Subject: SUPPORT.md: Un-shimmed 32-bit PV guests are no longer supported
From: George Dunlap <[email-masked]>

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 <[email-masked]>
Signed-off-by: George Dunlap <[email-masked]>
Signed-off-by: Jan Beulich <[email-masked]>
---

NB this patch should be considered a proposal to the community.  It
will not become effective until three weeks after the XSA-370 embargo
lifts, and only if there are no objections raised before that point.

TBD: Should we also default opt_pv32 to false when not running in shim
     mode?

The (forward) dependency on PV_SHIM isn't very useful especially when
configuring from scratch - we may want to re-order items down the road,
such that the prompt for PV_SHIM occurs ahead of that for PV32. Yet then
this conflicts with PV_SHIM also depending on GUEST.

v3:
- Add Kconfig adjustment.

v2:
- Port over changes in patch 1

--- 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
 
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -56,7 +56,7 @@ config PV
 config PV32
 	bool "Support for 32bit PV guests"
 	depends on PV
-	default y
+	default PV_SHIM
 	---help---
 	  The 32bit PV ABI uses Ring1, an area of the x86 architecture which
 	  was deprecated and mostly removed in the AMD64 spec.  As a result,
@@ -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.
 
 config PV_LINEAR_PT
        bool "Support for PV linear pagetables"