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Re: [Qemu-devel] [PATCH 2/4] json-streamer: Don't crash when input excee
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH 2/4] json-streamer: Don't crash when input exceeds nesting limit |
Date: |
Mon, 23 Nov 2015 18:21:36 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) |
Eric Blake <address@hidden> 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 <address@hidden>
>> Signed-off-by: Markus Armbruster <address@hidden>
>> ---
>> qobject/json-streamer.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Reviewed-by: Eric Blake <address@hidden>
>
> 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.
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Markus Armbruster <=