Re: [PATCH v2 2/2] crypto: aead, cipher - zeroize key buffer after use

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Apr 16, 2024 at 07:14:28PM +0000, Mothershead, Hailey wrote:
> > On 4/15/24, 3:50 PM, "Eric Biggers" <ebiggers@kernel.org <mailto:ebiggers@kernel.org>> wrote:
> >
> > On Mon, Apr 15, 2024 at 10:19:15PM +0000, Hailey Mothershead wrote:
> > > I.G 9.7.B for FIPS 140-3 specifies that variables temporarily holding
> > > cryptographic information should be zeroized once they are no longer
> > > needed. Accomplish this by using kfree_sensitive for buffers that
> > > previously held the private key.
> > >
> > > Signed-off-by: Hailey Mothershead <hailmo@amazon.com <mailto:hailmo@amazon.com>>
> > > ---
> > > crypto/aead.c | 3 +--
> > > crypto/cipher.c | 3 +--
> > > 2 files changed, 2 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/crypto/aead.c b/crypto/aead.c
> > > index 16991095270d..c4ece86c45bc 100644
> > > --- a/crypto/aead.c
> > > +++ b/crypto/aead.c
> > > @@ -35,8 +35,7 @@ static int setkey_unaligned(struct crypto_aead *tfm, const u8 *key,
> > > alignbuffer = (u8 *)ALIGN((unsigned long)buffer, alignmask + 1);
> > > memcpy(alignbuffer, key, keylen);
> > > ret = crypto_aead_alg(tfm)->setkey(tfm, alignbuffer, keylen);
> > > - memset(alignbuffer, 0, keylen);
> > > - kfree(buffer);
> > > + kfree_sensitive(buffer);
> > > return ret;
> > > }
> > >
> > > diff --git a/crypto/cipher.c b/crypto/cipher.c
> > > index b47141ed4a9f..395f0c2fbb9f 100644
> > > --- a/crypto/cipher.c
> > > +++ b/crypto/cipher.c
> > > @@ -34,8 +34,7 @@ static int setkey_unaligned(struct crypto_cipher *tfm, const u8 *key,
> > > alignbuffer = (u8 *)ALIGN((unsigned long)buffer, alignmask + 1);
> > > memcpy(alignbuffer, key, keylen);
> > > ret = cia->cia_setkey(crypto_cipher_tfm(tfm), alignbuffer, keylen);
> > > - memset(alignbuffer, 0, keylen);
> > > - kfree(buffer);
> > > + kfree_sensitive(buffer);
> > > return ret;
> >
> >
> > Well, the memset()s that you're removing already did the zeroization. This
> > patch seems worthwhile as a code simplification, but please don't characterize
> > it as a bug fix, because it's not.
> >
> >
> > - Eric
> 
> kfree_sensitive uses memzero_explicit() instead of the memset()s used
> above. The memzero_explicit header states -
> 
> * Note: usually using memset() is just fine (!), but in cases
> * where clearing out _local_ data at the end of a scope is
> * necessary, memzero_explicit() should be used instead in
> * order to prevent the compiler from optimising away zeroing.
> 
> It accomplishes this by calling memset() and then adding a barrier. Since
> FIPS requires this data be zeroed out, and the current memset() and
> kfree() don't guarantee this, I do not think the commit message is
> misleading. I can clarify the message with the above information if that
> is preferred.
> 

It's heap data, not local data.  So no, memzero_explicit() isn't actually needed
here.  It would convey the intent better, but it's not actually needed.

- Eric