Re: [PATCH v2] KEYS: asymmetric: Copy sig and digest in public_key_verify_signature()

[Roberto Sassu]

On Fri, 2022-12-09 at 11:04 -0800, Eric Biggers wrote:
> On Fri, Dec 09, 2022 at 04:06:33PM +0100, Roberto Sassu wrote:
> > +	/* key is used to store the sig and digest too. */
> > +	key = kmalloc(key_max_len, GFP_KERNEL);
> >  	if (!key)
> >  		goto error_free_req;
> 
> Maybe just call this 'buf', as the key is just one of the purposes the buffer is
> used for now.

Yes, better.

> > +	/* Cannot use one scatterlist. The first needs to be s->s_size long. */
> > +	sg_set_buf(&src_sg[0], key, sig->s_size);
> > +	sg_set_buf(&src_sg[1], key + sig->s_size, sig->digest_size);
> >  	akcipher_request_set_crypt(req, src_sg, NULL, sig->s_size,
> >  				   sig->digest_size);
> 
> AFAIK, none of the crypto APIs that operate on 'scatterlist' are supposed to
> care how the data is divided up into scatterlist elements.  So it sounds like
> there is another bug that needs to be fixed.  It should be fixed, not worked
> around.

The problem is a misalignment between req->src_len (set to sig->s_size
by akcipher_request_set_crypt()) and the length of the scatterlist (if
we set the latter to sig->s_size + sig->digest_size).

When rsa_enc() calls mpi_read_raw_from_sgl(), it passes req->src_len as
argument, and the latter allocates the MPI according to that. However,
it does parsing depending on the length of the scatterlist.

If there are two scatterlists, it is not a problem, there is no
misalignment. mpi_read_raw_from_sgl() picks the first. If there is just
one, mpi_read_raw_from_sgl() parses all data there.

Roberto