linux-kernel - Re: [PATCH] bpf: Fix integer overflow in argument calculation for bpf_map_area

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Date:   Thu, 28 Jan 2021 01:41:34 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Bui Quang Minh <minhquangbui99@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, hawk@...nel.org,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        kpsingh@...nel.org, Jakub Sitnicki <jakub@...udflare.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: Fix integer overflow in argument calculation for
 bpf_map_area_alloc

On 1/27/21 5:23 AM, Bui Quang Minh wrote:
> On Tue, Jan 26, 2021 at 09:36:57AM +0000, Lorenz Bauer wrote:
>> On Tue, 26 Jan 2021 at 08:26, Bui Quang Minh <minhquangbui99@...il.com> wrote:
>>>
>>> In 32-bit architecture, the result of sizeof() is a 32-bit integer so
>>> the expression becomes the multiplication between 2 32-bit integer which
>>> can potentially leads to integer overflow. As a result,
>>> bpf_map_area_alloc() allocates less memory than needed.
>>>
>>> Fix this by casting 1 operand to u64.
>>
>> Some quick thoughts:
>> * Should this have a Fixes tag?
> 
> Ok, I will add Fixes tag in later version patch.
> 
>> * Seems like there are quite a few similar calls scattered around
>> (cpumap, etc.). Did you audit these as well?
> 
[...]
> In cpumap,
> 
> 	static struct bpf_map *cpu_map_alloc(union bpf_attr *attr)
> 	{
> 		cmap->cpu_map = bpf_map_area_alloc(cmap->map.max_entries *
> 						   sizeof(struct bpf_cpu_map_entry *),
> 						   cmap->map.numa_node);
> 	}
> 
> I think this is safe because max_entries is not permitted to be larger than NR_CPUS.

Yes.

> In stackmap, there is a place that I'm not very sure about
> 
> 	static int prealloc_elems_and_freelist(struct bpf_stack_map *smap)
> 	{
> 		u32 elem_size = sizeof(struct stack_map_bucket) + smap->map.value_size;
> 		smap->elems = bpf_map_area_alloc(elem_size * smap->map.max_entries,
> 						 smap->map.numa_node);
> 	}
> 
> This is called after another bpf_map_area_alloc in stack_map_alloc(). In the first
> bpf_map_area_alloc() the argument is calculated in an u64 variable; so if in the second
> one, there is an integer overflow then the first one must be called with size > 4GB. I
> think the first one will probably fail (I am not sure about the actual limit of vmalloc()),
> so the second one might not be called.

I would sanity check this as well. Looks like k*alloc()/v*alloc() call sites typically
use array_size() which returns SIZE_MAX on overflow, 610b15c50e86 ("overflow.h: Add
allocation size calculation helpers").

Thanks,
Daniel