bpf: reject stores into ctx via st and xadd

Alexei found that verifier does not reject stores into context
via BPF_ST instead of BPF_STX. And while looking at it, we
also should not allow XADD variant of BPF_STX.

The context rewriter is only assuming either BPF_LDX_MEM- or
BPF_STX_MEM-type operations, thus reject anything other than
that so that assumptions in the rewriter properly hold. Add
test cases as well for BPF selftests.

Fixes: d691f9e8d4 ("bpf: allow programs to write to certain skb fields")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -978,6 +978,13 @@ static bool is_pointer_value(struct bpf_verifier_env *env, int regno)
 	return __is_pointer_value(env->allow_ptr_leaks, cur_regs(env) + regno);
 }
 
+static bool is_ctx_reg(struct bpf_verifier_env *env, int regno)
+{
+	const struct bpf_reg_state *reg = cur_regs(env) + regno;
+
+	return reg->type == PTR_TO_CTX;
+}
+
 static int check_pkt_ptr_alignment(struct bpf_verifier_env *env,
 				   const struct bpf_reg_state *reg,
 				   int off, int size, bool strict)
@@ -1258,6 +1265,12 @@ static int check_xadd(struct bpf_verifier_env *env, int insn_idx, struct bpf_ins
 		return -EACCES;
 	}
 
+	if (is_ctx_reg(env, insn->dst_reg)) {
+		verbose(env, "BPF_XADD stores into R%d context is not allowed\n",
+			insn->dst_reg);
+		return -EACCES;
+	}
+
 	/* check whether atomic_add can read the memory */
 	err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
 			       BPF_SIZE(insn->code), BPF_READ, -1);
@@ -3993,6 +4006,12 @@ static int do_check(struct bpf_verifier_env *env)
 			if (err)
 				return err;
 
+			if (is_ctx_reg(env, insn->dst_reg)) {
+				verbose(env, "BPF_ST stores into R%d context is not allowed\n",
+					insn->dst_reg);
+				return -EACCES;
+			}
+
 			/* check that memory (dst_reg + off) is writeable */
 			err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
 					       BPF_SIZE(insn->code), BPF_WRITE,

tools/testing/selftests/bpf/test_verifier.c

@@ -2592,6 +2592,29 @@ static struct bpf_test tests[] = {
 		.result = ACCEPT,
 		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
 	},
+	{
+		"context stores via ST",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_ST_MEM(BPF_DW, BPF_REG_1, offsetof(struct __sk_buff, mark), 0),
+			BPF_EXIT_INSN(),
+		},
+		.errstr = "BPF_ST stores into R1 context is not allowed",
+		.result = REJECT,
+		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
+	},
+	{
+		"context stores via XADD",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_RAW_INSN(BPF_STX | BPF_XADD | BPF_W, BPF_REG_1,
+				     BPF_REG_0, offsetof(struct __sk_buff, mark), 0),
+			BPF_EXIT_INSN(),
+		},
+		.errstr = "BPF_XADD stores into R1 context is not allowed",
+		.result = REJECT,
+		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
+	},
 	{
 		"direct packet access: test1",
 		.insns = {
@@ -4312,7 +4335,8 @@ static struct bpf_test tests[] = {
 		.fixup_map1 = { 2 },
 		.errstr_unpriv = "R2 leaks addr into mem",
 		.result_unpriv = REJECT,
-		.result = ACCEPT,
+		.result = REJECT,
+		.errstr = "BPF_XADD stores into R1 context is not allowed",
 	},
 	{
 		"leak pointer into ctx 2",
@@ -4326,7 +4350,8 @@ static struct bpf_test tests[] = {
 		},
 		.errstr_unpriv = "R10 leaks addr into mem",
 		.result_unpriv = REJECT,
-		.result = ACCEPT,
+		.result = REJECT,
+		.errstr = "BPF_XADD stores into R1 context is not allowed",
 	},
 	{
 		"leak pointer into ctx 3",