bpf: Temporarily disable atomic operations in BPF arena

Currently, the x86 JIT handling PROBE_MEM32 tagged accesses is not
equipped to handle atomic accesses into PTR_TO_ARENA, as no PROBE_MEM32
tagging is performed and no handling is enabled for them.

This will lead to unsafety as the offset into arena will dereferenced
directly without turning it into a base + offset access into the arena
region.

Since the changes to the x86 JIT will be fairly involved, for now,
temporarily disallow use of PTR_TO_ARENA as the destination operand for
atomics until support is added to the JIT backend.

Fixes: 2fe99eb0 ("bpf: Add x86-64 JIT support for PROBE_MEM32 pseudo instructions.")
Reported-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Message-ID: <20240314174931.98702-1-puranjay12@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -5682,6 +5682,13 @@ static bool is_flow_key_reg(struct bpf_verifier_env *env, int regno)
 	return reg->type == PTR_TO_FLOW_KEYS;
 }
 
+static bool is_arena_reg(struct bpf_verifier_env *env, int regno)
+{
+	const struct bpf_reg_state *reg = reg_state(env, regno);
+
+	return reg->type == PTR_TO_ARENA;
+}
+
 static u32 *reg2btf_ids[__BPF_REG_TYPE_MAX] = {
 #ifdef CONFIG_NET
 	[PTR_TO_SOCKET] = &btf_sock_ids[BTF_SOCK_TYPE_SOCK],
@@ -7019,7 +7026,8 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
 	if (is_ctx_reg(env, insn->dst_reg) ||
 	    is_pkt_reg(env, insn->dst_reg) ||
 	    is_flow_key_reg(env, insn->dst_reg) ||
-	    is_sk_reg(env, insn->dst_reg)) {
+	    is_sk_reg(env, insn->dst_reg) ||
+	    is_arena_reg(env, insn->dst_reg)) {
 		verbose(env, "BPF_ATOMIC stores into R%d %s is not allowed\n",
 			insn->dst_reg,
 			reg_type_str(env, reg_state(env, insn->dst_reg)->type));