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

[Vlastimil Babka]

On 10/14/24 09:52, Feng Tang wrote:
> On Fri, Oct 04, 2024 at 05:52:10PM +0800, Vlastimil Babka wrote:
> 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);

I think this will result in __ksize() doing
  skip_orig_size_check(folio_slab(folio)->slab_cache, object);
and we don't want that?

Also the checks below repeat some of the checks of ksize().

So I think in __do_krealloc() we should do things manually to determine ks
and not call ksize(). Just not break any of the cases ksize() handles
(kfence, large kmalloc).

> 
> 		/* 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
>> > 
>>