Code:  static int sanitize_ptr_alu(struct bpf_verifier_env *env,
 			    struct bpf_insn *insn,
 			    const struct bpf_reg_state *ptr_reg,
			    struct bpf_reg_state *dst_reg,
			    bool off_is_neg)
{
	struct bpf_verifier_state *vstate = env->cur_state;
	struct bpf_insn_aux_data *aux = cur_aux(env);
	bool ptr_is_dst_reg = ptr_reg == dst_reg;
	u8 opcode = BPF_OP(insn->code);
	u32 alu_state, alu_limit;
 	struct bpf_reg_state tmp;
 	bool ret;
 
	if (env->allow_ptr_leaks || BPF_SRC(insn->code) == BPF_K)
 		return 0;
 
 	/* We already marked aux for masking from non-speculative
	 * paths, thus we got here in the first place. We only care
	 * to explore bad access from here.
	 */
	if (vstate->speculative)
		goto do_sim;

	alu_state  = off_is_neg ? BPF_ALU_NEG_VALUE : 0;
	alu_state |= ptr_is_dst_reg ?
		     BPF_ALU_SANITIZE_SRC : BPF_ALU_SANITIZE_DST;
 
 	if (retrieve_ptr_limit(ptr_reg, &alu_limit, opcode, off_is_neg))
 		return 0;
	/* If we arrived here from different branches with different
	 * limits to sanitize, then this won't work.
	 */
	if (aux->alu_state &&
	    (aux->alu_state != alu_state ||
	     aux->alu_limit != alu_limit))
 		return -EACCES;
	/* Corresponding fixup done in fixup_bpf_calls(). */
	aux->alu_state = alu_state;
	aux->alu_limit = alu_limit;
 do_sim:
 	/* Simulate and find potential out-of-bounds access under
 	 * speculative execution from truncation as a result of
	 * masking when off was not within expected range. If off
	 * sits in dst, then we temporarily need to move ptr there
	 * to simulate dst (== 0) +/-= ptr. Needed, for example,
	 * for cases where we use K-based arithmetic in one direction
	 * and truncated reg-based in the other in order to explore
	 * bad access.
	 */
	if (!ptr_is_dst_reg) {
		tmp = *dst_reg;
		*dst_reg = *ptr_reg;
	}
	ret = push_stack(env, env->insn_idx + 1, env->insn_idx, true);
	if (!ptr_is_dst_reg)
		*dst_reg = tmp;
	return !ret ? -EFAULT : 0;
}

Vulnerability Type: 
CWE ID: CWE-189
Summary: kernel/bpf/verifier.c in the Linux kernel before 4.20.6 performs undesirable out-of-bounds speculation on pointer arithmetic in various cases, including cases of different branches with different state or limits to sanitize, leading to side-channel attacks.
Commit Message: bpf: fix sanitation of alu op with pointer / scalar type from different paths

While 979d63d50c0c ("bpf: prevent out of bounds speculation on pointer
arithmetic") took care of rejecting alu op on pointer when e.g. pointer
came from two different map values with different map properties such as
value size, Jann reported that a case was not covered yet when a given
alu op is used in both "ptr_reg += reg" and "numeric_reg += reg" from
different branches where we would incorrectly try to sanitize based
on the pointer's limit. Catch this corner case and reject the program
instead.

Fixes: 979d63d50c0c ("bpf: prevent out of bounds speculation on pointer arithmetic")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>