[PATCH net-next] crypto/krb5: Fix change to use SG miter to use offset

[David Howells <dhowells@redhat.com>]

[Note: Nothing in linus/master uses the krb5lib, though the bug is there,
 but it is used by AF_RXRPC's RxGK implementation in net-next, so can it go
 through the net-next tree rather than directly to Linus or through
 crypto?]

The recent patch to make the rfc3961 simplified code use sg_miter rather
than manually walking the scatterlist to hash the contents of a buffer
described by that scatterlist failed to take the starting offset into
account.

This is indicated by the selftests reporting:

    krb5: Running aes128-cts-hmac-sha256-128 mic
    krb5: !!! TESTFAIL crypto/krb5/selftest.c:446
    krb5: MIC mismatch

Fix this by calling sg_miter_skip() before doing the loop to advance by the
offset.

This only affects packet signing modes and not full encryption in RxGK
because, for full encryption, the message digest is handled inside the
authenc and krb5enc drivers.

Fixes: da6f9bf40ac2 ("crypto: krb5 - Use SG miter instead of doing it by hand")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: "David S. Miller" <davem@davemloft.net>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: linux-nfs@vger.kernel.org
cc: linux-crypto@vger.kernel.org
cc: netdev@vger.kernel.org
---
 crypto/krb5/rfc3961_simplified.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/crypto/krb5/rfc3961_simplified.c b/crypto/krb5/rfc3961_simplified.c
index 79180d28baa9..e49cbdec7c40 100644
--- a/crypto/krb5/rfc3961_simplified.c
+++ b/crypto/krb5/rfc3961_simplified.c
@@ -89,6 +89,7 @@ int crypto_shash_update_sg(struct shash_desc *desc, struct scatterlist *sg,
 
 	sg_miter_start(&miter, sg, sg_nents(sg),
 		       SG_MITER_FROM_SG | SG_MITER_LOCAL);
+	sg_miter_skip(&miter, offset);
 	for (i = 0; i < len; i += n) {
 		sg_miter_next(&miter);
 		n = min(miter.length, len - i);