Re: [PATCH] bpf: Fix integer overflow in prealloc_elems_and_freelist()

[Daniel Borkmann]

On 9/25/21 7:31 AM, Tatsuhiko Yasumatsu wrote:
> In prealloc_elems_and_freelist(), the multiplication to calculate the
> size passed to bpf_map_area_alloc() could lead to an integer overflow.
> As a result, out-of-bounds write could occur in pcpu_freelist_populate()
> as reported by KASAN:
>
> [...]
> [   16.968613] BUG: KASAN: slab-out-of-bounds in pcpu_freelist_populate+0xd9/0x100
> [   16.969408] Write of size 8 at addr ffff888104fc6ea0 by task crash/78
> [   16.970038]
> [   16.970195] CPU: 0 PID: 78 Comm: crash Not tainted 5.15.0-rc2+ #1
> [   16.970878] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
> [   16.972026] Call Trace:
> [   16.972306]  dump_stack_lvl+0x34/0x44
> [   16.972687]  print_address_description.constprop.0+0x21/0x140
> [   16.973297]  ? pcpu_freelist_populate+0xd9/0x100
> [   16.973777]  ? pcpu_freelist_populate+0xd9/0x100
> [   16.974257]  kasan_report.cold+0x7f/0x11b
> [   16.974681]  ? pcpu_freelist_populate+0xd9/0x100
> [   16.975190]  pcpu_freelist_populate+0xd9/0x100
> [   16.975669]  stack_map_alloc+0x209/0x2a0
> [   16.976106]  __sys_bpf+0xd83/0x2ce0
> [...]
>
> The possibility of this overflow was originally discussed in [0], but
> was overlooked.
>
> Fix the integer overflow by casting one operand to u64.
>
> [0] https://lore.kernel.org/bpf/728b238e-a481-eb50-98e9-b0f430ab01e7@xxxxxxxxx/
>
> Fixes: 557c0c6e7df8 ("bpf: convert stackmap to pre-allocation")
> Signed-off-by: Tatsuhiko Yasumatsu <th.yasumatsu@xxxxxxxxx>
> ---
>   kernel/bpf/stackmap.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 09a3fd97d329..8941dc83a769 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -66,7 +66,7 @@ static int prealloc_elems_and_freelist(struct bpf_stack_map *smap)
>   	u32 elem_size = sizeof(struct stack_map_bucket) + smap->map.value_size;

Thanks a lot for the fix, Tatsuhiko! Could we just change the above elem_size to u64 instead?

>   	int err;
>   
> -	smap->elems = bpf_map_area_alloc(elem_size * smap->map.max_entries,
> +	smap->elems = bpf_map_area_alloc((u64)elem_size * smap->map.max_entries,
>   					 smap->map.numa_node);
>   	if (!smap->elems)
>   		return -ENOMEM;

Best,
Daniel