Re: [PATCH] bpf: btf: Fix a missing-check bug

[Daniel Borkmann]

Hi Wenwen,

On 10/22/2018 05:57 PM, Y Song wrote:
> On Fri, Oct 19, 2018 at 3:30 PM Wenwen Wang <wang6495@xxxxxxx> wrote:
>>
>> In btf_parse(), the header of the user-space btf data 'btf_data' is firstly
>> parsed and verified through btf_parse_hdr(). In btf_parse_hdr(), the header
>> is copied from user-space 'btf_data' to kernel-space 'btf->hdr' and then
>> verified. If no error happens during the verification process, the whole
>> data of 'btf_data', including the header, is then copied to 'data' in
>> btf_parse(). It is obvious that the header is copied twice here. More
>> importantly, no check is enforced after the second copy to make sure the
>> headers obtained in these two copies are same. Given that 'btf_data'
>> resides in the user space, a malicious user can race to modify the header
>> between these two copies. By doing so, the user can inject inconsistent
>> data, which can cause undefined behavior of the kernel and introduce
>> potential security risk.
>>
>> To avoid the above issue, this patch rewrites the header after the second
>> copy, using 'btf->hdr', which is obtained in the first copy.
>>
>> Signed-off-by: Wenwen Wang <wang6495@xxxxxxx>
>> ---
>>  kernel/bpf/btf.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>> index 138f030..2a85f91 100644
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -2202,6 +2202,9 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
>>                 goto errout;
>>         }
>>
>> +       memcpy(data, &btf->hdr,
>> +               min_t(u32, btf->hdr.hdr_len, sizeof(btf->hdr)));
> 
> Could you restructure the code to memcpy the header followed by copying
> the rest of btf_data with copy_from_user? This way, each byte is only
> copied once.
> Could you add some comments right before memcpy so later people will know
> why we implement this way?

Thanks for the fix! Agree with Yonghong that we should rework this a bit, so
please respin a v2 with the feedback addressed, thanks.

>> +
>>         err = btf_parse_str_sec(env);
>>         if (err)
>>                 goto errout;
>> --
>> 2.7.4
>>