SUNRPC: Avoid relying on crypto API to derive CBC-CTS output IV
Subsystems (4)
PATCH 1/1 SUNRPC: Avoid relying on crypto API to derive CBC-CTS output IV
Scott reports SUNRPC self-test failures regarding the output IV on arm64 when using the SIMD accelerated implementation of AES in CBC mode with ciphertext stealing ("cts(cbc(aes))" in crypto API speak).
These failures are the result of the fact that, while RFC 3962 does specify what the output IV should be and includes test vectors for it, the general concept of an output IV is poorly defined, and generally, not specified by the various algorithms implemented by the crypto API. Only algorithms that support transparent chaining (e.g., CBC mode on a block boundary) have requirements on the output IV, but ciphertext stealing (CTS) is fundamentally about how to encapsulate CBC in a way where the length of the entire message may not be an integral multiple of the cipher block size, and the concept of an output IV does not exist here because it has no defined purpose past the end of the message.
The generic CTS template takes advantage of this chaining capability of the CBC implementations, and as a result, happens to return an output IV, simply because it passes its IV buffer directly to the encapsulated CBC implementation, which operates on full blocks only, and always returns an IV. This output IV happens to match how RFC 3962 defines it, even though the CTS template itself does not contain any output IV logic whatsoever, and, for this reason, lacks any test vectors that exercise this accidental output IV generation.
The arm64 SIMD implementation of cts(cbc(aes)) does not use the generic CTS template at all, but instead, implements the CBC mode and ciphertext stealing directly, and therefore does not encapsule a CBC implementation that returns an output IV in the same way. The arm64 SIMD implementation complies with the specification and passes all internal tests, but when invoked by the SUNRPC code, fails to produce the expected output IV and causes its selftests to fail.
Given that the output IV is defined as the penultimate block (where the final block may smaller than the block size), we can quite easily derive it in the caller by copying the appropriate slice of ciphertext after encryption.
--- net/sunrpc/auth_gss/gss_krb5_crypto.c | 7 +++++++ 1 file changed, 7 insertions(+)
net/sunrpc/auth_gss/gss_krb5_crypto.c +7 -0
@@ -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;
Signature · 1 line
2.39.2
Quoted 41 lines · Ard Biesheuvel
Scott reports SUNRPC self-test failures regarding the output IV on arm64 when using the SIMD accelerated implementation of AES in CBC mode with ciphertext stealing ("cts(cbc(aes))" in crypto API speak).
These failures are the result of the fact that, while RFC 3962 does specify what the output IV should be and includes test vectors for it, the general concept of an output IV is poorly defined, and generally, not specified by the various algorithms implemented by the crypto API. Only algorithms that support transparent chaining (e.g., CBC mode on a block boundary) have requirements on the output IV, but ciphertext stealing (CTS) is fundamentally about how to encapsulate CBC in a way where the length of the entire message may not be an integral multiple of the cipher block size, and the concept of an output IV does not exist here because it has no defined purpose past the end of the message.
The generic CTS template takes advantage of this chaining capability of the CBC implementations, and as a result, happens to return an output IV, simply because it passes its IV buffer directly to the encapsulated CBC implementation, which operates on full blocks only, and always returns an IV. This output IV happens to match how RFC 3962 defines it, even though the CTS template itself does not contain any output IV logic whatsoever, and, for this reason, lacks any test vectors that exercise this accidental output IV generation.
The arm64 SIMD implementation of cts(cbc(aes)) does not use the generic CTS template at all, but instead, implements the CBC mode and ciphertext stealing directly, and therefore does not encapsule a CBC implementation that returns an output IV in the same way. The arm64 SIMD implementation complies with the specification and passes all internal tests, but when invoked by the SUNRPC code, fails to produce the expected output IV and causes its selftests to fail.
Given that the output IV is defined as the penultimate block (where the final block may smaller than the block size), we can quite easily derive it in the caller by copying the appropriate slice of ciphertext after encryption.
