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 <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

7 files changed, 1 insertions, 52 deletions

diff --git a/arch/x86/include/asm/xen/hypercall.h b/arch/x86/include/asm/xen/hypercall.h
index 7d5f8ad66774..97771b9d33af 100644
--- a/arch/x86/include/asm/xen/hypercall.h
+++ b/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);
 
diff --git a/arch/x86/kernel/callthunks.c b/arch/x86/kernel/callthunks.c
index 465647456753..f17d16607882 100644
--- a/arch/x86/kernel/callthunks.c
+++ b/arch/x86/kernel/callthunks.c
@@ -143,11 +143,6 @@ static bool skip_addr(void *dest)
 	    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;
 }
 
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index fab3ac9a4574..6a17396c8174 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/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);
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 1887435af2fb..43dcd8c7badc 100644
--- a/arch/x86/xen/enlighten.c
+++ b/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);
 
diff --git a/arch/x86/xen/enlighten_hvm.c b/arch/x86/xen/enlighten_hvm.c
index 973a74fc966a..fe57ff85d004 100644
--- a/arch/x86/xen/enlighten_hvm.c
+++ b/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();
 
diff --git a/arch/x86/xen/enlighten_pvh.c b/arch/x86/xen/enlighten_pvh.c
index bf68c329fc01..0e3d930bcb89 100644
--- a/arch/x86/xen/enlighten_pvh.c
+++ b/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;
 
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index c173ba6740e9..9252652afe59 100644
--- a/arch/x86/xen/xen-head.S
+++ b/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")