SUNRPC: Improve Kerberos confounder generation
Other common Kerberos implementations use a fully random confounder
for encryption. The reason for this is explained in the new comment
added by this patch. The current get_random_bytes() implementation
does not exhaust system entropy.
Since confounder generation is part of Kerberos itself rather than
the GSS-API Kerberos mechanism, the function is renamed and moved.
Note that light top-down analysis shows that the SHA-1 transform
is by far the most CPU-intensive part of encryption. Thus we do not
expect this change to result in a significant performance impact.
However, eventually it might be necessary to generate an independent
stream of confounders for each Kerberos context to help improve I/O
parallelism.
Reviewed-by: Simo Sorce <simo@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
diff --git a/include/linux/sunrpc/gss_krb5.h b/include/linux/sunrpc/gss_krb5.h
index 51860e3..f0fac1e 100644
--- a/include/linux/sunrpc/gss_krb5.h
+++ b/include/linux/sunrpc/gss_krb5.h
@@ -311,7 +311,4 @@ gss_krb5_aes_decrypt(struct krb5_ctx *kctx, u32 offset, u32 len,
struct xdr_buf *buf, u32 *plainoffset,
u32 *plainlen);
-void
-gss_krb5_make_confounder(char *p, u32 conflen);
-
#endif /* _LINUX_SUNRPC_GSS_KRB5_H */
diff --git a/net/sunrpc/auth_gss/gss_krb5_crypto.c b/net/sunrpc/auth_gss/gss_krb5_crypto.c
index 8aa5610..7c06c11 100644
--- a/net/sunrpc/auth_gss/gss_krb5_crypto.c
+++ b/net/sunrpc/auth_gss/gss_krb5_crypto.c
@@ -47,10 +47,41 @@
#include <linux/sunrpc/gss_krb5.h>
#include <linux/sunrpc/xdr.h>
+#include "gss_krb5_internal.h"
+
#if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
# define RPCDBG_FACILITY RPCDBG_AUTH
#endif
+/**
+ * krb5_make_confounder - Generate a confounder string
+ * @p: memory location into which to write the string
+ * @conflen: string length to write, in octets
+ *
+ * RFCs 1964 and 3961 mention only "a random confounder" without going
+ * into detail about its function or cryptographic requirements. The
+ * assumed purpose is to prevent repeated encryption of a plaintext with
+ * the same key from generating the same ciphertext. It is also used to
+ * pad minimum plaintext length to at least a single cipher block.
+ *
+ * However, in situations like the GSS Kerberos 5 mechanism, where the
+ * encryption IV is always all zeroes, the confounder also effectively
+ * functions like an IV. Thus, not only must it be unique from message
+ * to message, but it must also be difficult to predict. Otherwise an
+ * attacker can correlate the confounder to previous or future values,
+ * making the encryption easier to break.
+ *
+ * Given that the primary consumer of this encryption mechanism is a
+ * network storage protocol, a type of traffic that often carries
+ * predictable payloads (eg, all zeroes when reading unallocated blocks
+ * from a file), our confounder generation has to be cryptographically
+ * strong.
+ */
+void krb5_make_confounder(u8 *p, int conflen)
+{
+ get_random_bytes(p, conflen);
+}
+
u32
krb5_encrypt(
struct crypto_sync_skcipher *tfm,
@@ -630,7 +661,7 @@ gss_krb5_aes_encrypt(struct krb5_ctx *kctx, u32 offset,
offset += GSS_KRB5_TOK_HDR_LEN;
if (xdr_extend_head(buf, offset, conflen))
return GSS_S_FAILURE;
- gss_krb5_make_confounder(buf->head[0].iov_base + offset, conflen);
+ krb5_make_confounder(buf->head[0].iov_base + offset, conflen);
offset -= GSS_KRB5_TOK_HDR_LEN;
if (buf->tail[0].iov_base != NULL) {
diff --git a/net/sunrpc/auth_gss/gss_krb5_internal.h b/net/sunrpc/auth_gss/gss_krb5_internal.h
new file mode 100644
index 0000000..16a83d5
--- /dev/null
+++ b/net/sunrpc/auth_gss/gss_krb5_internal.h
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0 or BSD-3-Clause */
+/*
+ * SunRPC GSS Kerberos 5 mechanism internal definitions
+ *
+ * Copyright (c) 2022 Oracle and/or its affiliates.
