SUNRPC: Fix crasher in unwrap_integ_data()

If a zero length is passed to kmalloc() it returns 0x10, which is
not a valid address. gss_verify_mic() subsequently crashes when it
attempts to dereference that pointer.

Instead of allocating this memory on every call based on an
untrusted size value, use a piece of dynamically-allocated scratch
memory that is always available.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>

net/sunrpc/auth_gss/svcauth_gss.c

@@ -49,11 +49,36 @@
 #include <linux/sunrpc/svcauth.h>
 #include <linux/sunrpc/svcauth_gss.h>
 #include <linux/sunrpc/cache.h>
+#include <linux/sunrpc/gss_krb5.h>
 
 #include <trace/events/rpcgss.h>
 
 #include "gss_rpc_upcall.h"
 
+/*
+ * Unfortunately there isn't a maximum checksum size exported via the
+ * GSS API. Manufacture one based on GSS mechanisms supported by this
+ * implementation.
+ */
+#define GSS_MAX_CKSUMSIZE (GSS_KRB5_TOK_HDR_LEN + GSS_KRB5_MAX_CKSUM_LEN)
+
+/*
+ * This value may be increased in the future to accommodate other
+ * usage of the scratch buffer.
+ */
+#define GSS_SCRATCH_SIZE GSS_MAX_CKSUMSIZE
+
+struct gss_svc_data {
+	/* decoded gss client cred: */
+	struct rpc_gss_wire_cred	clcred;
+	/* save a pointer to the beginning of the encoded verifier,
+	 * for use in encryption/checksumming in svcauth_gss_release: */
+	__be32				*verf_start;
+	struct rsc			*rsci;
+
+	/* for temporary results */
+	u8				gsd_scratch[GSS_SCRATCH_SIZE];
+};
 
 /* The rpcsec_init cache is used for mapping RPCSEC_GSS_{,CONT_}INIT requests
  * into replies.
@@ -887,13 +912,11 @@ read_u32_from_xdr_buf(struct xdr_buf *buf, int base, u32 *obj)
 static int
 unwrap_integ_data(struct svc_rqst *rqstp, struct xdr_buf *buf, u32 seq, struct gss_ctx *ctx)
 {
+	struct gss_svc_data *gsd = rqstp->rq_auth_data;
 	u32 integ_len, rseqno, maj_stat;
-	int stat = -EINVAL;
 	struct xdr_netobj mic;
 	struct xdr_buf integ_buf;
 
-	mic.data = NULL;
-
 	/* NFS READ normally uses splice to send data in-place. However
 	 * the data in cache can change after the reply's MIC is computed
 	 * but before the RPC reply is sent. To prevent the client from
@@ -917,11 +940,9 @@ unwrap_integ_data(struct svc_rqst *rqstp, struct xdr_buf *buf, u32 seq, struct g
 	/* copy out mic... */
 	if (read_u32_from_xdr_buf(buf, integ_len, &mic.len))
 		goto unwrap_failed;
-	if (mic.len > RPC_MAX_AUTH_SIZE)
-		goto unwrap_failed;
-	mic.data = kmalloc(mic.len, GFP_KERNEL);
-	if (!mic.data)
+	if (mic.len > sizeof(gsd->gsd_scratch))
 		goto unwrap_failed;
+	mic.data = gsd->gsd_scratch;
 	if (read_bytes_from_xdr_buf(buf, integ_len + 4, mic.data, mic.len))
 		goto unwrap_failed;
 	maj_stat = gss_verify_mic(ctx, &integ_buf, &mic);
@@ -932,20 +953,17 @@ unwrap_integ_data(struct svc_rqst *rqstp, struct xdr_buf *buf, u32 seq, struct g
 		goto bad_seqno;
 	/* trim off the mic and padding at the end before returning */
 	xdr_buf_trim(buf, round_up_to_quad(mic.len) + 4);
-	stat = 0;
-out:
-	kfree(mic.data);
-	return stat;
+	return 0;
 
 unwrap_failed:
 	trace_rpcgss_svc_unwrap_failed(rqstp);
-	goto out;
+	return -EINVAL;
 bad_seqno:
 	trace_rpcgss_svc_seqno_bad(rqstp, seq, rseqno);
-	goto out;
+	return -EINVAL;
 bad_mic:
 	trace_rpcgss_svc_mic(rqstp, maj_stat);
-	goto out;
+	return -EINVAL;
 }
 
 static inline int
@@ -1023,15 +1041,6 @@ unwrap_priv_data(struct svc_rqst *rqstp, struct xdr_buf *buf, u32 seq, struct gs
 	return -EINVAL;
 }
 
-struct gss_svc_data {
-	/* decoded gss client cred: */
-	struct rpc_gss_wire_cred	clcred;
-	/* save a pointer to the beginning of the encoded verifier,
-	 * for use in encryption/checksumming in svcauth_gss_release: */
-	__be32				*verf_start;
-	struct rsc			*rsci;
-};
-
 static int
 svcauth_gss_set_client(struct svc_rqst *rqstp)
 {