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Subject: Re: [PATCH] Bluetooth: Verify that l2cap_get_conf_opt provides large
 enough buffer
From:   Marcel Holtmann <marcel@holtmann.org>
In-Reply-To: <20190118125342.GA31025@kroah.com>
Date:   Fri, 18 Jan 2019 14:12:23 +0100
Cc:     linux-bluetooth@vger.kernel.org
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References: <20190118124319.12187-1-marcel@holtmann.org>
 <20190118125342.GA31025@kroah.com>
To:     Greg KH <gregkh@linuxfoundation.org>
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Hi Greg,

>> The function l2cap_get_conf_opt will return L2CAP_CONF_OPT_SIZE + opt->len
>> as length value. The opt->len however is in control over the remote user
>> and can be used by an attacker to gain access beyond the bounds of the
>> actual packet.
>> 
>> To prevent any potential leak of heap memory, it is enough to check that
>> the resulting len calculation after calling l2cap_get_conf_opt is not
>> below zero. A well formed packet will always return >= 0 here and will
>> end with the length value being zero after the last option has been
>> parsed. In case of malformed packets messing with the opt->len field the
>> length value will become negative. If that is the case, then just abort
>> and ignore the option.
>> 
>> In case an attacker uses a too short opt->len value, then garbage will
>> be parsed, but that is protected by the unknown option handling and also
>> the option parameter size checks.
>> 
>> Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
>> ---
>> net/bluetooth/l2cap_core.c | 6 ++++++
>> 1 file changed, 6 insertions(+)
>> 
>> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
>> index 77799e7d5a34..ccdc5c67d22a 100644
>> --- a/net/bluetooth/l2cap_core.c
>> +++ b/net/bluetooth/l2cap_core.c
>> @@ -3337,6 +3337,8 @@ static int l2cap_parse_conf_req(struct l2cap_chan *chan, void *data, size_t data
>> 
>> 	while (len >= L2CAP_CONF_OPT_SIZE) {
>> 		len -= l2cap_get_conf_opt(&req, &type, &olen, &val);
>> +		if (len < 0)
>> +			break;
> 
> <snip>
> 
> Patch looks good to me, thanks for fixing this all up:

it would be still good if we can get this verified by the reporter.

Regards

Marcel