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Commit f37a8cb8 authored by Daniel Borkmann's avatar Daniel Borkmann Committed by Alexei Starovoitov
Browse files

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: d691f9e8 ("bpf: allow programs to write to certain skb fields")
Reported-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent a2284d91

kernel/bpf/verifier.c

+19 −0
Original line number Diff line number Diff line
@@ -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

+27 −2
Original line number Diff line number Diff line
@@ -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",