On Wed, 24 Oct 2018, James Bottomley wrote:
+static void KDFa(u8 *key, int keylen, const char *label, u8 *u,
+u8 *v, int bytes, u8 *out)
Should this be in lower case? I would rename it as tpm_kdfa().
This one is defined as KDFa() in the standards and it's not TPM
specific (although some standards refer to it as KDFA). I'm not averse
to making them tpm_kdfe() and tpm_kdfa() but I was hoping that one day
the crypto subsystem would need them and we could move them in there
because KDFs are the new shiny in crypto primitives (TLS 1.2 started
using them, for instance).
I care more about tracing and debugging than naming and having 'tpm_' in
front of every TPM function makes tracing a lean process. AFAIK using
upper case letters is against kernel coding conventions. I'm not sure
why this would make an exception on that.
Why doesn't it matter here?
Because, as the comment says, it eventually gets overwritten by running
ecdh to derive the two co-ordinates. (pointers to these two
uninitialized areas are passed into the ecdh destination sg list).
Oh, I just misunderstood the comment. Wouldn't it be easier to say that
the data is initialized later?
+ buf_len = crypto_ecdh_key_len();
+ if (sizeof(encoded_key) < buf_len) {
+ dev_err(>dev, "salt buffer too small needs
%d\n",
+ buf_len);
+ goto out;
+ }
In what situation this can happen? Can sizeof(encoded_key) >=
buf_len?
Yes, but only if someone is trying to crack your ecdh. One of the
security issues in ecdh is if someone makes a very specific point
choice (usually in the cofactor space) that has a very short period,
the attacker can guess the input to KDFe. In this case if TPM genie
provided a specially crafted attack EC point, we'd detect it here
because the resulting buffer would be too short.
Right. Thank you for the explanation. Here some kind of comment might
not be a bad idea...
In general this function should have a clear explanation what it does
and maybe less these one character variables but instead variables
with more documenting names. Explain in high level what algorithms
are used and how the salt is calculated.
I'll try, but this is a rather complex function.
Understood. I do not expect perfection here and we can improve
documetation later on.
For anyone wanting to review James' patches and w/o much experience on
EC, I recommend reading this article:
https://arstechnica.com/information-technology/2013/10/a-relatively-easy-to-understand-primer-on-elliptic-curve-cryptography/
I read it few years ago and refreshed my memory few days ago by
re-reading it.
+
+/**
+ * tpm_buf_append_hmac_session() append a TPM session element
+ * @buf: The buffer to be appended
+ * @auth: the auth structure allocated by
tpm2_start_auth_session()
+ * @attributes: The session attributes
+ * @passphrase: The session authority (NULL if none)
+ * @passphraselen: The length of the session authority (0 if none)
The alignment.
the alignment of what?
We generally have parameter descriptions tab-aligned.
Why there would be trailing zeros?
Because TPM 1.2 mandated zero padded fixed size passphrases so the TPM
2.0 standard specifies a way of converting these to variable size
strings by eliminating the zero padding.
Ok.
James
I'm also looking forward for the CONTEXT_GAP patch based on the
yesterdays discussion. We do want it and I was stupid not to take it
couple years ago :-) Thanks.
/Jarkko
