[PATCH bpf-next 1/4] xdp: Support specifying expected existing program when attaching XDP

["Toke Høiland-Jørgensen" <toke@redhat.com>]

From: Toke Høiland-Jørgensen <toke@redhat.com>

While it is currently possible for userspace to specify that an existing
XDP program should not be replaced when attaching to an interface, there is
no mechanism to safely replace a specific XDP program with another.

This patch adds a new netlink attribute, IFLA_XDP_EXPECTED_FD, which can be
set along with IFLA_XDP_FD. If set, the kernel will check that the program
currently loaded on the interface matches the expected one, and fail the
operation if it does not. This corresponds to a 'cmpxchg' memory operation.

A new companion flag, XDP_FLAGS_EXPECT_FD, is also added to explicitly
request checking of the EXPECTED_FD attribute. This is needed for userspace
to discover whether the kernel supports the new attribute.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 include/linux/netdevice.h    |    2 +-
 include/uapi/linux/if_link.h |    4 +++-
 net/core/dev.c               |   25 ++++++++++++++++++++-----
 net/core/rtnetlink.c         |   11 +++++++++++
 4 files changed, 35 insertions(+), 7 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index b6fedd54cd8e..40b12bd93913 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3767,7 +3767,7 @@ struct sk_buff *dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 
 typedef int (*bpf_op_t)(struct net_device *dev, struct netdev_bpf *bpf);
 int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
-		      int fd, u32 flags);
+		      int fd, int expected_fd, u32 flags);
 u32 __dev_xdp_query(struct net_device *dev, bpf_op_t xdp_op,
 		    enum bpf_netdev_command cmd);
 int xdp_umem_query(struct net_device *dev, u16 queue_id);
diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 61e0801c82df..314173f8079e 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -972,11 +972,12 @@ enum {
 #define XDP_FLAGS_SKB_MODE		(1U << 1)
 #define XDP_FLAGS_DRV_MODE		(1U << 2)
 #define XDP_FLAGS_HW_MODE		(1U << 3)
+#define XDP_FLAGS_EXPECT_FD		(1U << 4)
 #define XDP_FLAGS_MODES			(XDP_FLAGS_SKB_MODE | \
 					 XDP_FLAGS_DRV_MODE | \
 					 XDP_FLAGS_HW_MODE)
 #define XDP_FLAGS_MASK			(XDP_FLAGS_UPDATE_IF_NOEXIST | \
-					 XDP_FLAGS_MODES)
+					 XDP_FLAGS_MODES | XDP_FLAGS_EXPECT_FD)
 
 /* These are stored into IFLA_XDP_ATTACHED on dump. */
 enum {
@@ -996,6 +997,7 @@ enum {
 	IFLA_XDP_DRV_PROG_ID,
 	IFLA_XDP_SKB_PROG_ID,
 	IFLA_XDP_HW_PROG_ID,
+	IFLA_XDP_EXPECTED_FD,
 	__IFLA_XDP_MAX,
 };
 
diff --git a/net/core/dev.c b/net/core/dev.c
index 25dab1598803..44095326b8d5 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -8654,15 +8654,17 @@ static void dev_xdp_uninstall(struct net_device *dev)
  *	@dev: device
  *	@extack: netlink extended ack
  *	@fd: new program fd or negative value to clear
+ *	@expected_fd: old program fd that userspace expects to replace or clear
  *	@flags: xdp-related flags
  *
  *	Set or clear a bpf program for a device
  */
 int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
-		      int fd, u32 flags)
+		      int fd, int expected_fd, u32 flags)
 {
 	const struct net_device_ops *ops = dev->netdev_ops;
 	enum bpf_netdev_command query;
+	u32 prog_id, expected_id = 0;
 	struct bpf_prog *prog = NULL;
 	bpf_op_t bpf_op, bpf_chk;
 	bool offload;
@@ -8683,15 +8685,28 @@ int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
 	if (bpf_op == bpf_chk)
 		bpf_chk = generic_xdp_install;
 
-	if (fd >= 0) {
-		u32 prog_id;
+	prog_id = __dev_xdp_query(dev, bpf_op, query);
+	if (expected_fd >= 0 || (flags & XDP_FLAGS_EXPECT_FD)) {
+		if (expected_fd >= 0) {
+			prog = bpf_prog_get_type_dev(expected_fd, BPF_PROG_TYPE_XDP,
+						     bpf_op == ops->ndo_bpf);
+			if (IS_ERR(prog))
+				return PTR_ERR(prog);
+			expected_id = prog->aux->id;
+			bpf_prog_put(prog);
+		}
 
+		if (prog_id != expected_id) {
+			NL_SET_ERR_MSG(extack, "Active program does not match expected");
+			return -EEXIST;
+		}
+	}
+	if (fd >= 0) {
 		if (!offload && __dev_xdp_query(dev, bpf_chk, XDP_QUERY_PROG)) {
 			NL_SET_ERR_MSG(extack, "native and generic XDP can't be active at the same time");
 			return -EEXIST;
 		}
 
-		prog_id = __dev_xdp_query(dev, bpf_op, query);
 		if ((flags & XDP_FLAGS_UPDATE_IF_NOEXIST) && prog_id) {
 			NL_SET_ERR_MSG(extack, "XDP program already attached");
 			return -EBUSY;
@@ -8714,7 +8729,7 @@ int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
 			return 0;
 		}
 	} else {
-		if (!__dev_xdp_query(dev, bpf_op, query))
+		if (!prog_id)
 			return 0;
 	}
 
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 14e6ea21c378..09c08980f6f6 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -1873,6 +1873,7 @@ static const struct nla_policy ifla_port_policy[IFLA_PORT_MAX+1] = {
 
 static const struct nla_policy ifla_xdp_policy[IFLA_XDP_MAX + 1] = {
 	[IFLA_XDP_FD]		= { .type = NLA_S32 },
+	[IFLA_XDP_EXPECTED_FD]	= { .type = NLA_S32 },
 	[IFLA_XDP_ATTACHED]	= { .type = NLA_U8 },
 	[IFLA_XDP_FLAGS]	= { .type = NLA_U32 },
 	[IFLA_XDP_PROG_ID]	= { .type = NLA_U32 },
@@ -2799,8 +2800,18 @@ static int do_setlink(const struct sk_buff *skb,
 		}
 
 		if (xdp[IFLA_XDP_FD]) {
+			int expected_fd = -1;
+
+			if (xdp[IFLA_XDP_EXPECTED_FD]) {
+				expected_fd = nla_get_s32(xdp[IFLA_XDP_EXPECTED_FD]);
+			} else if(xdp_flags & XDP_FLAGS_EXPECT_FD) {
+				err = -EINVAL;
+				goto errout;
+			}
+
 			err = dev_change_xdp_fd(dev, extack,
 						nla_get_s32(xdp[IFLA_XDP_FD]),
+						expected_fd,
 						xdp_flags);
 			if (err)
 				goto errout;