Re: [Qemu-devel] [PATCH 2/4] json-streamer: Don't crash when input exceeds nesting limit

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> On 10/29/2015 06:44 AM, Markus Armbruster wrote:
>> We limit nesting depth and input size to defend against input
>> triggering excessive heap or stack memory use (commit 29c75dd
>> json-streamer: limit the maximum recursion depth and maximum token
>> count).  However, when the nesting limit is exceeded,
>> parser_context_peek_token()'s assertion fails.
>> 
>> Broken in commit 65c0f1e "json-parser: don't replicate tokens at each
>> level of recursion".
>> 
>> To reproduce stuff 1025 open braces or brackets into QMP.
>> 
>> Fix by taking the error exit instead of the normal one.
>> 
>> Reported-by: Eric Blake <eblake@redhat.com>
>> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>> ---
>>  qobject/json-streamer.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> Reviewed-by: Eric Blake <eblake@redhat.com>
>
> However, a couple comments about the context:
>
>>    if (type == JSON_ERROR) {
>>        goto out_emit_bad;
>>    } else if (parser->brace_count < 0 ||
>>        parser->bracket_count < 0 ||
>>        (parser->brace_count == 0 &&
>>         parser->bracket_count == 0)) {
>>        goto out_emit;
>
> Should we go to out_emit_bad for brace_count/bracket_count < 0, and save
> out_emit only for the case where brace == bracket == 0?  Can we even
> trigger negative counts (probably by attempting unpaired "{]]", but will
> that trigger earlier errors?)

Fair questions.

Unpaired closing brace or bracket makes the count go negative.  A simple
test input is "}".  As far as I can tell, nothing breaks: we emit an
error, and the parser recovers.  Perhaps the error exit would be more
appropriate anyway, but I can't tell.  Let's leave it alone as long as
it works.

>>    } else if (parser->token_size > MAX_TOKEN_SIZE ||
>>               parser->bracket_count > MAX_NESTING ||
>>               parser->brace_count > MAX_NESTING) {
>>        /* Security consideration, we limit total memory allocated per object
>>         * and the maximum recursion depth that a message can force.
>>         */
>>        goto out_emit;
>>    }
>>
>>    return;
>>
>>out_emit_bad:
>>    /* clear out token list and tell the parser to emit and error
>
> Typo: s/and error/an error/

Fixing, thanks.