On Mon, Oct 26, 2026 at 12:12:13AM +0200, Jarkko Sakkinen wrote:
> On Sun, Oct 25, 2026 at 02:11:19PM -0700, Eric Biggers wrote:
> > crypto_kpp_set_secret() can fail, for example if the kernel is booted
> > with fips=1 and the broken-by-design FIPS RNG used for ECC private key
> > generation doesn't feel like generating random bytes at the moment.
> > Check for error, cancelling the TPM session creation rather than
> > proceeding with a zero or uninitialized private key.
> >
> > Fixes: 699e3efd6c64 ("tpm: Add HMAC session start and end functions")
> > Cc: [email protected]
> > Signed-off-by: Eric Biggers <[email protected]>
> > ---
> > drivers/char/tpm/tpm2-sessions.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/char/tpm/tpm2-sessions.c
> > b/drivers/char/tpm/tpm2-sessions.c
> > index ca1e2bf424e14..2aaaad69760dc 100644
> > --- a/drivers/char/tpm/tpm2-sessions.c
> > +++ b/drivers/char/tpm/tpm2-sessions.c
> > @@ -527,7 +527,9 @@ static int tpm_buf_append_salt(struct tpm_buf *buf,
> > struct tpm_chip *chip,
> > }
> > crypto_ecdh_encode_key(encoded_key, buf_len, &p);
> > /* this generates a random private key */
>
> Could you delete this comment while at it? It's not helping with
> anything.
>
> > - crypto_kpp_set_secret(kpp, encoded_key, buf_len);
> > + rc = crypto_kpp_set_secret(kpp, encoded_key, buf_len);
> > + if (rc)
> > + goto err_free_kpp;
> >
> > /* salt is now the public point of this private key */
> > req = kpp_request_alloc(kpp, GFP_KERNEL);
> >
I actually think the comment is helpful, because of how unreadable the
crypto_kpp API is. It's generating a private key, but to request that,
crypto_kpp apparently requires that an empty key be encoded into a byte
string. Then that key is "set", which actually generates a new key.
In comparison, the Curve25519 equivalent is
curve25519_generate_secret(), which is clearly named and also always
succeeds. At some point we need to fix the ECDH API...
- Eric
