On Thu, Nov 30, 2017 at 11:08 PM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> On Fri, Dec 1, 2017 at 5:04 AM, Willem de Bruijn
> <willemdebruijn.kernel@xxxxxxxxx> wrote:
>> On Thu, Nov 30, 2017 at 7:46 PM, Jann Horn <jannh@xxxxxxxxxx> 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@xxxxxxxxxx>
>>> ---
>>> 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.
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