Re: [PATCH v2 07/13] RISC-V: crypto: add accelerated AES-CBC/CTR/ECB/XTS implementations

Eric Biggers
2023-11-29T20:16:26Z

On Wed, Nov 29, 2023 at 03:57:25PM +0800, Jerry Shih wrote:
> On Nov 28, 2023, at 12:07, Eric Biggers <ebiggers@kernel.org> wrote:
> > On Mon, Nov 27, 2023 at 03:06:57PM +0800, Jerry Shih wrote:
> >> +typedef void (*aes_xts_func)(const u8 *in, u8 *out, size_t length,
> >> + const struct crypto_aes_ctx *key, u8 *iv,
> >> + int update_iv);
> >
> > There's no need for this indirection, because the function pointer can only have
> > one value.
> >
> > Note also that when Control Flow Integrity is enabled, assembly functions can
> > only be called indirectly when they use SYM_TYPED_FUNC_START. That's another
> > reason to avoid indirect calls that aren't actually necessary.
>
> We have two function pointers for encryption and decryption.
> static int xts_encrypt(struct skcipher_request *req)
> {
> return xts_crypt(req, rv64i_zvbb_zvkg_zvkned_aes_xts_encrypt);
> }
>
> static int xts_decrypt(struct skcipher_request *req)
> {
> return xts_crypt(req, rv64i_zvbb_zvkg_zvkned_aes_xts_decrypt);
> }
> The enc and dec path could be folded together into `xts_crypt()`, but we will have
> additional branches for enc/decryption path if we don't want to have the indirect calls.
> Use `SYM_TYPED_FUNC_START` in asm might be better.
>

Right. Normal branches are still more efficient and straightforward than
indirect calls, though, and they don't need any special considerations for CFI.
So I'd just add a 'bool encrypt' or 'bool decrypt' argument to xts_crypt(), and
make xts_crypt() call the appropriate assembly function based on that.

> > Did you consider writing xts_crypt() the way that arm64 and x86 do it? The
> > above seems to reinvent sort of the same thing from first principles. I'm
> > wondering if you should just copy the existing approach for now. Then there
> > would be no need to add the scatterwalk_next() function, and also the handling
> > of inputs that don't need ciphertext stealing would be a bit more streamlined.
>
> I will check the arm and x86's implementations.
> But the `scatterwalk_next()` proposed in this series does the same thing as the
> call `scatterwalk_ffwd()` in arm and x86's implementations.
> The scatterwalk_ffwd() iterates from the beginning of scatterlist(O(n)), but the
> scatterwalk_next() is just iterates from the end point of the last used
> scatterlist(O(1)).

Sure, but your scatterwalk_next() only matters when there are multiple
scatterlist entries and the AES-XTS message length isn't a multiple of the AES
block size. That's not an important case, so there's little need to
micro-optimize it. The case that actually matters for AES-XTS is a single-entry
scatterlist containing a whole number of AES blocks.

- Eric