Re: [PATCH v2] SUPPORT.md: Un-shimmed 32-bit PV guests are no longer supported

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, May 06, 2021 at 01:47:52PM +0100, George Dunlap wrote:
> The support status of 32-bit guests doesn't seem particularly useful.
> 
> With it changed to fully unsupported outside of PV-shim, adjust the PV32
> Kconfig default accordingly.
> 
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v2:
>  - add in Kconfig from advisory, ported over c/s d23d792478d
> ---
>  SUPPORT.md           | 9 +--------
>  xen/arch/x86/Kconfig | 7 +++++--
>  2 files changed, 6 insertions(+), 10 deletions(-)
> 
> diff --git a/SUPPORT.md b/SUPPORT.md
> index d0d4fc6f4f..a29680e04c 100644
> --- a/SUPPORT.md
> +++ b/SUPPORT.md
> @@ -86,14 +86,7 @@ No hardware requirements
>  
>      Status, x86_64: Supported
>      Status, x86_32, shim: Supported
> -    Status, x86_32, without shim: Supported, with caveats
> -
> -Due to architectural limitations,
> -32-bit PV guests must be assumed to be able to read arbitrary host memory
> -using speculative execution attacks.
> -Advisories will continue to be issued
> -for new vulnerabilities related to un-shimmed 32-bit PV guests
> -enabling denial-of-service attacks or privilege escalation attacks.
> +    Status, x86_32, without shim: Supported, not security supported
>  
>  ### x86/HVM
>  
> diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
> index e55e029b79..9b164db641 100644
> --- a/xen/arch/x86/Kconfig
> +++ b/xen/arch/x86/Kconfig
> @@ -55,7 +55,7 @@ config PV
>  config PV32
>       bool "Support for 32bit PV guests"
>       depends on PV
> -     default y
> +     default PV_SHIM
>       select COMPAT
>       ---help---
>         The 32bit PV ABI uses Ring1, an area of the x86 architecture which
> @@ -67,7 +67,10 @@ config PV32
>         reduction, or performance reasons.  Backwards compatibility can be
>         provided via the PV Shim mechanism.
>  
> -       If unsure, say Y.
> +       Note that outside of PV Shim, 32-bit PV guests are not security
> +       supported anymore.
> +
> +       If unsure, use the default setting.

While not opposed to this, I wonder whether we should give people some
time to adapt. We have in the past not blocked vulnerable
configurations by default (ie: the smt stuff for example).

It might be less disruptive for users to start by printing a warning
message at boot (either on the serial for dom0 or in the toolstack for
domU) and switch the default Kconfig slightly later?

Note I don't have any specific interest in 32bit PV, so I'm not going
to argue strongly against this if everyone else is fine with it.

I also wonder whether xl shouldn't try to boot PV 32bit guests by
default using the shim now if the hypervisor has been built without
CONFIG_PV32, or at least print a message so users know how to deal
with the fallout.

Roger.