Re: [PATCH] Bluetooth: Verify that l2cap_get_conf_opt provides large enough buffer

On Mon, Jan 21, 2019 at 03:51:11PM +0100, Marcel Holtmann wrote:

Hi Greg,

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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.

Sounds good to me, thanks.

greg k-h