+ */
+
+#ifndef _NET_SUNRPC_AUTH_GSS_KRB5_INTERNAL_H
+#define _NET_SUNRPC_AUTH_GSS_KRB5_INTERNAL_H
+
+void krb5_make_confounder(u8 *p, int conflen);
+
+#endif /* _NET_SUNRPC_AUTH_GSS_KRB5_INTERNAL_H */
diff --git a/net/sunrpc/auth_gss/gss_krb5_mech.c b/net/sunrpc/auth_gss/gss_krb5_mech.c
index 08a86ec..76a0d83 100644
--- a/net/sunrpc/auth_gss/gss_krb5_mech.c
+++ b/net/sunrpc/auth_gss/gss_krb5_mech.c
@@ -550,16 +550,15 @@ gss_import_sec_context_kerberos(const void *p, size_t len,
ret = gss_import_v1_context(p, end, ctx);
else
ret = gss_import_v2_context(p, end, ctx, gfp_mask);
-
- if (ret == 0) {
- ctx_id->internal_ctx_id = ctx;
- if (endtime)
- *endtime = ctx->endtime;
- } else
+ if (ret) {
kfree(ctx);
+ return ret;
+ }
- dprintk("RPC: %s: returning %d\n", __func__, ret);
- return ret;
+ ctx_id->internal_ctx_id = ctx;
+ if (endtime)
+ *endtime = ctx->endtime;
+ return 0;
}
static void
diff --git a/net/sunrpc/auth_gss/gss_krb5_wrap.c b/net/sunrpc/auth_gss/gss_krb5_wrap.c
index bd068e9..66e65e4 100644
--- a/net/sunrpc/auth_gss/gss_krb5_wrap.c
+++ b/net/sunrpc/auth_gss/gss_krb5_wrap.c
@@ -32,9 +32,10 @@
#include <linux/types.h>
#include <linux/jiffies.h>
#include <linux/sunrpc/gss_krb5.h>
-#include <linux/random.h>
#include <linux/pagemap.h>
+#include "gss_krb5_internal.h"
+
#if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
# define RPCDBG_FACILITY RPCDBG_AUTH
#endif
@@ -113,39 +114,6 @@ gss_krb5_remove_padding(struct xdr_buf *buf, int blocksize)
return 0;
}
-void
-gss_krb5_make_confounder(char *p, u32 conflen)
-{
- static u64 i = 0;
- u64 *q = (u64 *)p;
-
- /* rfc1964 claims this should be "random". But all that's really
- * necessary is that it be unique. And not even that is necessary in
- * our case since our "gssapi" implementation exists only to support
- * rpcsec_gss, so we know that the only buffers we will ever encrypt
- * already begin with a unique sequence number. Just to hedge my bets
- * I'll make a half-hearted attempt at something unique, but ensuring
- * uniqueness would mean worrying about atomicity and rollover, and I
- * don't care enough. */
-
- /* initialize to random value */
- if (i == 0) {
- i = get_random_u32();
- i = (i << 32) | get_random_u32();
- }
-
- switch (conflen) {
- case 16:
- *q++ = i++;
- fallthrough;
- case 8:
- *q++ = i++;
- break;
- default:
- BUG();
- }
-}
-
/* Assumptions: the head and tail of inbuf are ours to play with.
* The pages, however, may be real pages in the page cache and we replace
* them with scratch pages from **pages before writing to them. */
@@ -211,7 +179,7 @@ gss_wrap_kerberos_v1(struct krb5_ctx *kctx, int offset,
ptr[6] = 0xff;
ptr[7] = 0xff;
- gss_krb5_make_confounder(msg_start, conflen);
+ krb5_make_confounder(msg_start, conflen);
if (kctx->gk5e->keyed_cksum)
cksumkey = kctx->cksum;
