bpf: arsh is not supported in 32 bit alu thus reject it

The following snippet was throwing an 'unknown opcode cc' warning
in BPF interpreter:

  0: (18) r0 = 0x0
  2: (7b) *(u64 *)(r10 -16) = r0
  3: (cc) (u32) r0 s>>= (u32) r0
  4: (95) exit

Although a number of JITs do support BPF_ALU | BPF_ARSH | BPF_{K,X}
generation, not all of them do and interpreter does neither. We can
leave existing ones and implement it later in bpf-next for the
remaining ones, but reject this properly in verifier for the time
being.

Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)")
Reported-by: syzbot+93c4904c5c70348a6890@syzkaller.appspotmail.com
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -2493,6 +2493,11 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 			return -EINVAL;
 		}
 
+		if (opcode == BPF_ARSH && BPF_CLASS(insn->code) != BPF_ALU64) {
+			verbose(env, "BPF_ARSH not supported for 32 bit ALU\n");
+			return -EINVAL;
+		}
+
 		if ((opcode == BPF_LSH || opcode == BPF_RSH ||
 		     opcode == BPF_ARSH) && BPF_SRC(insn->code) == BPF_K) {
 			int size = BPF_CLASS(insn->code) == BPF_ALU64 ? 64 : 32;

tools/testing/selftests/bpf/test_verifier.c

@@ -272,6 +272,46 @@ static struct bpf_test tests[] = {
 		.errstr = "invalid bpf_ld_imm64 insn",
 		.result = REJECT,
 	},
+	{
+		"arsh32 on imm",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 1),
+			BPF_ALU32_IMM(BPF_ARSH, BPF_REG_0, 5),
+			BPF_EXIT_INSN(),
+		},
+		.result = REJECT,
+		.errstr = "BPF_ARSH not supported for 32 bit ALU",
+	},
+	{
+		"arsh32 on reg",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 1),
+			BPF_MOV64_IMM(BPF_REG_1, 5),
+			BPF_ALU32_REG(BPF_ARSH, BPF_REG_0, BPF_REG_1),
+			BPF_EXIT_INSN(),
+		},
+		.result = REJECT,
+		.errstr = "BPF_ARSH not supported for 32 bit ALU",
+	},
+	{
+		"arsh64 on imm",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 1),
+			BPF_ALU64_IMM(BPF_ARSH, BPF_REG_0, 5),
+			BPF_EXIT_INSN(),
+		},
+		.result = ACCEPT,
+	},
+	{
+		"arsh64 on reg",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 1),
+			BPF_MOV64_IMM(BPF_REG_1, 5),
+			BPF_ALU64_REG(BPF_ARSH, BPF_REG_0, BPF_REG_1),
+			BPF_EXIT_INSN(),
+		},
+		.result = ACCEPT,
+	},
 	{
 		"no bpf_exit",
 		.insns = {