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 <minhquangbui99@xxxxxxxxx> 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