Subject: Re: [PATCH net] bpf: fix allocation warnings in bpf maps and integer overflow

From: Daniel Borkmann <[email-masked]>
Date: Mon, 30 Nov 2015 15:34:35 +0100

On 11/30/2015 02:52 PM, Daniel Borkmann wrote:
> On 11/30/2015 01:59 AM, Alexei Starovoitov wrote:
> [...]
>> For large map->value_size the user space can trigger memory allocation warnings like:
> [...]
>
>> To avoid never succeeding kmalloc with order >= MAX_ORDER check that
>> elem->value_size and computed elem_size are within limits for both hash and
>> array type maps.
> [...]
>
>> Large value_size can cause integer overflows in elem_size and map.pages
>> formulas, so check for that as well.
> [...]
>
>> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
>> index 3f4c99e06c6b..b1e53b79c586 100644
>> --- a/kernel/bpf/arraymap.c
>> +++ b/kernel/bpf/arraymap.c
>> @@ -28,11 +28,17 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
>>           attr->value_size == 0)
>>           return ERR_PTR(-EINVAL);
>>
>> +    if (attr->value_size >= 1 << (KMALLOC_SHIFT_MAX - 1))
>> +        /* if value_size is bigger, the user space won't be able to
>> +         * access the elements.
>> +         */
>> +        return ERR_PTR(-E2BIG);
>> +
>
> Bit confused, given that in array map, we try kzalloc() with __GFP_NOWARN already
> and if that fails, we fall back to vzalloc(), it shouldn't trigger memory allocation
> warnings here ...
>
> Then, integer overflow in elem_size with round_up(attr->value_size, 8) could only
> result in 0, which is already tested below.
>
>>       elem_size = round_up(attr->value_size, 8);
>>
>>       /* check round_up into zero and u32 overflow */
>>       if (elem_size == 0 ||
>> -        attr->max_entries > (U32_MAX - sizeof(*array)) / elem_size)
>> +        attr->max_entries > (U32_MAX - PAGE_SIZE - sizeof(*array)) / elem_size)
>>           return ERR_PTR(-ENOMEM);
>
> ... and this change seems to be needed for the integer overflow in map.pages?
>
> So if the first check above intends to check for some size overflow (?), how is it
> then related to KMALLOC_SHIFT_MAX?

Ok, I see. The check and comment is related to the fact that when we do bpf(2)
syscall to lookup an element:

We call map_lookup_elem(), which does kmalloc() on the value_size.

So an individual entry lookup could fail with kmalloc() there, unrelated to an
individual map implementation.

Hmm, seems this patch fixes many things at once, maybe makes sense to split it?