| Message ID | 20230615153157.444-3-alejandro.vallejo@cloud.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | Introduce a REQUIRE_NX Kconfig option | expand |
On 15/06/2023 4:31 pm, Alejandro Vallejo wrote: > diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig > index 406445a358..fa97d4cccc 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 bit support" "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 ce62eae6f3..ec1e80ef68 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_bit_msg: .asciz "ERR: Not an NX-bit capable CPU!" Still two too many "bit"'s in this line. With these two adjusted, Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
On 15.06.2023 17:31, Alejandro Vallejo wrote: > 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> Just one nit on top of Andrew's comments: > @@ -697,9 +708,11 @@ trampoline_setup: > jnc .Lno_nx_bit > > .Lhas_nx_bit: > +#ifndef CONFIG_REQUIRE_NX > /* Adjust EFER is NX is present */ > orb $EFER_NXE >> 8, 1 + sym_esi(trampoline_efer) > .Lno_nx_bit: > +#endif In the comment the first "is" likely was meant to be "if". Jan
diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig index 406445a358..fa97d4cccc 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 bit 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 ce62eae6f3..ec1e80ef68 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_bit_msg: .asciz "ERR: Not an NX-bit capable CPU!" .section .init.data, "aw", @progbits .align 4 @@ -151,6 +152,11 @@ not_multiboot: .Lnot_aligned: add $sym_offs(.Lbag_alg_msg),%esi # Error message jmp .Lget_vtb +#ifdef CONFIG_REQUIRE_NX +.Lno_nx_bit: + add $sym_offs(.Lno_nx_bit_msg),%esi + jmp .Lget_vtb +#endif .Lmb2_no_st: /* * Here we are on EFI platform. vga_text_buffer was zapped earlier @@ -651,7 +657,12 @@ 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 .Lhas_nx_bit @@ -697,9 +708,11 @@ trampoline_setup: jnc .Lno_nx_bit .Lhas_nx_bit: +#ifndef CONFIG_REQUIRE_NX /* Adjust EFER is NX is present */ orb $EFER_NXE >> 8, 1 + sym_esi(trampoline_efer) .Lno_nx_bit: +#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..84700559bb 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 Xen build requires NX bit 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 ace31e3b1f..610532da43 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)
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> --- 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(-)