Re: [PATCH] sctp: fix a potential missing-check bug

[Wenwen Wang]

Hi Marcelo,

I guess I worked on an old version of the kernel. I will re-submit the
patch. Sorry :(

Wenwen

On Wed, May 2, 2018 at 6:23 PM, Marcelo Ricardo Leitner
<marcelo.leitner@xxxxxxxxx> wrote:
> Hi Wenwen,
>
> On Wed, May 02, 2018 at 05:12:45PM -0500, Wenwen Wang wrote:
>> In sctp_setsockopt_maxseg(), the integer 'val' is compared against min_len
>> and max_len to check whether it is in the appropriate range. If it is not,
>> an error code -EINVAL will be returned. This is enforced by a security
>> check. But, this check is only executed when 'val' is not 0. In fact, if
>
> Which makes sense, no? Especially if considering that 0 should be an
> allowed value as it turns off the user limit.
>
>> 'val' is 0, it will be assigned with a new value (if the return value of
>> the function sctp_id2assoc() is not 0) in the following execution. However,
>> this new value of 'val' is not checked before it is used to assigned to
>
> Which 'new value'? val is not set to something new during the
> function. It always contains the user supplied value.
>
>> asoc->user_frag. That means it is possible that the new value of 'val'
>> could be out of the expected range. This can cause security issues
>> such as buffer overflows, e.g., the new value of 'val' is used as an index
>> to access a buffer.
>>
>> This patch inserts a check for the new value of 'val' to see if it is in
>> the expected range. If it is not, an error code -EINVAL will be returned.
>>
>> Signed-off-by: Wenwen Wang <wang6495@xxxxxxx>
>> ---
>>  net/sctp/socket.c | 21 ++++++++++-----------
>>  1 file changed, 10 insertions(+), 11 deletions(-)
>>
>> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
>> index 80835ac..2beb601 100644
>> --- a/net/sctp/socket.c
>> +++ b/net/sctp/socket.c
>> @@ -3212,6 +3212,7 @@ static int sctp_setsockopt_maxseg(struct sock *sk, char __user *optval, unsigned
>>       struct sctp_af *af = sp->pf->af;
>>       struct sctp_assoc_value params;
>>       struct sctp_association *asoc;
>> +     int min_len, max_len;
>>       int val;
>>
>>       if (optlen == sizeof(int)) {
>> @@ -3231,19 +3232,15 @@ static int sctp_setsockopt_maxseg(struct sock *sk, char __user *optval, unsigned
>>               return -EINVAL;
>>       }
>>
>> -     if (val) {
>> -             int min_len, max_len;
>> +     min_len = SCTP_DEFAULT_MINSEGMENT - af->net_header_len;
>> +     min_len -= af->ip_options_len(sk);
>> +     min_len -= sizeof(struct sctphdr) +
>> +                sizeof(struct sctp_data_chunk);
>
> On which tree did you base your patch on? Your patch lacks a tag so it
> defaults to net-next, and I reworked this section on current net-next
> and these MTU calculcations are now handled by sctp_mtu_payload().
>
> But even for net tree, I don't understand which issue you're fixing
> here. Actually it seems to me that both codes seems to do the same
> thing.
>
>>
>> -             min_len = SCTP_DEFAULT_MINSEGMENT - af->net_header_len;
>> -             min_len -= af->ip_options_len(sk);
>> -             min_len -= sizeof(struct sctphdr) +
>> -                        sizeof(struct sctp_data_chunk);
>> +     max_len = SCTP_MAX_CHUNK_LEN - sizeof(struct sctp_data_chunk);
>>
>> -             max_len = SCTP_MAX_CHUNK_LEN - sizeof(struct sctp_data_chunk);
>> -
>> -             if (val < min_len || val > max_len)
>> -                     return -EINVAL;
>> -     }
>> +     if (val && (val < min_len || val > max_len))
>> +             return -EINVAL;
>>
>>       asoc = sctp_id2assoc(sk, params.assoc_id);
>>       if (asoc) {
>> @@ -3253,6 +3250,8 @@ static int sctp_setsockopt_maxseg(struct sock *sk, char __user *optval, unsigned
>>                       val -= sizeof(struct sctphdr) +
>>                              sctp_datachk_len(&asoc->stream);
>>               }
>> +             if (val < min_len || val > max_len)
>> +                     return -EINVAL;
>>               asoc->user_frag = val;
>>               asoc->frag_point = sctp_frag_point(asoc, asoc->pathmtu);
>>       } else {
>> --
>> 2.7.4
>>
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