s390/zcrypt: Improve EP11 CPRB length and overflow checks

The xcrb_msg_to_type6_ep11cprb_msgx() function lacks proper input
validation, creating security vulnerabilities:
1. Missing minimum size validation: The ep11_cprb structure and
   subsequent payload fields (pld_tag, pld_lenfmt) are copied from
   userspace without verifying sufficient buffer length.
2. Arithmetic overflow in length calculations: CEIL4 alignment could
   overflow, bypassing size checks and enabling buffer overflows.
3. The payload is asn1 encoded but the function just uses a simple c
   struct overlay to access some fields of the payload.

Fix by using size_t for length calculations, adding U32_MAX boundary
checks after alignment, and validating minimum request size and
minimum reply size before copying from userspace. Do a very simple
asn1 parsing of the payload up to the function value field.

Fixes: e2c6d91eb8 ("s390/zcrypt: Rework domain processing within zcrypt device driver")
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Cc: stable@vger.kernel.org # 7.1+
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

drivers/s390/crypto/zcrypt_msgtype6.c

@@ -19,6 +19,7 @@
 #include <linux/slab.h>
 #include <linux/atomic.h>
 #include <linux/uaccess.h>
+#include <linux/unaligned.h>
 
 #include "ap_bus.h"
 #include "zcrypt_api.h"
@@ -34,6 +35,9 @@
 #define CEXXC_RESPONSE_TYPE_XCRB 1
 #define CEXXC_RESPONSE_TYPE_EP11 2
 
+/* smallest possible EP11 payload size */
+#define MIN_EP11_PAYLOAD_SIZE 5
+
 MODULE_AUTHOR("IBM Corporation");
 MODULE_DESCRIPTION("Cryptographic Coprocessor (message type 6), " \
 		   "Copyright IBM Corp. 2001, 2023");
@@ -438,12 +442,59 @@ static int xcrb_msg_to_type6cprb_msgx(bool userspace, struct ap_message *ap_msg,
 	return 0;
 }
 
+/*
+ * Simple asn1 int reader/decoder helper function
+ * Returns number of bytes processed or < 0 on failure
+ * Only accepts int length values of 1, 2 or 4.
+ */
+static inline int asn1_int_decode(const u8 *buf, size_t intlen, u32 *u)
+{
+	switch (intlen) {
+	case 1:
+		*u = (u32)(*buf);
+		return 1;
+	case 2:
+		*u = (u32)get_unaligned_be16(buf);
+		return 2;
+	case 4:
+		*u = (u32)get_unaligned_be32(buf);
+		return 4;
+	default:
+		return -EINVAL;
+	}
+}
+
+/*
+ * Simple asn1 length parse helper function
+ * Returns number of bytes processed or < 0 on failure
+ * Only accepts length encoded within the length octet
+ * or for long form 1, 2 or 4 octet length bytes.
+ */
+static inline int asn1_length_decode(const u8 *buf, size_t buflen, u32 *u)
+{
+	int i;
+
+	if (buflen < 1)
+		return -EINVAL;
+
+	if (*buf < 128) {
+		*u = (u32)(*buf & 0x7F);
+		return 1;
+	}
+
+	i = *buf & 0x7F;
+	if (--buflen < i)
+		return -EINVAL;
+	i = asn1_int_decode(++buf, i, u);
+
+	return i < 0 ? i : i + 1;
+}
+
 static int xcrb_msg_to_type6_ep11cprb_msgx(bool userspace, struct ap_message *ap_msg,
 					   struct ep11_urb *xcrb,
 					   unsigned int *fcode,
 					   unsigned int *domain)
 {
-	unsigned int lfmt;
 	static struct type6_hdr static_type6_ep11_hdr = {
 		.type		=  0x06,
 		.rqid		= {0x00, 0x01},
@@ -455,34 +506,32 @@ static int xcrb_msg_to_type6_ep11cprb_msgx(bool userspace, struct ap_message *ap
 	struct {
 		struct type6_hdr hdr;
 		union {
-			struct {
-				struct ep11_cprb cprbx;
-				unsigned char pld_tag;    /* fixed value 0x30 */
-				unsigned char pld_lenfmt; /* length format */
-			} __packed;
+			struct ep11_cprb cprbx;
 			DECLARE_FLEX_ARRAY(u8, userdata);
 		};
 	} __packed * msg = ap_msg->msg;
 
