Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

[Ard Biesheuvel]

On Fri, 15 Apr 2022 at 14:19, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
> On Fri, Apr 15, 2022 at 10:05:21AM +0200, Ard Biesheuvel wrote:
> > On Fri, 15 Apr 2022 at 09:52, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
> > > On Fri, Apr 15, 2022 at 09:49:12AM +0200, Ard Biesheuvel wrote:
> > > > I'm not sure I understand what would go wrong if that assumption no
> > > > longer holds.
> > >
> > > It's very simple, we don't do anything to the pointer returned
> > > by kmalloc before returning it as a tfm or other object with
> > > an alignment of CRYPTO_MINALIGN.  IOW if kmalloc starts returning
> > > pointers that are not aligned to CRYPTO_MINALIGN then we'd be
> > > lying to the compiler.
> >
> > I guess that should be fixable. GIven that this is about padding
> > rather than alignment, we could do something like
> >
> > struct crypto_request {
> >   union {
> >       struct {
> >         ... fields ...
> >       };
> >       u8 __padding[ARCH_DMA_MINALIGN];
> >    };
> >     void __ctx[]  __align(CRYPTO_MINALIGN);
> > };
> >
> > And then hopefully, we can get rid of the padding once we fix drivers
> > doing non-cache coherent inbound DMA into those structures.
>
> But if we keep CRYPTO_MINALIGN as 128, don't we get the padding
> automatically?
>

I suppose, yes.

> struct crypto_request {
>         ...
>         void *__ctx[] CRYPTO_MINALIGN_ATTR;
> };
>
> __alignof__(struct crypto_request) == 128;
> sizeof(struct crypto_request) == N * 128
>
> The same alignment and size is true for a structure like:
>
> struct crypto_alg {
>         ...
> } CRYPTO_MINALIGN_ATTR;
>
> Any kmalloc() of sizeof(the above structures) will return a pointer
> aligned to 128, irrespective of what ARCH_KMALLOC_MINALIGN is.
>
> The problem is if you have a structure without any alignment attribute
> (just ABI default), making its sizeof() smaller than ARCH_DMA_MINALIGN.
> In this case kmalloc() could return a pointer aligned to something
> smaller. Is this the case in the crypto code today? I can see it uses
> the right alignment annotations already, no need for kmalloc() hacks.
>

As long as CRYPTO_MINALIGN >= ARCH_KMALLOC_MINALIGN, we won't be lying
to the compiler when casting kmalloc buffers to these struct types.

I'd still like to fix the bad DMA behavior but I suppose it is a separate issue.