On Fri, 28 Apr 2023 at 17:18, Chuck Lever III <chuck.lever@xxxxxxxxxx> wrote:
>
>
>
> > On Apr 28, 2023, at 12:09 PM, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> >
> > On Fri, 28 Apr 2023 at 13:59, Chuck Lever III <chuck.lever@xxxxxxxxxx> wrote:
> >>
> >>
> >>
> >>> On Apr 28, 2023, at 5:57 AM, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> >>>
> >>> On Fri, 28 Apr 2023 at 10:44, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
> >>>>
> >>>> Scott Mayhew <smayhew@xxxxxxxxxx> wrote:
> >>>>>
> >>>>> diff --git a/arch/arm64/crypto/aes-modes.S b/arch/arm64/crypto/aes-modes.S
> >>>>> index 0e834a2c062c..477605fad76b 100644
> >>>>> --- a/arch/arm64/crypto/aes-modes.S
> >>>>> +++ b/arch/arm64/crypto/aes-modes.S
> >>>>> @@ -268,6 +268,7 @@ AES_FUNC_START(aes_cbc_cts_encrypt)
> >>>>> add x4, x0, x4
> >>>>> st1 {v0.16b}, [x4] /* overlapping stores */
> >>>>> st1 {v1.16b}, [x0]
> >>>>> + st1 {v1.16b}, [x5]
> >>>>> ret
> >>>>> AES_FUNC_END(aes_cbc_cts_encrypt)
> >>>>>
> >>>>> But I don't know if that change is at all correct! (I've never even
> >>>>> looked at arm64 asm before). If someone who's knowledgeable about this
> >>>>> code could chime in, I'd appreciate it.
> >>>>
> >>>> Ard, could you please take a look at this?
> >>>>
> >>>
> >>> The issue seems to be that the caller expects iv_out to have been
> >>> populated even when doing ciphertext stealing.
> >>>
> >>> This is meaningless, because the output IV can only be used to chain
> >>> additional encrypted data if the split is at a block boundary. The
> >>> ciphertext stealing fundamentally terminates the encryption, and
> >>> produces a block of ciphertext that is shorter than the block size, so
> >>> what the output IV should be is actually unspecified.
> >>>
> >>> IOW, test cases having plain/ciphertext vectors whose size is not a
> >>> multiple of the cipher block size should not specify an expected value
> >>> for the output IV.
> >>
> >> The test cases are extracted from RFC 3962 Appendix B. The
> >> standard clearly expects there to be a non-zero next IV for
> >> plaintext sizes that are not block-aligned.
> >>
> >
> > OK, so this is the Kerberos V specific spec on how to use AES in CBC
> > mode, which appears to describe how to chain multiple CBC encryptions
> > together.
> >
> > CBC-CTS itself does not define this: the IV is an input only, and the
> > reason we happen to return the IV is because a single CBC operation
> > may be broken up into several ones, which can only be done on block
> > boundaries. This is why CBC-CTS itself passes all its tests: a single
> > CBC-CTS encryption only performs ciphertext stealing at the very end,
> > and the next IV is never used in that case. (This is why the CTS mode
> > tests in crypto/testmgr.h don't have iv_out vectors)
> >
> > Note that RFC3962 defines that the penultimate block of CBC-CTS
> > ciphertext is used as the next IV. CBC returns the last ciphertext
> > block as the output IV. It is a happy coincidence that the generic CTS
> > template encapsulates CBC in a way where its output IV ends up in the
> > right place.
> >
> >> Also, these test cases pass on other hardware platforms.
> >>
> >
> > Fair enough.
> >
> > I am not opposed to fixing this, but given that it is the RFC3962 spec
> > that defines that the next IV is the penultimate full block of the
> > previous CBC-CTS ciphertext, we might consider doing the memcpy() in
> > the Kerberos code not in the CBC-CTS implementations.
>
> Interesting thought. I'm all about proper layering, so I think this
> is worth considering. Can you send an RFC patch?
>
I'm not that familiar with kunit so this is just an off hand
suggestion, but I imagine something like the below would suffice
--- a/net/sunrpc/auth_gss/gss_krb5_crypto.c
+++ b/net/sunrpc/auth_gss/gss_krb5_crypto.c
@@ -639,6 +639,13 @@ gss_krb5_cts_crypt(struct crypto_sync_skcipher
*cipher, struct xdr_buf *buf,
ret = write_bytes_to_xdr_buf(buf, offset, data, len);
+ /*
+ * CBC-CTS does not define an output IV but RFC 3962 defines it as the
+ * penultimate block of ciphertext, so copy that into the IV buffer
+ * before returning.
+ */
+ if (encrypt)
+ memcpy(iv, data, crypto_sync_skcipher_ivsize(cipher));
out:
kfree(data);
return ret;
