Re: [PATCH] bpf: Fix integer overflow in argument calculation for bpf_map_area_alloc

[Daniel Borkmann]

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 <minhquangbu...@gmail.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