Quoted 7 lines
Cc Trond Myklebust <[email protected]>Cc Anna Schumaker <[email protected]>Cc Chuck Lever <[email protected]>Cc Jeff Layton <[email protected]>Reported-by Scott Mayhew <[email protected]>Tested-by Scott Mayhew <[email protected]>Signed-off-by Ard Biesheuvel <[email protected]>
Thanks Ard. I always like a patch description with lots of context. I agree that looking at the output IV is a GSS Kerberos idiosyncrasy.
Since I'm responsible for the GSS Kerberos Kunit tests, I can take this one for v6.4-rc1.
Quoted 19 lines
---
net/sunrpc/auth_gss/gss_krb5_crypto.c| 7 +++++++1 file changed, 7 insertions(+)net/sunrpc/auth_gss/gss_krb5_crypto.c +7
-0@@ -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));
Quoted 6 lines
out:
kfree(data);
return ret;Signature · 1 line
2.39.2
--
Chuck Lever
Quoted 8 lines
-----Original Message-----
From: Ard Biesheuvel <[email protected]>
Sent: Monday, May 1, 2023 9:04 AM
Subject: [PATCH] SUNRPC: Avoid relying on crypto API to derive CBC-CTS output IV
...
Quoted 15 lines
+++
b/net/sunrpc/auth_gss/gss_krb5_crypto.cnet/sunrpc/auth_gss/gss_krb5_crypto.c +7
-0@@ -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);
Quoted 3 lines
return ret;
--
2.39.2
What about the decrypt (encrypt == 0) case?
That function supports both encrypt and decrypt operations, and both of its callers mention this IV expectation:
gss_krb5_aes_encrypt:
/* Make sure IV carries forward from any CBC results. */ err = gss_krb5_cts_crypt(cipher, buf,
offset + GSS_KRB5_TOK_HDR_LEN + cbcbytes, desc.iv, pages, 1);gss_krb5_aes_decrypt:
/* Make sure IV carries forward from any CBC results. */
ret = gss_krb5_cts_crypt(cipher, &subbuf, cbcbytes, desc.iv, NULL, 0);Quoted 34 lines · Robert (Servers)
Quoted 9 lines
-----Original Message-----
From: Ard Biesheuvel <[email protected]>
Sent: Monday, May 1, 2023 9:04 AM
Subject: [PATCH] SUNRPC: Avoid relying on crypto API to derive CBC-CTS output IV...
Quoted 16 lines
+++
b/net/sunrpc/auth_gss/gss_krb5_crypto.cnet/sunrpc/auth_gss/gss_krb5_crypto.c +7
-0@@ -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;Quoted 2 lines
--
2.39.2What about the decrypt (encrypt == 0) case?
That function supports both encrypt and decrypt operations, and both of its callers mention this IV expectation:
Quoted 9 lines
gss_krb5_aes_encrypt:
/* Make sure IV carries forward from any CBC results. */ err = gss_krb5_cts_crypt(cipher, buf,
offset +GSS_KRB5_TOK_HDR_LEN+ cbcbytes, desc.iv, pages, 1);gss_krb5_aes_decrypt:/* Make sure IV carries forward from any CBC results. */ ret = gss_krb5_cts_crypt(cipher, &subbuf, cbcbytes, desc.iv, NULL, 0);
This is something different: for some reason, this code chooses to split the input into two parts, and passes the first into plain CBC, and then passes the rest (which needs ciphertext stealing *) into CBC-CTS configured with the same key. The net result should be precisely the same, so I'm not sure why this is implemented like this, but the consequence of this is that the final CBC ciphertext block should be passed to the CTS-CBC routine as its input IV, and our CBC implementations happen to return this via the IV buffer (to support transparent chaining inside the crypto API implementations)
This patch is about what gss_krb5_cts_crypt() /returns/ to its caller via desc.iv, which, as I explained, has no semantic significance, but is covered by RFC 3962 test vectors nonetheless.
* note that the CS3 ciphertext stealing scheme we use in the crypto API always reorders the final two blocks, even if the input length is an integral multiple of the block size, so we always need the CTS logic to take effect.