Re: [PATCH v3 0/7] crypto: x86: Fix indirect function call casts

[Joao Moreira]

On 5/8/19 11:04 PM, Eric Biggers wrote:
> On Wed, May 08, 2019 at 02:08:25PM -0700, Kees Cook wrote:
> > On Wed, May 8, 2019 at 6:36 AM Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
> > > On Tue, May 07, 2019 at 02:50:46PM -0700, Eric Biggers wrote:
> > > > I don't know yet.  It's difficult to read the code with 2 layers of macros.
> > > >
> > > > Hence why I asked why you didn't just change the prototypes to be compatible.
> > >
> > > I agree.  Kees, since you're changing this anyway please make it
> > > look better not worse.
> >
> > Do you mean I should use the typedefs in the new macros? I'm not aware
> > of a way to use a typedef to declare a function body, so I had to
> > repeat them. I'm open to suggestions!
> >
> > As far as "fixing the prototypes", the API is agnostic of the context
> > type, and uses void *. And also it provides a way to call the same
> > function with different pointer types on the other arguments:
> >
> > For example, quoting the existing code:
> >
> > asmlinkage void twofish_dec_blk(struct twofish_ctx *ctx, u8 *dst,
> >                                  const u8 *src);
> >
> > Which is used for ecb and cbc:
> >
> > #define GLUE_FUNC_CAST(fn) ((common_glue_func_t)(fn))
> > #define GLUE_CBC_FUNC_CAST(fn) ((common_glue_cbc_func_t)(fn))
> > ...
> > static const struct common_glue_ctx twofish_dec = {
> > ...
> >                  .fn_u = { .ecb = GLUE_FUNC_CAST(twofish_dec_blk) }
> >
> > static const struct common_glue_ctx twofish_dec_cbc = {
> > ...
> >                  .fn_u = { .cbc = GLUE_CBC_FUNC_CAST(twofish_dec_blk) }
> >
> > which have different prototypes:
> >
> > typedef void (*common_glue_func_t)(void *ctx, u8 *dst, const u8 *src);
> > typedef void (*common_glue_cbc_func_t)(void *ctx, u128 *dst, const u128 *src);
> > ...
> > struct common_glue_func_entry {
> >          unsigned int num_blocks; /* number of blocks that @fn will process */
> >          union {
> >                  common_glue_func_t ecb;
> >                  common_glue_cbc_func_t cbc;
> >                  common_glue_ctr_func_t ctr;
> >                  common_glue_xts_func_t xts;
> >          } fn_u;
> > };
>
> As Herbert said, the ctx parameters could be made 'void *'.

This is how things were done in the original patch set, but some 
concerns were raised about this approach:

https://lkml.org/lkml/2018/4/16/74

Tks,
Joao.

> And I also asked whether indirect calls to asm code are even allowed with CFI.
> IIRC, the AOSP kernels have been patched to remove them from arm64.  It would be
> helpful if you would answer that question, since it would inform the best
> approach here.
>
> As for the "ecb" functions taking 'u8 *' but the "cbc" ones taking 'u128 *' and
> the same function being used in the blocks==1 case, you could just pick one of
> the types to use for both.  'u8 *' probably makes more sense since both ecb and
> cbc operate on blocks of 16 bytes but don't interpret them as 128-bit integers.
>
> - Eric