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Commit f531fbb0 authored by Daniel Borkmann's avatar Daniel Borkmann Committed by Greg Kroah-Hartman
Browse files

bpf: reject stores into ctx via st and xadd 



[ upstream commit f37a8cb84cce18762e8f86a70bd6a49a66ab964c ]

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>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 265d7657

kernel/bpf/verifier.c

+19 −0
Original line number Diff line number Diff line
@@ -702,6 +702,13 @@ static bool is_pointer_value(struct bpf_verifier_env *env, int regno) 
	return __is_pointer_value(env->allow_ptr_leaks, &env->cur_state.regs[regno]);

}



static bool is_ctx_reg(struct bpf_verifier_env *env, int regno)

{

	const struct bpf_reg_state *reg = &env->cur_state.regs[regno];



	return reg->type == PTR_TO_CTX;

}



static int check_ptr_alignment(struct bpf_verifier_env *env,

			       struct bpf_reg_state *reg, int off, int size)

{
@@ -896,6 +903,12 @@ static int check_xadd(struct bpf_verifier_env *env, struct bpf_insn *insn) 
		return -EACCES;

	}



	if (is_ctx_reg(env, insn->dst_reg)) {

		verbose("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->dst_reg, insn->off,

			       BPF_SIZE(insn->code), BPF_READ, -1);
@@ -3012,6 +3025,12 @@ static int do_check(struct bpf_verifier_env *env) 
			if (err)

				return err;



			if (is_ctx_reg(env, insn->dst_reg)) {

				verbose("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->dst_reg, insn->off,

					       BPF_SIZE(insn->code), BPF_WRITE,