x86: Add Kconfig option to require NX bit support

This option hardens Xen by forcing it to write secure (NX-enhanced) PTEs
regardless of the runtime NX feature bit in boot_cpu_data. This prevents an
attacker with partial write support from affecting Xen's PTE generation
logic by overriding the NX feature flag. The patch asserts support for the
NX bit in PTEs at boot time and if so short-circuits the cpu_has_nx macro
to 1.

It has the nice benefit of replacing many instances of runtime checks with
folded constants. This has several knock-on effects that improve codegen,
saving 2.5KiB off the text section.

The config option defaults to OFF for compatibility with previous
behaviour.

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/Kconfig

@@ -307,6 +307,22 @@ config MEM_SHARING
 	bool "Xen memory sharing support (UNSUPPORTED)" if UNSUPPORTED
 	depends on HVM
 
+config REQUIRE_NX
+	bool "Require NX (No eXecute) support"
+	help
+	  No-eXecute (also called XD "eXecute Disable" and DEP "Data
+	  Execution Prevention") is a security feature designed originally
+	  to combat buffer overflow attacks by marking regions of memory
+	  which the CPU must not interpret as instructions.
+
+	  The NX feature exists in every 64bit CPU except for some very
+	  early Pentium 4 Prescott machines.
+
+	  Enabling this option will improve Xen's security by removing
+	  cases where Xen could be tricked into thinking that the feature
+	  was unavailable. However, if enabled, Xen will no longer boot on
+	  any CPU which is lacking NX support.
+
 endmenu
 
 source "common/Kconfig"

xen/arch/x86/boot/head.S

@@ -123,6 +123,7 @@ multiboot2_header:
 .Lbad_ldr_nih: .asciz "ERR: EFI ImageHandle is not provided by bootloader!"
 .Lbad_efi_msg: .asciz "ERR: EFI IA-32 platforms are not supported!"
 .Lbag_alg_msg: .asciz "ERR: Xen must be loaded at a 2Mb boundary!"
+.Lno_nx_msg:   .asciz "ERR: Not an NX-capable CPU!"
 
         .section .init.data, "aw", @progbits
         .align 4
@@ -153,6 +154,11 @@ early_error: /* Here to improve the disassembly. */
 .Lnot_aligned:
         add     $sym_offs(.Lbag_alg_msg), %esi
         jmp     .Lget_vtb
+#ifdef CONFIG_REQUIRE_NX
+.Lno_nx:
+        add     $sym_offs(.Lno_nx_msg), %esi
+        jmp     .Lget_vtb
+#endif
 .Lmb2_no_st:
         /*
          * Here we are on EFI platform. vga_text_buffer was zapped earlier
@@ -656,7 +662,12 @@ trampoline_setup:
         bt      $cpufeat_bit(X86_FEATURE_LM),%edx
         jnc     .Lbad_cpu
 
-        /* Check for NX */
+        /*
+         * Check for NX
+         *   - If Xen was compiled requiring it simply assert it's
+         *     supported. The trampoline already has the right constant.
+         *   - Otherwise, update the trampoline EFER mask accordingly.
+         */
         bt      $cpufeat_bit(X86_FEATURE_NX), %edx
         jc     .Lgot_nx
 
@@ -695,9 +706,11 @@ trampoline_setup:
         jnc     .Lno_nx
 
 .Lgot_nx:
+#ifndef CONFIG_REQUIRE_NX
         /* Adjust EFER given that NX is present */
         orb     $EFER_NXE >> 8, 1 + sym_esi(trampoline_efer)
 .Lno_nx:
+#endif
 
         /* Stash TSC to calculate a good approximation of time-since-boot */
         rdtsc

xen/arch/x86/boot/trampoline.S

@@ -147,7 +147,8 @@ GLOBAL(trampoline_misc_enable_off)
 
 /* EFER OR-mask for boot paths.  SCE conditional on PV support, NX added when available. */
 GLOBAL(trampoline_efer)
-        .long   EFER_LME | (EFER_SCE * IS_ENABLED(CONFIG_PV))
+        .long   EFER_LME | (EFER_SCE * IS_ENABLED(CONFIG_PV)) | \
+                (EFER_NXE * IS_ENABLED(CONFIG_REQUIRE_NX))
 
 GLOBAL(trampoline_xen_phys_start)
         .long   0

xen/arch/x86/efi/efi-boot.h

@@ -751,6 +751,15 @@ static void __init efi_arch_cpu(void)
     {
         caps[FEATURESET_e1d] = cpuid_edx(0x80000001);
 
+        /*
+         * This check purposefully doesn't use cpu_has_nx because
+         * cpu_has_nx bypasses the boot_cpu_data read if Xen was compiled
+         * with CONFIG_REQUIRE_NX
+         */
+        if ( IS_ENABLED(CONFIG_REQUIRE_NX) &&
+             !boot_cpu_has(X86_FEATURE_NX) )
+            blexit(L"This build of Xen requires NX support");
+
         if ( cpu_has_nx )
             trampoline_efer |= EFER_NXE;
     }

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

@@ -91,7 +91,8 @@ static inline bool boot_cpu_has(unsigned int feat)
 #define cpu_has_hypervisor      boot_cpu_has(X86_FEATURE_HYPERVISOR)
 
 /* CPUID level 0x80000001.edx */
-#define cpu_has_nx              boot_cpu_has(X86_FEATURE_NX)
+#define cpu_has_nx              (IS_ENABLED(CONFIG_REQUIRE_NX) || \
+                                 boot_cpu_has(X86_FEATURE_NX))
 #define cpu_has_page1gb         boot_cpu_has(X86_FEATURE_PAGE1GB)
 #define cpu_has_rdtscp          boot_cpu_has(X86_FEATURE_RDTSCP)
 #define cpu_has_3dnow_ext       boot_cpu_has(X86_FEATURE_3DNOWEXT)