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

[Bui Quang Minh]

On 1/28/21 7:41 AM, Daniel Borkmann wrote:
> 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").

Hi,

I almost forget about this patch, I have checked the bpf_map_area_alloc 
in in stackmap.c and I can see that integer overflow cannot happen in 
this stackmap.c case.

In stack_map_alloc(),

	u64 cost;
	...
	cost = n_buckets * sizeof(struct stack_map_bucket *) + sizeof(*smap);
	cost += n_buckets * (value_size + sizeof(struct stack_map_bucket));
	smap = bpf_map_area_alloc(cost, bpf_map_attr_numa_node(attr)); (1)
	...
	prealloc_elems_and_freelist(smap);

In prealloc_elems_and_freelist(),

	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); (2)

Argument calculation at (1) is safe. Argument calculation at (2) can 
potentially result in an integer overflow in 32-bit architecture. 
However, if the integer overflow happens, it means argument at (1) must 
be 2**32, which cannot pass the SIZE_MAX check in __bpf_map_area_alloc()

In __bpf_map_area_alloc()

	if (size >= SIZE_MAX)
		return NULL;

So I think the original patch has fixed instances of this bug pattern.

Thank you,
Quang Minh.