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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: <20190118150044.GA31656@kroah.com>
Date:   Mon, 21 Jan 2019 15:51:11 +0100
Cc:     linux-bluetooth@vger.kernel.org
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References: <20190118124319.12187-1-marcel@holtmann.org>
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 <93BDB9CB-55F3-415A-8EED-DD532C36BC8A@holtmann.org>
 <20190118150044.GA31656@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.
> 
> The "reporter" seems to have disappeared once they reported this stuff,
> so I would not count on them doing anything here, we asked numerous
> times :(
> 
> If the patches look correct, I recommend just merging it and I can
> backport it to the stable releases and the distros can pick it up from
> there.

I think that I have Johan merge them into bluetooth-next first and let them sit a little so we can verify they do not break anything else. You can pick them up later into -stable if nobody complained and we didn’t break qualification.

Regards

Marcel