan_disable_current();
		if (orig_size && orig_size < new_size)
			memset((void *)p + orig_size, 0, new_size - orig_size);
		else
			memset((void *)p + new_size, 0, ks - new_size);
		kasan_enable_current();
	}

	/* Setup kmalloc redzone when needed */
	if (s && slub_debug_orig_size(s) && !is_kfence_address(p)) {
		set_orig_size(s, (void *)p, new_size);
		if (s->flags & SLAB_RED_ZONE && new_size < ks)
			memset_no_sanitize_memory((void *)p + new_size,
						SLUB_RED_ACTIVE, ks - new_size);
	}

	p = kasan_krealloc((void *)p, new_size, flags);
	return (void *)p;

alloc_new:
	ret = kmalloc_node_track_caller_noprof(new_size, flags, NUMA_NO_NODE, _RET_IP_);
	if (ret && p) {
		/* Disable KASAN checks as the object's redzone is accessed. */
		kasan_disable_current();
		memcpy(ret, kasan_reset_tag(p), orig_size ?: ks);
		kasan_enable_current();
	}

	return ret;
}

I've run it with the reproducer of syzbot, so far the issue hasn't been
reproduced on my local machine.

Thanks,
Feng

> 
> But either way means rewriting 2 commits. I think it's indeed better to drop
> the series now from -next and submit a v3.
> 
> Vlastimil
> 
> >> Thanks,
> >> -- Marco
> > 
>