[PATCH 5.13 069/104] net: llc: fix skb_over_panic

Greg Kroah-Hartman
2021-08-02T14:12:59Z

From: Pavel Skripkin <paskripkin@gmail.com>

[ Upstream commit c7c9d2102c9c098916ab9e0ab248006107d00d6c ]

Syzbot reported skb_over_panic() in llc_pdu_init_as_xid_cmd(). The
problem was in wrong LCC header manipulations.

Syzbot's reproducer tries to send XID packet. llc_ui_sendmsg() is
doing following steps:

1. skb allocation with size = len + header size
len is passed from userpace and header size
is 3 since addr->sllc_xid is set.

2. skb_reserve() for header_len = 3
3. filling all other space with memcpy_from_msg()

Ok, at this moment we have fully loaded skb, only headers needs to be
filled.

Then code comes to llc_sap_action_send_xid_c(). This function pushes 3
bytes for LLC PDU header and initializes it. Then comes
llc_pdu_init_as_xid_cmd(). It initalizes next 3 bytes *AFTER* LLC PDU
header and call skb_push(skb, 3). This looks wrong for 2 reasons:

1. Bytes rigth after LLC header are user data, so this function
was overwriting payload.

2. skb_push(skb, 3) call can cause skb_over_panic() since
all free space was filled in llc_ui_sendmsg(). (This can
happen is user passed 686 len: 686 + 14 (eth header) + 3 (LLC
header) = 703. SKB_DATA_ALIGN(703) = 704)

So, in this patch I added 2 new private constansts: LLC_PDU_TYPE_U_XID
and LLC_PDU_LEN_U_XID. LLC_PDU_LEN_U_XID is used to correctly reserve
header size to handle LLC + XID case. LLC_PDU_TYPE_U_XID is used by
llc_pdu_header_init() function to push 6 bytes instead of 3. And finally
I removed skb_push() call from llc_pdu_init_as_xid_cmd().

This changes should not affect other parts of LLC, since after
all steps we just transmit buffer.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-and-tested-by: syzbot+5e5a981ad7cc54c4b2b4@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/net/llc_pdu.h | 31 +++++++++++++++++++++++--------
net/llc/af_llc.c | 10 +++++++++-
net/llc/llc_s_ac.c | 2 +-
3 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/include/net/llc_pdu.h b/include/net/llc_pdu.h
index c0f0a13ed818..49aa79c7b278 100644
--- a/include/net/llc_pdu.h
+++ b/include/net/llc_pdu.h
@@ -15,9 +15,11 @@
#include <linux/if_ether.h>

/* Lengths of frame formats */
-#define LLC_PDU_LEN_I 4 /* header and 2 control bytes */
-#define LLC_PDU_LEN_S 4
-#define LLC_PDU_LEN_U 3 /* header and 1 control byte */
+#define LLC_PDU_LEN_I 4 /* header and 2 control bytes */
+#define LLC_PDU_LEN_S 4
+#define LLC_PDU_LEN_U 3 /* header and 1 control byte */
+/* header and 1 control byte and XID info */
+#define LLC_PDU_LEN_U_XID (LLC_PDU_LEN_U + sizeof(struct llc_xid_info))
/* Known SAP addresses */
#define LLC_GLOBAL_SAP 0xFF
#define LLC_NULL_SAP 0x00 /* not network-layer visible */
@@ -50,9 +52,10 @@
#define LLC_PDU_TYPE_U_MASK 0x03 /* 8-bit control field */
#define LLC_PDU_TYPE_MASK 0x03

-#define LLC_PDU_TYPE_I 0 /* first bit */
-#define LLC_PDU_TYPE_S 1 /* first two bits */
-#define LLC_PDU_TYPE_U 3 /* first two bits */
+#define LLC_PDU_TYPE_I 0 /* first bit */
+#define LLC_PDU_TYPE_S 1 /* first two bits */
+#define LLC_PDU_TYPE_U 3 /* first two bits */
+#define LLC_PDU_TYPE_U_XID 4 /* private type for detecting XID commands */

#define LLC_PDU_TYPE_IS_I(pdu) \
((!(pdu->ctrl_1 & LLC_PDU_TYPE_I_MASK)) ? 1 : 0)
@@ -230,9 +233,18 @@ static inline struct llc_pdu_un *llc_pdu_un_hdr(struct sk_buff *skb)
static inline void llc_pdu_header_init(struct sk_buff *skb, u8 type,
u8 ssap, u8 dsap, u8 cr)
{
- const int hlen = type == LLC_PDU_TYPE_U ? 3 : 4;
+ int hlen = 4; /* default value for I and S types */
struct llc_pdu_un *pdu;

+ switch (type) {
+ case LLC_PDU_TYPE_U:
+ hlen = 3;
+ break;
+ case LLC_PDU_TYPE_U_XID:
+ hlen = 6;
+ break;
+ }
+
skb_push(skb, hlen);
skb_reset_network_header(skb);
pdu = llc_pdu_un_hdr(skb);
@@ -374,7 +386,10 @@ static inline void llc_pdu_init_as_xid_cmd(struct sk_buff *skb,
xid_info->fmt_id = LLC_XID_FMT_ID; /* 0x81 */
xid_info->type = svcs_supported;
xid_info->rw = rx_window << 1; /* size of receive window */
- skb_put(skb, sizeof(struct llc_xid_info));
+
+ /* no need to push/put since llc_pdu_header_init() has already
+ * pushed 3 + 3 bytes
+ */
}

/**
diff --git a/net/llc/af_llc.c b/net/llc/af_llc.c
index 7180979114e4..ac5cadd02cfa 100644
--- a/net/llc/af_llc.c
+++ b/net/llc/af_llc.c
@@ -98,8 +98,16 @@ static inline u8 llc_ui_header_len(struct sock *sk, struct sockaddr_llc *addr)
{
u8 rc = LLC_PDU_LEN_U;

- if (addr->sllc_test || addr->sllc_xid)
+ if (addr->sllc_test)
rc = LLC_PDU_LEN_U;
+ else if (addr->sllc_xid)
+ /* We need to expand header to sizeof(struct llc_xid_info)
+ * since llc_pdu_init_as_xid_cmd() sets 4,5,6 bytes of LLC header
+ * as XID PDU. In llc_ui_sendmsg() we reserved header size and then
+ * filled all other space with user data. If we won't reserve this
+ * bytes, llc_pdu_init_as_xid_cmd() will overwrite user data
+ */
+ rc = LLC_PDU_LEN_U_XID;
else if (sk->sk_type == SOCK_STREAM)
rc = LLC_PDU_LEN_I;
return rc;
diff --git a/net/llc/llc_s_ac.c b/net/llc/llc_s_ac.c
index b554f26c68ee..79d1cef8f15a 100644
--- a/net/llc/llc_s_ac.c
+++ b/net/llc/llc_s_ac.c
@@ -79,7 +79,7 @@ int llc_sap_action_send_xid_c(struct llc_sap *sap, struct sk_buff *skb)
struct llc_sap_state_ev *ev = llc_sap_ev(skb);
int rc;

- llc_pdu_header_init(skb, LLC_PDU_TYPE_U, ev->saddr.lsap,
+ llc_pdu_header_init(skb, LLC_PDU_TYPE_U_XID, ev->saddr.lsap,
ev->daddr.lsap, LLC_PDU_CMD);
llc_pdu_init_as_xid_cmd(skb, LLC_XID_NULL_CLASS_2, 0);
rc = llc_mac_hdr_init(skb, ev->saddr.mac, ev->daddr.mac);
--
2.30.2