bpf: Fix a verifier failure with xor

bpf selftest test_progs/test_sk_assign failed with llvm 11 and llvm 12.
Compared to llvm 10, llvm 11 and 12 generates xor instruction which
is not handled properly in verifier. The following illustrates the
problem:

  16: (b4) w5 = 0
  17: ... R5_w=inv0 ...
  ...
  132: (a4) w5 ^= 1
  133: ... R5_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) ...
  ...
  37: (bc) w8 = w5
  38: ... R5=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff))
          R8_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) ...
  ...
  41: (bc) w3 = w8
  42: ... R3_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) ...
  45: (56) if w3 != 0x0 goto pc+1
   ... R3_w=inv0 ...
  46: (b7) r1 = 34
  47: R1_w=inv34 R7=pkt(id=0,off=26,r=38,imm=0)
  47: (0f) r7 += r1
  48: R1_w=invP34 R3_w=inv0 R7_w=pkt(id=0,off=60,r=38,imm=0)
  48: (b4) w9 = 0
  49: R1_w=invP34 R3_w=inv0 R7_w=pkt(id=0,off=60,r=38,imm=0)
  49: (69) r1 = *(u16 *)(r7 +0)
  invalid access to packet, off=60 size=2, R7(id=0,off=60,r=38)
  R7 offset is outside of the packet

At above insn 132, w5 = 0, but after w5 ^= 1, we give a really conservative
value of w5. At insn 45, in reality the condition should be always false.
But due to conservative value for w3, the verifier evaluates it could be
true and this later leads to verifier failure complaining potential
packet out-of-bound access.

This patch implemented proper XOR support in verifier.
In the above example, we have:
  132: R5=invP0
  132: (a4) w5 ^= 1
  133: R5_w=invP1
  ...
  37: (bc) w8 = w5
  ...
  41: (bc) w3 = w8
  42: R3_w=invP1
  ...
  45: (56) if w3 != 0x0 goto pc+1
  47: R3_w=invP1
  ...
  processed 353 insns ...
and the verifier can verify the program successfully.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200825064608.2017937-1-yhs@fb.com

kernel/bpf/verifier.c

@@ -5829,6 +5829,67 @@ static void scalar_min_max_or(struct bpf_reg_state *dst_reg,
 	__update_reg_bounds(dst_reg);
 }
 
+static void scalar32_min_max_xor(struct bpf_reg_state *dst_reg,
+				 struct bpf_reg_state *src_reg)
+{
+	bool src_known = tnum_subreg_is_const(src_reg->var_off);
+	bool dst_known = tnum_subreg_is_const(dst_reg->var_off);
+	struct tnum var32_off = tnum_subreg(dst_reg->var_off);
+	s32 smin_val = src_reg->s32_min_value;
+
+	/* Assuming scalar64_min_max_xor will be called so it is safe
+	 * to skip updating register for known case.
+	 */
+	if (src_known && dst_known)
+		return;
+
+	/* We get both minimum and maximum from the var32_off. */
+	dst_reg->u32_min_value = var32_off.value;
+	dst_reg->u32_max_value = var32_off.value | var32_off.mask;
+
+	if (dst_reg->s32_min_value >= 0 && smin_val >= 0) {
+		/* XORing two positive sign numbers gives a positive,
+		 * so safe to cast u32 result into s32.
+		 */
+		dst_reg->s32_min_value = dst_reg->u32_min_value;
+		dst_reg->s32_max_value = dst_reg->u32_max_value;
+	} else {
+		dst_reg->s32_min_value = S32_MIN;
+		dst_reg->s32_max_value = S32_MAX;
+	}
+}
+
+static void scalar_min_max_xor(struct bpf_reg_state *dst_reg,
+			       struct bpf_reg_state *src_reg)
+{
+	bool src_known = tnum_is_const(src_reg->var_off);
+	bool dst_known = tnum_is_const(dst_reg->var_off);
+	s64 smin_val = src_reg->smin_value;
+
+	if (src_known && dst_known) {
+		/* dst_reg->var_off.value has been updated earlier */
+		__mark_reg_known(dst_reg, dst_reg->var_off.value);
+		return;
+	}
+
+	/* We get both minimum and maximum from the var_off. */
+	dst_reg->umin_value = dst_reg->var_off.value;
+	dst_reg->umax_value = dst_reg->var_off.value | dst_reg->var_off.mask;
+
+	if (dst_reg->smin_value >= 0 && smin_val >= 0) {
+		/* XORing two positive sign numbers gives a positive,
+		 * so safe to cast u64 result into s64.
+		 */
+		dst_reg->smin_value = dst_reg->umin_value;
+		dst_reg->smax_value = dst_reg->umax_value;
+	} else {
+		dst_reg->smin_value = S64_MIN;
+		dst_reg->smax_value = S64_MAX;
+	}
+
+	__update_reg_bounds(dst_reg);
+}
+
 static void __scalar32_min_max_lsh(struct bpf_reg_state *dst_reg,
 				   u64 umin_val, u64 umax_val)
 {
@@ -6137,6 +6198,11 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 		scalar32_min_max_or(dst_reg, &src_reg);
 		scalar_min_max_or(dst_reg, &src_reg);
 		break;
+	case BPF_XOR:
+		dst_reg->var_off = tnum_xor(dst_reg->var_off, src_reg.var_off);
+		scalar32_min_max_xor(dst_reg, &src_reg);
+		scalar_min_max_xor(dst_reg, &src_reg);
+		break;
 	case BPF_LSH:
 		if (umax_val >= insn_bitness) {
 			/* Shifts greater than 31 or 63 are undefined.