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

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Date:   Wed, 8 May 2019 14:08:25 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Joao Moreira <jmoreira@...e.de>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
        linux-crypto <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3 0/7] crypto: x86: Fix indirect function call casts

On Wed, May 8, 2019 at 6:36 AM Herbert Xu <herbert@...dor.apana.org.au> 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;
};

What CFI dislikes is calling a func(void *ctx, ...) when the actual
function is, for example, func(struct twofish_ctx *ctx, ...).

This needs to be fixed at the call site, not the static initializers,
and since the call site is void, there needs to be a static inline
that will satisfy the types.

I'm open to suggestions! :)

Thanks,

-- 
Kees Cook