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

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Message-ID: <8c9a53b9-12e6-3380-21c8-4fe85342f0ac@suse.de>
Date:   Thu, 9 May 2019 00:12:54 -0300
From:   Joao Moreira <jmoreira@...e.de>
To:     Eric Biggers <ebiggers@...nel.org>,
        Kees Cook <keescook@...omium.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        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 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@...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;
>> };
>>
> 
> 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
>