Re: [PATCH bpf-next v2 4/5] bpf: lru: Fix unintended eviction when updating lru hash maps

[Leon Hwang <leon.hwang@linux.dev>]

On 2026/1/5 23:43, bot+bpf-ci@kernel.org wrote:
>> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
>> index d029690246f8..8665eb6b8a7d 100644
>> --- a/kernel/bpf/hashtab.c
>> +++ b/kernel/bpf/hashtab.c
> 
> [ ... ]
> 
>> +static int htab_lru_map_update_elem_in_place(struct bpf_htab *htab, void *key, void *value,
>> +					     u64 map_flags, struct bucket *b,
>> +					     struct hlist_nulls_head *head, u32 hash,
>> +					     bool percpu, bool onallcpus)
>> +{
>> +	struct htab_elem *l_new, *l_old, *l_free;
>> +	struct bpf_map *map = &htab->map;
>> +	u32 key_size = map->key_size;
>> +	struct bpf_lru_node *node;
>> +	unsigned long flags;
>> +	void *l_val;
>> +	int ret;
>> +
>> +	node = bpf_lru_pop_extra(&htab->lru);
>> +	if (!node)
>> +		return -ENOENT;
>> +
>> +	l_new = container_of(node, struct htab_elem, lru_node);
>> +	l_new->hash = hash;
>> +	memcpy(l_new->key, key, key_size);
>> +	if (!percpu) {
>> +		l_val = htab_elem_value(l_new, map->key_size);
>> +		copy_map_value(map, l_val, value);
>> +		bpf_obj_free_fields(map->record, l_val);
>> +	}
> 
> Should bpf_obj_free_fields() be called here on l_new? The value was just
> copied into l_new and l_new will be inserted into the hash table when
> l_old is found. Calling bpf_obj_free_fields() on l_new's value frees the
> special fields (kptr, bpf_list_head, etc.) that were just copied, then
> l_new is inserted with freed fields. This appears to be a use-after-free
> for maps with special fields.
> 
> Looking at htab_lru_map_update_elem(), the pattern is to copy the value
> and NOT call bpf_obj_free_fields() on the new element. The fields are
> only freed when an element is removed from the hash table.
> 

You're wrong here, that the copied value must be cleaned by
bpf_obj_free_fields().

>> +
>> +	ret = htab_lock_bucket(b, &flags);
>> +	if (ret)
>> +		goto err_lock_bucket;
>> +
>> +	l_old = lookup_elem_raw(head, hash, key, key_size);
>> +
>> +	ret = check_flags(htab, l_old, map_flags);
>> +	if (ret)
>> +		goto err;
>> +
>> +	if (l_old) {
>> +		bpf_lru_node_set_ref(&l_new->lru_node);
>> +		if (percpu) {
>> +			/* per-cpu hash map can update value in-place.
>> +			 * Keep the same logic in __htab_lru_percpu_map_update_elem().
>> +			 */
>> +			pcpu_copy_value(htab, htab_elem_get_ptr(l_old, key_size),
>> +					value, onallcpus);
>> +			l_free = l_new;
>> +		} else {
>> +			hlist_nulls_add_head_rcu(&l_new->hash_node, head);
>> +			hlist_nulls_del_rcu(&l_old->hash_node);
>> +			l_free = l_old;
>> +		}
>> +	} else {
>> +		ret = -ENOENT;
>> +	}
>> +
>> +err:
>> +	htab_unlock_bucket(b, flags);
>> +
>> +err_lock_bucket:
>> +	bpf_lru_push_free(&htab->lru, ret ? node : &l_free->lru_node);
>                                               ^^^^^^^^^^^^^^^^^^
> 
> When ret == 0 and !percpu, l_free points to l_old which is being removed
> from the hash table. The code calls bpf_lru_push_free() directly instead
> of htab_lru_push_free(). Looking at htab_lru_push_free():
> 
>     static void htab_lru_push_free(struct bpf_htab *htab, struct htab_elem *elem)
>     {
>         check_and_free_fields(htab, elem);
>         bpf_map_dec_elem_count(&htab->map);
>         bpf_lru_push_free(&htab->lru, &elem->lru_node);
>     }
> 
> It calls check_and_free_fields() to free special fields in the element
> before pushing to the free list. The code here bypasses that, which would
> leak special fields (kptr, etc.) in l_old when it has them.
> 
> Should this use htab_lru_push_free() instead of bpf_lru_push_free() for
> the non-error path where l_old is being freed?
> 

You're right here, that I did not free the special fields of l_old. But
htab_lru_push_free() shouldn't be used here for the non-error path,
because bpf_map_dec_elem_count() was not required here.

I'll fix it in the next revision.

Thanks,
Leon

>> +
>> +	return ret;
>> +}
> 
> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/20720201621