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From: "Roger Pau Monné" <roger.pau@citrix.com>
To: George Dunlap <george.dunlap@citrix.com>
Cc: <security@xenproject.org>, <xen-devel@lists.xenproject.org>,
	Jann Horn <jannh@google.com>, Jan Beulich <jbeulich@suse.com>
Subject: Re: [PATCH v2] SUPPORT.md: Un-shimmed 32-bit PV guests are no longer supported
Date: Fri, 7 May 2021 12:26:15 +0200	[thread overview]
Message-ID: <YJUVx8WMn/4f0gMS@Air-de-Roger> (raw)
In-Reply-To: <20210506124752.65844-1-george.dunlap@citrix.com>

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@google.com>
> Signed-off-by: George Dunlap <george.dunlap@citrix.com>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> 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.


  parent reply	other threads:[~2021-05-07 10:26 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-06 12:47 George Dunlap
2021-05-06 13:09 ` Jan Beulich
2021-05-06 14:32   ` Andrew Cooper
2021-05-07 10:26 ` Roger Pau Monné [this message]
2021-05-07 11:05   ` Jan Beulich
2021-06-04 13:14 ` George Dunlap
2021-06-07  9:18 ` Jan Beulich

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