Re: [PATCH v5 01/11] ima-evm-utils: Make sure sig buffer is always MAX_SIGNATURE_SIZE

[Mimi Zohar <zohar@linux.ibm.com>]

On Fri, 2019-06-21 at 09:59 +0300, Vitaly Chikunov wrote:
> On Thu, Jun 20, 2019 at 05:42:18PM -0400, Mimi Zohar wrote:
> > On Tue, 2019-06-18 at 16:56 +0300, Vitaly Chikunov wrote:
> > > Fix off-by-one error of the output buffer passed to sign_hash().
> > > 
> > > Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
> > > ---
> > >  src/evmctl.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/src/evmctl.c b/src/evmctl.c
> > > index 15a7226..03f41fe 100644
> > > --- a/src/evmctl.c
> > > +++ b/src/evmctl.c
> > > @@ -510,7 +510,7 @@ static int calc_evm_hash(const char *file, unsigned char *hash)
> > >  static int sign_evm(const char *file, const char *key)
> > >  {
> > >  	unsigned char hash[MAX_DIGEST_SIZE];
> > > -	unsigned char sig[MAX_SIGNATURE_SIZE];
> > > +	unsigned char sig[MAX_SIGNATURE_SIZE + 1];
> > >  	int len, err;
> > > 
> > >  	len = calc_evm_hash(file, hash);
> > > @@ -519,7 +519,7 @@ static int sign_evm(const char *file, const char *key)
> > >  		return len;
> > > 
> > >  	len = sign_hash(params.hash_algo, hash, len, key, NULL, sig + 1);
> > > -	assert(len < sizeof(sig));
> > > +	assert(len <= MAX_SIGNATURE_SIZE);
> > >  	if (len <= 1)
> > >  		return len;
> > > 
> > 
> > A similar problem occurs in sign_ima.  Without these changes
> > sign_hash() succeeds, returning a length of 520 for
> > sha256/streebog256. 
> 
> I will add it. Also, I found more similar errors and will fix them together.

The first byte of sig is reserved for the type of signature.  The
remaining buffer is for the signature itself.  The existing
"assert(len < sizeof(sig))" is therefore correct.  The sig size being
returned is less than 1023, so why is this change needed?

Mimi