Re: [PATCH] x86: re-initialize RNG seed when selecting kernel

[Peter Maydell]

On Mon, 26 Sept 2022 at 17:53, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> On both x86 and dtb-based archs, the seed in memory is zeroed out by the
> kernel after reading. So, as far as the guest is concerned, there's
> forward secrecy. Except! Except if the guest has someway of
> re-requesting that seed from the host. This patch prevents that from
> happening through fw_cfg on x86. Somebody told me last week that device
> tree archs don't use fw_cfg, so this won't be a problem there. I haven't
> yet looked to verify that yet, though, or looked if there are other
> mechanisms.

I am leaping in here with no context, so I may well have
the wrong end of the stick, but:

"does this system have a fw_cfg device" and "does this system
pass a device tree to the kernel" are orthogonal questions:

 fw_cfg, no device tree: classic x86 pc; arm virt board when
   booting UEFI firmware in the guest
 fw_cfg, device tree: arm virt board booting a kernel directly
 no fw_cfg, device tree: arm vexpress-a15 board (or any other
   just-emulating-hardware machine type)
 no fw_cfg, no device tree: arm sbsa-ref board (and probably
   lots of non-arm architecture machines too)

PS: if we're going to give FW_CFG_KERNEL_DATA magic side
effect behaviour, is there somewhere we can document that magic?

thanks
-- PMM