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

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Wed, Nov 30, 2022 at 05:13:18PM +0100, Arnd Bergmann wrote:
> On Wed, Nov 30, 2022, at 16:47, Jason A. Donenfeld wrote:
> 
> >> > 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?
> 
> The typical problem is someone doing a copy_to_user() of an in-kernel
> structure into the userspace side, which would then copy the
> padding as well. If the source is on the stack, a malicious caller
> can trick the another syscall into leaving sensitive data at this
> exact stack location.

I'm quite aware of this infoleak, having made use of it countless times
over the years. It just doesn't seem relevant to the vvar page.

Jason