Re: [PATCH] dccp: handle invalid feature options length

[Gerrit Renker <gerrit@erg.abdn.ac.uk>]

Quoting Dan Rosenberg:
| A length of zero (after subtracting two for the type and len fields) for
| the DCCPO_{CHANGE,CONFIRM}_{L,R} options will cause an underflow due to
| the subtraction.  The subsequent code may read past the end of the
| options value buffer when parsing.  I'm unsure of what the consequences
| of this might be, but it's probably not good.
| 
Can you please check again: did you experience this condition, to me it 
seems the patch is based on reading this code.

In this case, please consider the condition before the switch statement:

                /* Check if this isn't a single byte option */
                if (opt > DCCPO_MAX_RESERVED) {
                        if (opt_ptr == opt_end)
                                goto out_nonsensical_length;

                        len = *opt_ptr++;
                        if (len < 2)
                                goto out_nonsensical_length;
                         
The described range (DCCPO_CHANGE_L = 32 ... DCCPO_CONFIRM_R = 35) is after DCCPO_MAX_RESERVED = 31,
so that the above test applies.

Hence for these option types the len is always at least 1, so that (len - 1) >= 0.

| --- a/net/dccp/options.c
| +++ b/net/dccp/options.c
| @@ -123,6 +123,8 @@ int dccp_parse_options(struct sock *sk, struct dccp_request_sock *dreq,
|  		case DCCPO_CHANGE_L ... DCCPO_CONFIRM_R:
|  			if (pkt_type == DCCP_PKT_DATA)      /* RFC 4340, 6 */
|  				break;
| +			if (len == 0)
| +				goto out_invalid_option;
|  			rc = dccp_feat_parse_options(sk, dreq, mandatory, opt,
|  						    *value, value + 1, len - 1);
|  			if (rc)
| 
| 

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