Re: [PATCH] netfilter: add overflow checks in xt_bpf.c

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Thu, Nov 30, 2017 at 11:08 PM, Jann Horn <jannh@google.com> wrote:
> On Fri, Dec 1, 2017 at 5:04 AM, Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
>> On Thu, Nov 30, 2017 at 7:46 PM, Jann Horn <jannh@google.com> wrote:
>>> Check whether inputs from userspace are too long (explicit length field too
>>> big or string not null-terminated) to avoid out-of-bounds reads.
>>>
>>> As far as I can tell, this can at worst lead to very limited kernel heap
>>> memory disclosure or oopses.
>>>
>>> This bug can be triggered by an unprivileged user even if the xt_bpf module
>>> is not loaded: iptables is available in network namespaces, and the xt_bpf
>>> module can be autoloaded.
>>>
>>> Triggering the bug with a classic BPF filter with fake length 0x1000 causes
>>> the following KASAN report:
>>>
>>> ==================================================================
>>> BUG: KASAN: slab-out-of-bounds in bpf_prog_create+0x84/0xf0
>>> Read of size 32768 at addr ffff8801eff2c494 by task test/4627
>>>
>>> CPU: 0 PID: 4627 Comm: test Not tainted 4.15.0-rc1+ #1
>>> [...]
>>> Call Trace:
>>>  dump_stack+0x5c/0x85
>>>  print_address_description+0x6a/0x260
>>>  kasan_report+0x254/0x370
>>>  ? bpf_prog_create+0x84/0xf0
>>>  memcpy+0x1f/0x50
>>>  bpf_prog_create+0x84/0xf0
>>>  bpf_mt_check+0x90/0xd6 [xt_bpf]
>>> [...]
>>> Allocated by task 4627:
>>>  kasan_kmalloc+0xa0/0xd0
>>>  __kmalloc_node+0x47/0x60
>>>  xt_alloc_table_info+0x41/0x70 [x_tables]
>>> [...]
>>> The buggy address belongs to the object at ffff8801eff2c3c0
>>>                 which belongs to the cache kmalloc-2048 of size 2048
>>> The buggy address is located 212 bytes inside of
>>>                 2048-byte region [ffff8801eff2c3c0, ffff8801eff2cbc0)
>>> [...]
>>> ==================================================================
>>>
>>> Fixes: e6f30c731718 ("netfilter: x_tables: add xt_bpf match")
>>> Signed-off-by: Jann Horn <jannh@google.com>
>>> ---
>>>  net/netfilter/xt_bpf.c | 6 ++++++
>>>  1 file changed, 6 insertions(+)
>>>
>>> diff --git a/net/netfilter/xt_bpf.c b/net/netfilter/xt_bpf.c
>>> index 041da0d9c06f..1f7fbd3c7e5a 100644
>>> --- a/net/netfilter/xt_bpf.c
>>> +++ b/net/netfilter/xt_bpf.c
>>> @@ -27,6 +27,9 @@ static int __bpf_mt_check_bytecode(struct sock_filter *insns, __u16 len,
>>>  {
>>>         struct sock_fprog_kern program;
>>>
>>> +       if (len > XT_BPF_MAX_NUM_INSTR)
>>> +               return -EINVAL;
>>> +
>>>         program.len = len;
>>>         program.filter = insns;
>>
>> Next, this calls bpf_prog_create, which calls bpf_check_basics_ok to verify len.
>
> Irrelevant:
>
>  - see the KASAN splat in the commit message
>  - bpf_check_basics_ok checks against BPF_MAXINSNS (4096), but a check against
>    XT_BPF_MAX_NUM_INSTR (64) is needed because that's the size of the
> member in the
>    input struct

Argh, of course. Thanks.