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?
I spotted another bug after re-auditting. In hashtab, there ares 2 places using
the same calls
static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
{
/* ... snip ... */
if (htab->n_buckets == 0 ||
htab->n_buckets > U32_MAX / sizeof(struct bucket))
goto free_htab;
htab->buckets = bpf_map_area_alloc(htab->n_buckets *
sizeof(struct bucket),
htab->map.numa_node);
}
This is safe because of the above check.
static int prealloc_init(struct bpf_htab *htab)
{
u32 num_entries = htab->map.max_entries;
htab->elems = bpf_map_area_alloc(htab->elem_size * num_entries,
htab->map.numa_node);
}
This is not safe since there is no limit check in elem_size.
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.
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.
Overall, I think it is error prone in this pattern, maybe we should use typecasting in all
similar calls or make a comment why we don't use typecasting. As I see typecasting is not so
expensive and we can typecast the sizeof() operand so this change only affect 32-bit
architecture.
> * I'd prefer a calloc style version of bpf_map_area_alloc although
> that might conflict with Fixes tag.
Yes, I think the calloc style will prevent this kind of integer overflow bug.
Thank you,
Quang Minh.
