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Subject[PATCH v2] socket: Provide put_cmsg_whitelist() for constant size copies
Most callers of put_cmsg() use a "sizeof(foo)" for the length argument.
But within put_cmsg(), the copy_to_user() call is made with a dynamic
length, as a result of the cmsg header calculations. This means that
hardened usercopy will examine the copy, even though it was technically
a fixed size and should be implicitly whitelisted.

Most callers of put_cmsg() are copying out of stack or kmalloc, so these
cases aren't a problem for hardened usercopy. However, some try to copy
out of the skbuff_head_cache slab, including the "cb" region. Since
whitelisting the slab area would leave other protocol definition of the
"cb" region exposed to usercopy bugs, this creates put_cmsg_whitelist(),
which internally uses sizeof() to provide a constant-sized length and
a stack bounce buffer, in order to explicitly whitelist an otherwise
disallowed slab region.

Original report was:

Bad or missing usercopy whitelist? Kernel memory exposure attempt detected from SLAB object 'skbuff_head_cache' (offset 64, size 16)!
WARNING: CPU: 0 PID: 3663 at mm/usercopy.c:81 usercopy_warn+0xdb/0x100 mm/usercopy.c:76
__check_heap_object+0x89/0xc0 mm/slab.c:4426
check_heap_object mm/usercopy.c:236 [inline]
__check_object_size+0x272/0x530 mm/usercopy.c:259
check_object_size include/linux/thread_info.h:112 [inline]
check_copy_size include/linux/thread_info.h:143 [inline]
copy_to_user include/linux/uaccess.h:154 [inline]
put_cmsg+0x233/0x3f0 net/core/scm.c:242
sock_recv_errqueue+0x200/0x3e0 net/core/sock.c:2913
packet_recvmsg+0xb2e/0x17a0 net/packet/af_packet.c:3296
sock_recvmsg_nosec net/socket.c:803 [inline]
sock_recvmsg+0xc9/0x110 net/socket.c:810
___sys_recvmsg+0x2a4/0x640 net/socket.c:2179
__sys_recvmmsg+0x2a9/0xaf0 net/socket.c:2287
SYSC_recvmmsg net/socket.c:2368 [inline]
SyS_recvmmsg+0xc4/0x160 net/socket.c:2352

Fixes: 6d07d1cd300f ("usercopy: Restrict non-usercopy caches to size 0")
Signed-off-by: Kees Cook <>
include/linux/socket.h | 8 ++++++++
net/core/sock.c | 4 +---
net/iucv/af_iucv.c | 5 ++---
net/netlink/af_netlink.c | 4 ++--
net/socket.c | 4 ++--
5 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index 9286a5a8c60c..1f52e998068b 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -343,6 +343,14 @@ struct ucred {

extern int move_addr_to_kernel(void __user *uaddr, int ulen, struct sockaddr_storage *kaddr);
extern int put_cmsg(struct msghdr*, int level, int type, int len, void *data);
+ * Provide a bounce buffer for copying cmsg data to userspace when the
+ * target memory isn't already whitelisted for hardened usercopy.
+ */
+#define put_cmsg_whitelist(_msg, _level, _type, _ptr) ({ \
+ typeof(*(_ptr)) _val = *(_ptr); \
+ put_cmsg(_msg, _level, _type, sizeof(_val), &_val); \
+ })

struct timespec;

diff --git a/net/core/sock.c b/net/core/sock.c
index f39206b41b32..d8a3228acfd0 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2879,7 +2879,6 @@ void sock_enable_timestamp(struct sock *sk, int flag)
int sock_recv_errqueue(struct sock *sk, struct msghdr *msg, int len,
int level, int type)
- struct sock_exterr_skb *serr;
struct sk_buff *skb;
int copied, err;

@@ -2899,8 +2898,7 @@ int sock_recv_errqueue(struct sock *sk, struct msghdr *msg, int len,

sock_recv_timestamp(msg, sk, skb);

- serr = SKB_EXT_ERR(skb);
- put_cmsg(msg, level, type, sizeof(serr->ee), &serr->ee);
+ put_cmsg_whitelist(msg, level, type, &SKB_EXT_ERR(skb)->ee);

msg->msg_flags |= MSG_ERRQUEUE;
err = copied;
diff --git a/net/iucv/af_iucv.c b/net/iucv/af_iucv.c
index 148533169b1d..676c019ba357 100644
--- a/net/iucv/af_iucv.c
+++ b/net/iucv/af_iucv.c
@@ -1407,9 +1407,8 @@ static int iucv_sock_recvmsg(struct socket *sock, struct msghdr *msg,
/* create control message to store iucv msg target class:
* get the trgcls from the control buffer of the skb due to
* fragmentation of original iucv message. */
- err = put_cmsg(msg, SOL_IUCV, SCM_IUCV_TRGCLS,
- sizeof(IUCV_SKB_CB(skb)->class),
- (void *)&IUCV_SKB_CB(skb)->class);
+ err = put_cmsg_whitelist(msg, SOL_IUCV, SCM_IUCV_TRGCLS,
+ &IUCV_SKB_CB(skb)->class);
if (err) {
if (!(flags & MSG_PEEK))
skb_queue_head(&sk->sk_receive_queue, skb);
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index b9e0ee4e22f5..4420dba35a44 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1781,8 +1781,8 @@ static void netlink_cmsg_listen_all_nsid(struct sock *sk, struct msghdr *msg,
if (!NETLINK_CB(skb).nsid_is_set)

- put_cmsg(msg, SOL_NETLINK, NETLINK_LISTEN_ALL_NSID, sizeof(int),
- &NETLINK_CB(skb).nsid);
+ put_cmsg_whitelist(msg, SOL_NETLINK, NETLINK_LISTEN_ALL_NSID,
+ &NETLINK_CB(skb).nsid);

static int netlink_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
diff --git a/net/socket.c b/net/socket.c
index 42d8e9c9ccd5..cb03ae055eb1 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -787,8 +787,8 @@ static inline void sock_recv_drops(struct msghdr *msg, struct sock *sk,
struct sk_buff *skb)
if (sock_flag(sk, SOCK_RXQ_OVFL) && skb && SOCK_SKB_CB(skb)->dropcount)
- put_cmsg(msg, SOL_SOCKET, SO_RXQ_OVFL,
- sizeof(__u32), &SOCK_SKB_CB(skb)->dropcount);
+ put_cmsg_whitelist(msg, SOL_SOCKET, SO_RXQ_OVFL,
+ &SOCK_SKB_CB(skb)->dropcount);

void __sock_recv_ts_and_drops(struct msghdr *msg, struct sock *sk,

Kees Cook
Pixel Security

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