Re: [PATCH v10 3/4] random: introduce generic vDSO getrandom() implementation

[Jason A. Donenfeld]

Hi Arnd,

On Wed, Nov 30, 2022 at 4:29 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > I think it does address the issue. CONFIG_64BIT is a .config setting,
> > not a compiler-derived setting. So a 64-bit kernel will get a u64 in
> > kernel mode, and then it will get a u64 for the 64-bit vdso usermode
> > compile, and finally it will get a u64 for the 32-bit vdso usermode
> > compile. So in all three cases, the size is the same.
>
> I see what you mean now. However this means your vdso32 copies
> are different between 32-bit and 64-bit kernels. If you need to
> access one of the fields from assembler, it even ends up
> different at source level, which adds a bit of complexity.
>
> Making the interface configuration-independent makes it obvious
> to the reader that none of these problems can happen.

Except ideally, these are word-sized accesses (where only compat code
has to suffer I suppose).

> >> > struct vdso_rng_data {
> >> >       vdso_kernel_ulong       generation;
> >> >       bool                    is_ready;
> >> > };
> >>
> >> There is another problem with this: you have implicit padding
> >> in the structure because the two members have different size
> >> and alignment requirements. The easiest fix is to make them
> >> both u64, or you could have a u32 is_ready and an explit u32
> >> for the padding.
> >
> > There's padding at the end of the structure, yes. But both
> > `generation` and `is_ready` will be at the same offset. If the
> > structure grows, then sure, that'll have to be taken into account. But
> > that's not a problem because this is a private implementation detail
> > between the vdso code and the kernel.
>
> I was not concerned about incompatibility here, but rather about
> possibly leaking kernel data to the vdso page.

The vvar page starts out zeroed, no?

Jason