[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [PATCH 59.5/65] x86: Introduce helpers/checks for endbr64 instructions
... to prevent the optimiser creating unsafe code. See the code comment for full details. Also add a build time check for endbr64 embedded in imm32 operands, which catches the obvious cases where the optimiser has done an unsafe thing. Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> --- CC: Jan Beulich <JBeulich@xxxxxxxx> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx> CC: Wei Liu <wl@xxxxxxx> --- xen/arch/x86/Makefile | 4 ++++ xen/include/asm-x86/endbr.h | 55 +++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 59 insertions(+) create mode 100644 xen/include/asm-x86/endbr.h diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile index 69b6cfaded25..64a5c0d20018 100644 --- a/xen/arch/x86/Makefile +++ b/xen/arch/x86/Makefile @@ -190,6 +190,10 @@ $(TARGET)-syms: prelink.o xen.lds $(MAKE) -f $(BASEDIR)/Rules.mk efi-y= $(@D)/.$(@F).1.o $(LD) $(XEN_LDFLAGS) -T xen.lds -N prelink.o $(build_id_linker) \ $(@D)/.$(@F).1.o -o $@ +ifeq ($(CONFIG_XEN_IBT),y) + $(OBJDUMP) -d $@ | grep 0xfa1e0ff3 >/dev/null && \ + { echo "Found embedded endbr64 instructions" >&2; false; } || : +endif $(NM) -pa --format=sysv $(@D)/$(@F) \ | $(BASEDIR)/tools/symbols --all-symbols --xensyms --sysv --sort \ >$(@D)/$(@F).map diff --git a/xen/include/asm-x86/endbr.h b/xen/include/asm-x86/endbr.h new file mode 100644 index 000000000000..47f766024c12 --- /dev/null +++ b/xen/include/asm-x86/endbr.h @@ -0,0 +1,55 @@ +/****************************************************************************** + * include/asm-x86/endbr.h + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation; either version 2 of the License, or + * (at your option) any later version. + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License + * along with this program; If not, see <http://www.gnu.org/licenses/>. + * + * Copyright (c) 2021 Citrix Systems Ltd. + */ +#ifndef XEN_ASM_ENDBR_H +#define XEN_ASM_ENDBR_H + +#include <xen/compiler.h> + +/* + * In some cases we need to inspect/insert endbr64 instructions. + * + * The naive way, mem{cmp,cpy}(ptr, "\xf3\x0f\x1e\xfa", 4), optimises unsafely + * by placing 0xfa1e0ff3 in an imm32 operand, which marks a legal indirect + * branch target as far as the CPU is concerned. + * + * gen_endbr64() is written deliberately to avoid the problematic operand, and + * marked __const__ as it is safe for the optimiser to hoist/merge/etc. + */ +static inline uint32_t __attribute_const__ gen_endbr64(void) +{ + uint32_t res; + + asm ( "mov $~0xfa1e0ff3, %[res]\n\t" + "not %[res]\n\t" + : [res] "=r" (res) ); + + return res; +} + +static inline bool is_endbr64(const void *ptr) +{ + return *(const uint32_t *)ptr == gen_endbr64(); +} + +static inline void place_endbr64(void *ptr) +{ + *(uint32_t *)ptr = gen_endbr64(); +} + +#endif /* XEN_ASM_ENDBR_H */ -- 2.11.0
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