x86/xen: remove hypercall page

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]>

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/kernel/callthunks.c

@@ -142,11 +142,6 @@ static bool skip_addr(void *dest)
 	if (dest >= (void *)relocate_kernel &&
 	    dest < (void*)relocate_kernel + KEXEC_CONTROL_CODE_MAX_SIZE)
 		return true;
-#endif
-#ifdef CONFIG_XEN
-	if (dest >= (void *)hypercall_page &&
-	    dest < (void*)hypercall_page + PAGE_SIZE)
-		return true;
 #endif
 	return false;
 }

arch/x86/kernel/vmlinux.lds.S

@@ -519,14 +519,10 @@ INIT_PER_CPU(irq_stack_backing_store);
  * linker will never mark as relocatable. (Using just ABSOLUTE() is not
  * sufficient for that).
  */
-#ifdef CONFIG_XEN
 #ifdef CONFIG_XEN_PV
 xen_elfnote_entry_value =
 	ABSOLUTE(xen_elfnote_entry) + ABSOLUTE(startup_xen);
 #endif
-xen_elfnote_hypercall_page_value =
-	ABSOLUTE(xen_elfnote_hypercall_page) + ABSOLUTE(hypercall_page);
-#endif
 #ifdef CONFIG_PVH
 xen_elfnote_phys32_entry_value =
 	ABSOLUTE(xen_elfnote_phys32_entry) + ABSOLUTE(pvh_start_xen - LOAD_OFFSET);

arch/x86/xen/enlighten.c

@@ -22,8 +22,6 @@
 
 #include "xen-ops.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

@@ -106,15 +106,8 @@ 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

@@ -129,17 +129,10 @@ static void __init pvh_arch_setup(void)
 
 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));
-
 	x86_init.oem.arch_setup = pvh_arch_setup;
 	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)
@@ -199,8 +177,6 @@ SYM_FUNC_END(xen_hypercall_intel)
 #else
 # define FEATURES_DOM0 0
 #endif
-	ELFNOTE(Xen, XEN_ELFNOTE_HYPERCALL_PAGE, .globl xen_elfnote_hypercall_page;
-		xen_elfnote_hypercall_page: _ASM_PTR xen_elfnote_hypercall_page_value - .)
 	ELFNOTE(Xen, XEN_ELFNOTE_SUPPORTED_FEATURES,
 		.long FEATURES_PV | FEATURES_PVH | FEATURES_DOM0)
 	ELFNOTE(Xen, XEN_ELFNOTE_LOADER,         .asciz "generic")