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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [xen master] x86: Add Kconfig option to require NX bit support
commit 098e27578b0b1743831877787fc7ded591dfd8d6
Author: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
AuthorDate: Thu Jun 29 13:17:13 2023 +0100
Commit: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Jun 30 14:18:28 2023 +0100
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@xxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
xen/arch/x86/Kconfig | 16 ++++++++++++++++
xen/arch/x86/boot/head.S | 15 ++++++++++++++-
xen/arch/x86/boot/trampoline.S | 3 ++-
xen/arch/x86/efi/efi-boot.h | 9 +++++++++
xen/arch/x86/include/asm/cpufeature.h | 3 ++-
5 files changed, 43 insertions(+), 3 deletions(-)
diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 406445a358..92f3a627da 100644
--- a/xen/arch/x86/Kconfig
+++ b/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"
diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 6685d3e504..e03f52c755 100644
--- a/xen/arch/x86/boot/head.S
+++ b/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
diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S
index c6005fa33d..b8ab0ffdcb 100644
--- a/xen/arch/x86/boot/trampoline.S
+++ b/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
diff --git a/xen/arch/x86/efi/efi-boot.h b/xen/arch/x86/efi/efi-boot.h
index c94e53d139..64c1a02cf1 100644
--- a/xen/arch/x86/efi/efi-boot.h
+++ b/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;
}
diff --git a/xen/arch/x86/include/asm/cpufeature.h
b/xen/arch/x86/include/asm/cpufeature.h
index e2cb8f3cc7..64e1dad225 100644
--- a/xen/arch/x86/include/asm/cpufeature.h
+++ b/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)
--
generated by git-patchbot for /home/xen/git/xen.git#master
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