From: Oliver Hartkopp socketcan@hartkopp.net

Muyu Yu provided a POC where user root with CAP_NET_ADMIN can create a CAN frame modification rule that makes the data length code a higher value than the available CAN frame data size. In combination with a configured checksum calculation where the result is stored relatively to the end of the data (e.g. cgw_csum_xor_rel) the tail of the skb (e.g. frag_list pointer in skb_shared_info) can be rewritten which finally can cause a system crash.

Michael Kubecek suggested to drop frames that have a DLC exceeding the available space after the modification process and provided a patch that can handle CAN FD frames too. Within this patch we also limit the length for the checksum calculations to the maximum of Classic CAN data length (8).

CAN frames that are dropped by these additional checks are counted with the CGW_DELETED counter which indicates misconfigurations in can-gw rules.

This fixes CVE-2019-3701.

Reported-by: Muyu Yu ieatmuttonchuan@gmail.com Reported-by: Marcus Meissner meissner@suse.de Suggested-by: Michal Kubecek mkubecek@suse.cz Tested-by: Muyu Yu ieatmuttonchuan@gmail.com Tested-by: Oliver Hartkopp socketcan@hartkopp.net Signed-off-by: Oliver Hartkopp socketcan@hartkopp.net Cc: linux-stable stable@vger.kernel.org # >= v3.2 Signed-off-by: Marc Kleine-Budde mkl@pengutronix.de --- Hello,

I've removed the else from dlc length check. Keeps the code and the patch more readable.

Marc

Changes since v2: - add newline after goto (Tnx, Oliver) Changes since v1: - remove else from dlc length check.

net/can/gw.c | 30 +++++++++++++++++++++++++++--- 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/net/can/gw.c b/net/can/gw.c index faa3da88a127..53859346dc9a 100644 --- a/net/can/gw.c +++ b/net/can/gw.c @@ -416,13 +416,29 @@ static void can_can_gw_rcv(struct sk_buff *skb, void *data) while (modidx < MAX_MODFUNCTIONS && gwj->mod.modfunc[modidx]) (*gwj->mod.modfunc[modidx++])(cf, &gwj->mod);

- /* check for checksum updates when the CAN frame has been modified */ + /* Has the CAN frame been modified? */ if (modidx) { - if (gwj->mod.csumfunc.crc8) + /* get available space for the processed CAN frame type */ + int max_len = nskb->len - offsetof(struct can_frame, data); + + /* dlc may have changed, make sure it fits to the CAN frame */ + if (cf->can_dlc > max_len) + goto out_delete; + + /* check for checksum updates in classic CAN length only */ + if (gwj->mod.csumfunc.crc8) { + if (cf->can_dlc > 8) + goto out_delete; + (*gwj->mod.csumfunc.crc8)(cf, &gwj->mod.csum.crc8); + } + + if (gwj->mod.csumfunc.xor) { + if (cf->can_dlc > 8) + goto out_delete;

- if (gwj->mod.csumfunc.xor) (*gwj->mod.csumfunc.xor)(cf, &gwj->mod.csum.xor); + } }

/* clear the skb timestamp if not configured the other way */ @@ -434,6 +450,14 @@ static void can_can_gw_rcv(struct sk_buff *skb, void *data) gwj->dropped_frames++; else gwj->handled_frames++; + + return; + + out_delete: + /* delete frame due to misconfiguration */ + gwj->deleted_frames++; + kfree_skb(nskb); + return; }

static inline int cgw_register_filter(struct net *net, struct cgw_job *gwj)