[PATCH bpf-next v7 03/13] bpf: stop unsafely accessing TCP fields in bpf callbacks

[Jason Xing <kerneljasonxing@gmail.com>]

The "allow_tcp_access" flag is added to indicate that the callback
site has a tcp_sock locked.

Applying the new member allow_tcp_access in the existing callbacks
where is_fullsock is set to 1 can help us stop UDP socket accessing
struct tcp_sock and stop TCP socket without sk lock protecting does
the similar thing, or else it could be catastrophe leading to panic.

To keep it simple, instead of distinguishing between read and write
access, we disallow all read/write access to the tcp_sock through
the older bpf_sock_ops ctx. The new timestamping callbacks can use
newer helpers to read everything from a sk (e.g. bpf_core_cast), so
nothing is lost.

Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
---
 include/linux/filter.h | 5 +++++
 include/net/tcp.h      | 1 +
 net/core/filter.c      | 8 ++++----
 net/ipv4/tcp_input.c   | 2 ++
 net/ipv4/tcp_output.c  | 2 ++
 5 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index a3ea46281595..1569e9f31a8c 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1508,6 +1508,11 @@ struct bpf_sock_ops_kern {
 	void	*skb_data_end;
 	u8	op;
 	u8	is_fullsock;
+	u8	allow_tcp_access;	/* Indicate that the callback site
+					 * has a tcp_sock locked. Then it
+					 * would be safe to access struct
+					 * tcp_sock.
+					 */
 	u8	remaining_opt_len;
 	u64	temp;			/* temp and everything after is not
 					 * initialized to 0 before calling
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 5b2b04835688..293047694710 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -2649,6 +2649,7 @@ static inline int tcp_call_bpf(struct sock *sk, int op, u32 nargs, u32 *args)
 	memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
 	if (sk_fullsock(sk)) {
 		sock_ops.is_fullsock = 1;
+		sock_ops.allow_tcp_access = 1;
 		sock_owned_by_me(sk);
 	}
 
diff --git a/net/core/filter.c b/net/core/filter.c
index 1c6c07507a78..dc0e67c5776a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -10381,10 +10381,10 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
 		}							      \
 		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(			      \
 						struct bpf_sock_ops_kern,     \
-						is_fullsock),		      \
+						allow_tcp_access),	      \
 				      fullsock_reg, si->src_reg,	      \
 				      offsetof(struct bpf_sock_ops_kern,      \
-					       is_fullsock));		      \
+					       allow_tcp_access));	      \
 		*insn++ = BPF_JMP_IMM(BPF_JEQ, fullsock_reg, 0, jmp);	      \
 		if (si->dst_reg == si->src_reg)				      \
 			*insn++ = BPF_LDX_MEM(BPF_DW, reg, si->src_reg,	      \
@@ -10469,10 +10469,10 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
 					       temp));			      \
 		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(			      \
 						struct bpf_sock_ops_kern,     \
-						is_fullsock),		      \
+						allow_tcp_access),	      \
 				      reg, si->dst_reg,			      \
 				      offsetof(struct bpf_sock_ops_kern,      \
-					       is_fullsock));		      \
+					       allow_tcp_access));	      \
 		*insn++ = BPF_JMP_IMM(BPF_JEQ, reg, 0, 2);		      \
 		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(			      \
 						struct bpf_sock_ops_kern, sk),\
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index eb82e01da911..77185479ed5e 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -169,6 +169,7 @@ static void bpf_skops_parse_hdr(struct sock *sk, struct sk_buff *skb)
 	memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
 	sock_ops.op = BPF_SOCK_OPS_PARSE_HDR_OPT_CB;
 	sock_ops.is_fullsock = 1;
+	sock_ops.allow_tcp_access = 1;
 	sock_ops.sk = sk;
 	bpf_skops_init_skb(&sock_ops, skb, tcp_hdrlen(skb));
 
@@ -185,6 +186,7 @@ static void bpf_skops_established(struct sock *sk, int bpf_op,
 	memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
 	sock_ops.op = bpf_op;
 	sock_ops.is_fullsock = 1;
+	sock_ops.allow_tcp_access = 1;
 	sock_ops.sk = sk;
 	/* sk with TCP_REPAIR_ON does not have skb in tcp_finish_connect */
 	if (skb)
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 0e5b9a654254..695749807c09 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -522,6 +522,7 @@ static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
 		sock_owned_by_me(sk);
 
 		sock_ops.is_fullsock = 1;
+		sock_ops.allow_tcp_access = 1;
 		sock_ops.sk = sk;
 	}
 
@@ -567,6 +568,7 @@ static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
 		sock_owned_by_me(sk);
 
 		sock_ops.is_fullsock = 1;
+		sock_ops.allow_tcp_access = 1;
 		sock_ops.sk = sk;
 	}
 
-- 
2.43.5