bpf: allow programs to write to certain skb fields

allow programs read/write skb->mark, tc_index fields and
((struct qdisc_skb_cb *)cb)->data.

mark and tc_index are generically useful in TC.
cb[0]-cb[4] are primarily used to pass arguments from one
program to another called via bpf_tail_call() which can
be seen in sockex3_kern.c example.

All fields of 'struct __sk_buff' are readable to socket and tc_cls_act progs.
mark, tc_index are writeable from tc_cls_act only.
cb[0]-cb[4] are writeable by both sockets and tc_cls_act.

Add verifier tests and improve sample code.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 28 insertions, 9 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index cfd9a40b9a5a..039d866fd36a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1692,6 +1692,8 @@ static int do_check(struct verifier_env *env)
 			}
 
 		} else if (class == BPF_STX) {
+			enum bpf_reg_type dst_reg_type;
+
 			if (BPF_MODE(insn->code) == BPF_XADD) {
 				err = check_xadd(env, insn);
 				if (err)
@@ -1700,11 +1702,6 @@ static int do_check(struct verifier_env *env)
 				continue;
 			}
 
-			if (BPF_MODE(insn->code) != BPF_MEM ||
-			    insn->imm != 0) {
-				verbose("BPF_STX uses reserved fields\n");
-				return -EINVAL;
-			}
 			/* check src1 operand */
 			err = check_reg_arg(regs, insn->src_reg, SRC_OP);
 			if (err)
@@ -1714,6 +1711,8 @@ static int do_check(struct verifier_env *env)
 			if (err)
 				return err;
 
+			dst_reg_type = regs[insn->dst_reg].type;
+
 			/* check that memory (dst_reg + off) is writeable */
 			err = check_mem_access(env, insn->dst_reg, insn->off,
 					       BPF_SIZE(insn->code), BPF_WRITE,
@@ -1721,6 +1720,15 @@ static int do_check(struct verifier_env *env)
 			if (err)
 				return err;
 
+			if (insn->imm == 0) {
+				insn->imm = dst_reg_type;
+			} else if (dst_reg_type != insn->imm &&
+				   (dst_reg_type == PTR_TO_CTX ||
+				    insn->imm == PTR_TO_CTX)) {
+				verbose("same insn cannot be used with different pointers\n");
+				return -EINVAL;
+			}
+
 		} else if (class == BPF_ST) {
 			if (BPF_MODE(insn->code) != BPF_MEM ||
 			    insn->src_reg != BPF_REG_0) {
@@ -1839,12 +1847,18 @@ static int replace_map_fd_with_map_ptr(struct verifier_env *env)
 
 	for (i = 0; i < insn_cnt; i++, insn++) {
 		if (BPF_CLASS(insn->code) == BPF_LDX &&
-		    (BPF_MODE(insn->code) != BPF_MEM ||
-		     insn->imm != 0)) {
+		    (BPF_MODE(insn->code) != BPF_MEM || insn->imm != 0)) {
 			verbose("BPF_LDX uses reserved fields\n");
 			return -EINVAL;
 		}
 
+		if (BPF_CLASS(insn->code) == BPF_STX &&
+		    ((BPF_MODE(insn->code) != BPF_MEM &&
+		      BPF_MODE(insn->code) != BPF_XADD) || insn->imm != 0)) {
+			verbose("BPF_STX uses reserved fields\n");
+			return -EINVAL;
+		}
+
 		if (insn[0].code == (BPF_LD | BPF_IMM | BPF_DW)) {
 			struct bpf_map *map;
 			struct fd f;
@@ -1967,12 +1981,17 @@ static int convert_ctx_accesses(struct verifier_env *env)
 	struct bpf_prog *new_prog;
 	u32 cnt;
 	int i;
+	enum bpf_access_type type;
 
 	if (!env->prog->aux->ops->convert_ctx_access)
 		return 0;
 
 	for (i = 0; i < insn_cnt; i++, insn++) {
-		if (insn->code != (BPF_LDX | BPF_MEM | BPF_W))
+		if (insn->code == (BPF_LDX | BPF_MEM | BPF_W))
+			type = BPF_READ;
+		else if (insn->code == (BPF_STX | BPF_MEM | BPF_W))
+			type = BPF_WRITE;
+		else
 			continue;
 
 		if (insn->imm != PTR_TO_CTX) {
@@ -1982,7 +2001,7 @@ static int convert_ctx_accesses(struct verifier_env *env)
 		}
 
 		cnt = env->prog->aux->ops->
-			convert_ctx_access(insn->dst_reg, insn->src_reg,
+			convert_ctx_access(type, insn->dst_reg, insn->src_reg,
 					   insn->off, insn_buf);
 		if (cnt == 0 || cnt >= ARRAY_SIZE(insn_buf)) {
 			verbose("bpf verifier is misconfigured\n");