[PATCH v1 2/2] s390/zcrypt: Fix out-of-bounds ptr advance in cca_query_crypto_facility()

[Harald Freudenberger <freude@linux.ibm.com>]

The FQ reply parser code blindly advanced the walk pointer by a length
value read directly from the hardware reply payload without checking
that the advance stayed within the allocated reply buffer. A corrupt
or malicious device response could push ptr beyond the cprbmem region,
causing an out-of-bounds dereference or kernel memory exposure via the
subsequent memcpy().

Fix this by tracking the remaining reply buffer space in a variable
and validating each device-supplied length field against it before
advancing or dereferencing the pointer.

Fixes: 2004b57cde6b ("s390/zcrypt: code cleanup")
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Cc: stable@vger.kernel.org
---
 drivers/s390/crypto/zcrypt_ccamisc.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/s390/crypto/zcrypt_ccamisc.c b/drivers/s390/crypto/zcrypt_ccamisc.c
index 19909bf43dc9..862947431164 100644
--- a/drivers/s390/crypto/zcrypt_ccamisc.c
+++ b/drivers/s390/crypto/zcrypt_ccamisc.c
@@ -1627,6 +1627,7 @@ int cca_query_crypto_facility(u16 cardnr, u16 domain,
 		u8  subfunc_code[2];
 		u8  lvdata[];
 	} __packed * prepparm;
+	size_t datalen;
 
 	/* get already prepared memory for 2 cprbs with param block each */
 	rc = alloc_and_prep_cprbmem(parmbsize, &mem,
@@ -1673,27 +1674,44 @@ int cca_query_crypto_facility(u16 cardnr, u16 domain,
 	prepcblk->rpl_parmb = (u8 __user *)ptr;
 	prepparm = (struct fqrepparm *)ptr;
 	ptr = prepparm->lvdata;
+	datalen = parmbsize - 2 * sizeof(u8);
 
 	/* check and possibly copy reply rule array */
 	len = *((u16 *)ptr);
+	if (len > datalen) {
+		ZCRYPT_DBF_ERR("%s reply rule array len %u exceeds datalen %zu\n",
+			       __func__, len, datalen);
+		rc = -EIO;
+		goto out;
+	}
+	datalen -= sizeof(u16);
+	ptr += sizeof(u16);
 	if (len > sizeof(u16)) {
-		ptr += sizeof(u16);
 		len -= sizeof(u16);
 		if (rarray && rarraylen && *rarraylen > 0) {
 			*rarraylen = (len > *rarraylen ? *rarraylen : len);
 			memcpy(rarray, ptr, *rarraylen);
 		}
+		datalen -= len;
 		ptr += len;
 	}
 	/* check and possible copy reply var array */
 	len = *((u16 *)ptr);
+	if (len > datalen) {
+		ZCRYPT_DBF_ERR("%s reply var array len %u exceeds datalen %zu\n",
+			       __func__, len, datalen);
+		rc = -EIO;
+		goto out;
+	}
+	datalen -= sizeof(u16);
+	ptr += sizeof(u16);
 	if (len > sizeof(u16)) {
-		ptr += sizeof(u16);
 		len -= sizeof(u16);
 		if (varray && varraylen && *varraylen > 0) {
 			*varraylen = (len > *varraylen ? *varraylen : len);
 			memcpy(varray, ptr, *varraylen);
 		}
+		datalen -= len;
 		ptr += len;
 	}
 
-- 
2.43.0