Input: cyttsp4 - avoid overflows when calculating memory sizes

There are several places to perform subtraction to calculate buffer
size such as:

si->si_ofs.cydata_size = si->si_ofs.test_ofs - si->si_ofs.cydata_ofs;
...
p = krealloc(si->si_ptrs.cydata, si->si_ofs.cydata_size, GFP_KERNEL);

Actually, data types of above variables during subtraction are size_t, so
it is unsigned. That means if second operand(si->si_ofs.cydata_ofs) is
greater than the first operand(si->si_ofs.test_ofs), then resulting
si->si_ofs.cydata_size could result in an unsigned integer wrap which is
not desirable.

The proper way to correct this problem is to perform a test of both
operands to avoid having unsigned wrap.

Signed-off-by: Vince Kim <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/touchscreen/cyttsp4_core.c

···
 	void *p;
 	int rc;
 
+	if (si->si_ofs.test_ofs <= si->si_ofs.cydata_ofs) {
+		dev_err(cd->dev,
+			"%s: invalid offset test_ofs: %zu, cydata_ofs: %zu\n",
+			__func__, si->si_ofs.test_ofs, si->si_ofs.cydata_ofs);
+		return -EINVAL;
+	}
+
 	si->si_ofs.cydata_size = si->si_ofs.test_ofs - si->si_ofs.cydata_ofs;
 	dev_dbg(cd->dev, "%s: cydata size: %zd\n", __func__,
 			si->si_ofs.cydata_size);
 
 	p = krealloc(si->si_ptrs.cydata, si->si_ofs.cydata_size, GFP_KERNEL);
 	if (p == NULL) {
-		dev_err(cd->dev, "%s: fail alloc cydata memory\n", __func__);
+		dev_err(cd->dev, "%s: failed to allocate cydata memory\n",
+			__func__);
 		return -ENOMEM;
 	}
 	si->si_ptrs.cydata = p;
···
 	void *p;
 	int rc;
 
+	if (si->si_ofs.pcfg_ofs <= si->si_ofs.test_ofs) {
+		dev_err(cd->dev,
+			"%s: invalid offset pcfg_ofs: %zu, test_ofs: %zu\n",
+			__func__, si->si_ofs.pcfg_ofs, si->si_ofs.test_ofs);
+		return -EINVAL;
+	}
+
 	si->si_ofs.test_size = si->si_ofs.pcfg_ofs - si->si_ofs.test_ofs;
 
 	p = krealloc(si->si_ptrs.test, si->si_ofs.test_size, GFP_KERNEL);
 	if (p == NULL) {
-		dev_err(cd->dev, "%s: fail alloc test memory\n", __func__);
+		dev_err(cd->dev, "%s: failed to allocate test memory\n",
+			__func__);
 		return -ENOMEM;
 	}
 	si->si_ptrs.test = p;
···
 	void *p;
 	int rc;
 
+	if (si->si_ofs.opcfg_ofs <= si->si_ofs.pcfg_ofs) {
+		dev_err(cd->dev,
+			"%s: invalid offset opcfg_ofs: %zu, pcfg_ofs: %zu\n",
+			__func__, si->si_ofs.opcfg_ofs, si->si_ofs.pcfg_ofs);
+		return -EINVAL;
+	}
+
 	si->si_ofs.pcfg_size = si->si_ofs.opcfg_ofs - si->si_ofs.pcfg_ofs;
 
 	p = krealloc(si->si_ptrs.pcfg, si->si_ofs.pcfg_size, GFP_KERNEL);
 	if (p == NULL) {
-		rc = -ENOMEM;
-		dev_err(cd->dev, "%s: fail alloc pcfg memory r=%d\n",
-			__func__, rc);
-		return rc;
+		dev_err(cd->dev, "%s: failed to allocate pcfg memory\n",
+			__func__);
+		return -ENOMEM;
 	}
 	si->si_ptrs.pcfg = p;
 
···
 	void *p;
 	int rc;
 
+	if (si->si_ofs.ddata_ofs <= si->si_ofs.opcfg_ofs) {
+		dev_err(cd->dev,
+			"%s: invalid offset ddata_ofs: %zu, opcfg_ofs: %zu\n",
+			__func__, si->si_ofs.ddata_ofs, si->si_ofs.opcfg_ofs);
+		return -EINVAL;
+	}
+
 	si->si_ofs.opcfg_size = si->si_ofs.ddata_ofs - si->si_ofs.opcfg_ofs;
 
 	p = krealloc(si->si_ptrs.opcfg, si->si_ofs.opcfg_size, GFP_KERNEL);
 	if (p == NULL) {
-		dev_err(cd->dev, "%s: fail alloc opcfg memory\n", __func__);
-		rc = -ENOMEM;
-		goto cyttsp4_si_get_opcfg_data_exit;
+		dev_err(cd->dev, "%s: failed to allocate opcfg memory\n",
+			__func__);
+		return -ENOMEM;
 	}
 	si->si_ptrs.opcfg = p;
 
···
 	if (rc < 0) {
 		dev_err(cd->dev, "%s: fail read opcfg data r=%d\n",
 			__func__, rc);
-		goto cyttsp4_si_get_opcfg_data_exit;
+		return rc;
 	}
 	si->si_ofs.cmd_ofs = si->si_ptrs.opcfg->cmd_ofs;
 	si->si_ofs.rep_ofs = si->si_ptrs.opcfg->rep_ofs;
···
 	cyttsp4_pr_buf(cd->dev, cd->pr_buf, (u8 *)si->si_ptrs.opcfg,
 		si->si_ofs.opcfg_size, "sysinfo_opcfg_data");
 
-cyttsp4_si_get_opcfg_data_exit:
-	return rc;
+	return 0;
 }
 
 static int cyttsp4_si_get_ddata(struct cyttsp4 *cd)