Merge branch 'bpf-fix-verifier-min-max-handling-in-BPF_SUB'

Edward Cree says:

====================
bpf: fix verifier min/max handling in BPF_SUB

I managed to come up with a test for the swapped bounds in BPF_SUB, so here
 it is along with a patch that fixes it, separated out from my 'rewrite
 everything' series so it can go to -stable.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/verifier.c

@@ -1865,10 +1865,12 @@ static void adjust_reg_min_max_vals(struct bpf_verifier_env *env,
 	 * do our normal operations to the register, we need to set the values
 	 * to the min/max since they are undefined.
 	 */
-	if (min_val == BPF_REGISTER_MIN_RANGE)
-		dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
-	if (max_val == BPF_REGISTER_MAX_RANGE)
-		dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
+	if (opcode != BPF_SUB) {
+		if (min_val == BPF_REGISTER_MIN_RANGE)
+			dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
+		if (max_val == BPF_REGISTER_MAX_RANGE)
+			dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
+	}
 
 	switch (opcode) {
 	case BPF_ADD:
@@ -1879,10 +1881,17 @@ static void adjust_reg_min_max_vals(struct bpf_verifier_env *env,
 		dst_reg->min_align = min(src_align, dst_align);
 		break;
 	case BPF_SUB:
+		/* If one of our values was at the end of our ranges, then the
+		 * _opposite_ value in the dst_reg goes to the end of our range.
+		 */
+		if (min_val == BPF_REGISTER_MIN_RANGE)
+			dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
+		if (max_val == BPF_REGISTER_MAX_RANGE)
+			dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
 		if (dst_reg->min_value != BPF_REGISTER_MIN_RANGE)
-			dst_reg->min_value -= min_val;
+			dst_reg->min_value -= max_val;
 		if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
-			dst_reg->max_value -= max_val;
+			dst_reg->max_value -= min_val;
 		dst_reg->min_align = min(src_align, dst_align);
 		break;
 	case BPF_MUL:

tools/testing/selftests/bpf/test_verifier.c

@@ -5980,6 +5980,34 @@ static struct bpf_test tests[] = {
 		.result = REJECT,
 		.result_unpriv = REJECT,
 	},
+	{
+		"subtraction bounds (map value)",
+		.insns = {
+			BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+			BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+			BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
+			BPF_LD_MAP_FD(BPF_REG_1, 0),
+			BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
+				     BPF_FUNC_map_lookup_elem),
+			BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 9),
+			BPF_LDX_MEM(BPF_B, BPF_REG_1, BPF_REG_0, 0),
+			BPF_JMP_IMM(BPF_JGT, BPF_REG_1, 0xff, 7),
+			BPF_LDX_MEM(BPF_B, BPF_REG_3, BPF_REG_0, 1),
+			BPF_JMP_IMM(BPF_JGT, BPF_REG_3, 0xff, 5),
+			BPF_ALU64_REG(BPF_SUB, BPF_REG_1, BPF_REG_3),
+			BPF_ALU64_IMM(BPF_RSH, BPF_REG_1, 56),
+			BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1),
+			BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.fixup_map1 = { 3 },
+		.errstr_unpriv = "R0 pointer arithmetic prohibited",
+		.errstr = "R0 min value is negative, either use unsigned index or do a if (index >=0) check.",
+		.result = REJECT,
+		.result_unpriv = REJECT,
+	},
 };
 
 static int probe_filter_length(const struct bpf_insn *fp)