-	struct pld_hdr {
-		unsigned char	func_tag;	/* fixed value 0x4 */
-		unsigned char	func_len;	/* fixed value 0x4 */
-		unsigned int	func_val;	/* function ID	   */
-		unsigned char	dom_tag;	/* fixed value 0x4 */
-		unsigned char	dom_len;	/* fixed value 0x4 */
-		unsigned int	dom_val;	/* domain id	   */
-	} __packed * payload_hdr = NULL;
+	size_t req_len, rep_len, pld_len;
+	unsigned char *pld;
+	int offs = 0, i;
+	unsigned int u;
 
-	if (CEIL4(xcrb->req_len) < xcrb->req_len)
-		return -EINVAL; /* overflow after alignment*/
-
-	/* length checks */
-	ap_msg->len = sizeof(struct type6_hdr) + CEIL4(xcrb->req_len);
+	/* request length and overflow checks */
+	if (xcrb->req_len < sizeof(struct ep11_cprb) + MIN_EP11_PAYLOAD_SIZE)
+		return -EINVAL;
+	req_len = CEIL4(xcrb->req_len);
+	if (req_len < xcrb->req_len || req_len > U32_MAX)
+		return -EINVAL;
+	ap_msg->len = sizeof(struct type6_hdr) + req_len;
 	if (ap_msg->len > ap_msg->bufsize)
 		return -EINVAL;
 
-	if (CEIL4(xcrb->resp_len) < xcrb->resp_len)
-		return -EINVAL; /* overflow after alignment*/
+	/* reply length and overflow checks */
+	if (xcrb->resp_len < sizeof(struct ep11_cprb))
+		return -EINVAL;
+	rep_len = CEIL4(xcrb->resp_len);
+	if (rep_len < xcrb->resp_len || rep_len > U32_MAX)
+		return -EINVAL;
 
 	/* prepare type6 header */
 	msg->hdr = static_type6_ep11_hdr;
@@ -491,26 +540,51 @@ static int xcrb_msg_to_type6_ep11cprb_msgx(bool userspace, struct ap_message *ap
 
 	/* Import CPRB data from the ioctl input parameter */
 	if (z_copy_from_user(userspace, msg->userdata,
-			     (char __force __user *)xcrb->req, xcrb->req_len)) {
+			     (char __force __user *)xcrb->req, xcrb->req_len))
 		return -EFAULT;
-	}
 
-	if ((msg->pld_lenfmt & 0x80) == 0x80) { /*ext.len.fmt 2 or 3*/
-		switch (msg->pld_lenfmt & 0x03) {
-		case 1:
-			lfmt = 2;
-			break;
-		case 2:
-			lfmt = 3;
-			break;
-		default:
-			return -EINVAL;
-		}
-	} else {
-		lfmt = 1; /* length format #1 */
-	}
-	payload_hdr = (struct pld_hdr *)((&msg->pld_lenfmt) + lfmt);
-	*fcode = payload_hdr->func_val & 0xFFFF;
+	pld = msg->userdata + sizeof(struct ep11_cprb);
+	pld_len = msg->cprbx.payload_len;
+	if (pld_len != xcrb->req_len - sizeof(struct ep11_cprb))
+		return -EINVAL;
+	/*
+	 * At this point pld_len is always >= MIN_EP11_PAYLOAD_SIZE
+	 * and the smallest supported asn1 payload is:
+	 *   payload tag (1 octet)
+	 *   payload length (1-5 octets)
+	 *   function tag (1 octet)
+	 *   function length (1-5 octets)
+	 *   function value (1-4 octets)
+	 */
+
+	/* payload tag */
+	if (pld[offs++] != 0x30)
+		return -EINVAL;
+	/* payload length field */
+	i = asn1_length_decode(pld + offs, pld_len - offs, &u);
+	if (i < 0)
+		return -EINVAL;
+	offs += i;
+	if (offs >= pld_len || u > pld_len - offs)
+		return -EINVAL;
+	/* function tag */
+	if (pld[offs++] != 0x04)
+		return -EINVAL;
+	/* function length */
+	if (offs >= pld_len)
+		return -EINVAL;
+	i = asn1_length_decode(pld + offs, pld_len - offs, &u);
+	if (i < 0)
+		return -EINVAL;
+	offs += i;
+	if (offs >= pld_len || u > pld_len - offs)
+		return -EINVAL;
+	/* function value */
+	i = asn1_int_decode(pld + offs, u, &u);
+	if (i < 0)
+		return -EINVAL;
+	offs += i;
+	*fcode = 0xFFFF & u;
 
 	/* enable special processing based on the cprbs flags special bit */
 	if (msg->cprbx.flags & 0x20)