x86/xen: remove hypercall page

commit 7fa0da5373685e7ed249af3fa317ab1e1ba8b0a6 upstream.

The hypercall page is no longer needed. It can be removed, as from the
Xen perspective it is optional.

But, from Linux's perspective, it removes naked RET instructions that
escape the speculative protections that Call Depth Tracking and/or
Untrain Ret are trying to achieve.

This is part of XSA-466 / CVE-2024-53241.

Reported-by: Andrew Cooper <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>
Reviewed-by: Andrew Cooper <[email protected]>
Reviewed-by: Jan Beulich <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: jgross1

arch/x86/include/asm/xen/hypercall.h

@@ -88,8 +88,6 @@ struct xen_dm_op_buf;
  * there aren't more than 5 arguments...)
  */
 
-extern struct { char _entry[32]; } hypercall_page[];
-
 void xen_hypercall_func(void);
 DECLARE_STATIC_CALL(xen_hypercall, xen_hypercall_func);
 

arch/x86/xen/enlighten.c

@@ -26,8 +26,6 @@
 #include "smp.h"
 #include "pmu.h"
 
-EXPORT_SYMBOL_GPL(hypercall_page);
-
 DEFINE_STATIC_CALL(xen_hypercall, xen_hypercall_hvm);
 EXPORT_STATIC_CALL_TRAMP(xen_hypercall);
 

arch/x86/xen/enlighten_hvm.c

@@ -108,15 +108,8 @@ static void __init init_hvm_pv_info(void)
 	/* PVH set up hypercall page in xen_prepare_pvh(). */
 	if (xen_pvh_domain())
 		pv_info.name = "Xen PVH";
-	else {
-		u64 pfn;
-		uint32_t msr;
-
+	else
 		pv_info.name = "Xen HVM";
-		msr = cpuid_ebx(base + 2);
-		pfn = __pa(hypercall_page);
-		wrmsr_safe(msr, (u32)pfn, (u32)(pfn >> 32));
-	}
 
 	xen_setup_features();
 

arch/x86/xen/enlighten_pvh.c

@@ -27,17 +27,10 @@ EXPORT_SYMBOL_GPL(xen_pvh);
 
 void __init xen_pvh_init(struct boot_params *boot_params)
 {
-	u32 msr;
-	u64 pfn;
-
 	xen_pvh = 1;
 	xen_domain_type = XEN_HVM_DOMAIN;
 	xen_start_flags = pvh_start_info.flags;
 
-	msr = cpuid_ebx(xen_cpuid_base() + 2);
-	pfn = __pa(hypercall_page);
-	wrmsr_safe(msr, (u32)pfn, (u32)(pfn >> 32));
-
 	if (xen_initial_domain())
 		x86_init.oem.arch_setup = xen_add_preferred_consoles;
 	x86_init.oem.banner = xen_banner;

arch/x86/xen/xen-head.S

@@ -22,28 +22,6 @@
 #include <xen/interface/xen-mca.h>
 #include <asm/xen/interface.h>
 
-.pushsection .noinstr.text, "ax"
-	.balign PAGE_SIZE
-SYM_CODE_START(hypercall_page)
-	.rept (PAGE_SIZE / 32)
-		UNWIND_HINT_FUNC
-		ANNOTATE_NOENDBR
-		ANNOTATE_UNRET_SAFE
-		ret
-		/*
-		 * Xen will write the hypercall page, and sort out ENDBR.
-		 */
-		.skip 31, 0xcc
-	.endr
-
-#define HYPERCALL(n) \
-	.equ xen_hypercall_##n, hypercall_page + __HYPERVISOR_##n * 32; \
-	.type xen_hypercall_##n, @function; .size xen_hypercall_##n, 32
-#include <asm/xen-hypercalls.h>
-#undef HYPERCALL
-SYM_CODE_END(hypercall_page)
-.popsection
-
 #ifdef CONFIG_XEN_PV
 	__INIT
 SYM_CODE_START(startup_xen)
@@ -176,7 +154,6 @@ SYM_FUNC_END(xen_hypercall_intel)
 #ifdef CONFIG_XEN_PV
 	ELFNOTE(Xen, XEN_ELFNOTE_ENTRY,          _ASM_PTR startup_xen)
 #endif
-	ELFNOTE(Xen, XEN_ELFNOTE_HYPERCALL_PAGE, _ASM_PTR hypercall_page)
 	ELFNOTE(Xen, XEN_ELFNOTE_FEATURES,
 		.ascii "!writable_page_tables|pae_pgdir_above_4gb")
 	ELFNOTE(Xen, XEN_ELFNOTE_SUPPORTED_FEATURES,