Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
patch looks generally good, but does not apply anymore.
Are you planning to update it later to support the more secure "map_shadow_stack" syscall, if/when it is added?
---
meson.build | 16 +++++++--
util/coroutine-asm.c | 82 ++++++++++++++++++++++++++++++++++++++++++--
2 files changed, 93 insertions(+), 5 deletions(-)
diff --git a/meson.build b/meson.build
index 0121ccab78dd..17e4a3bc582e 100644
--- a/meson.build
+++ b/meson.build
@@ -328,6 +328,10 @@ elif coroutine_backend not in supported_backends
.format(coroutine_backend, ', '.join(supported_backends)))
endif
+if cfi_mode == 'hw' and coroutine_backend != 'asm'
+ error('Hardware control-flow integrity requires the "asm" coroutine backend.')
+endif
+
# Compiles if SafeStack *not* enabled
safe_stack_probe = '''
int main(void)
@@ -469,16 +473,22 @@ if cfi_mode == 'sw'
endif
endif
elif cfi_mode in ['hw', 'auto']
- if cfi_mode == 'hw'
- error('Hardware CFI is not supported yet')
+ if cpu in ['x86', 'x86_64']
+ cfi_flags += cc.get_supported_arguments('-fcf-protection=full')
+ if cfi_mode == 'hw'
+ error('C compiler does not support -fcf-protection')
+ endif
+ elif cfi_mode == 'hw'
+ error('Hardware CFI is only supported on x86')
endif
if cfi_flags == [] and cfi_mode == 'auto'
cfi_mode = 'disabled'
endif
endif
-if cpu in ['x86', 'x86_64']
+if cpu in ['x86', 'x86_64'] and cfi_mode != 'hw'
cfi_flags += cc.get_supported_arguments('-fcf-protection=branch')
endif
+
add_global_arguments(cfi_flags, native: false, language: all_languages)
add_global_link_arguments(cfi_flags, native: false, language: all_languages)
diff --git a/util/coroutine-asm.c b/util/coroutine-asm.c
index a06ecbcb0a07..771b1d4a0fc9 100644
--- a/util/coroutine-asm.c
+++ b/util/coroutine-asm.c
@@ -22,6 +22,13 @@
#include "qemu/osdep.h"
#include "qemu-common.h"
#include "qemu/coroutine_int.h"
+#include "qemu/error-report.h"
+
+#ifdef CONFIG_CF_PROTECTION
+#include <asm/prctl.h>
+#include <sys/prctl.h>
+int arch_prctl(int code, unsigned long addr);
+#endif
#ifdef CONFIG_VALGRIND_H
#include <valgrind/valgrind.h>
@@ -39,10 +46,14 @@
typedef struct {
Coroutine base;
void *sp;
+ void *ssp;
void *stack;
size_t stack_size;
+ /* x86: CET shadow stack */
+ void *sstack;
+ size_t sstack_size;
#ifdef CONFIG_VALGRIND_H
unsigned int valgrind_stack_id;
#endif
@@ -77,6 +88,35 @@ static void start_switch_fiber(void **fake_stack_save,
#endif
}
+static bool have_sstack(void)
+{
+#if defined CONFIG_CF_PROTECTION && defined __x86_64__
+ uint64_t ssp;
+ asm ("xor %0, %0; rdsspq %0\n" : "=r" (ssp));
+ return !!ssp;
+#else
+ return 0;
+#endif
+}
+
+static void *alloc_sstack(size_t sz)
+{
+#if defined CONFIG_CF_PROTECTION && defined __x86_64__
+#ifndef ARCH_X86_CET_ALLOC_SHSTK
+#define ARCH_X86_CET_ALLOC_SHSTK 0x3004
+#endif
+
+ uint64_t arg = sz;
+ if (arch_prctl(ARCH_X86_CET_ALLOC_SHSTK, (unsigned long) &arg) < 0) {
+ abort();
+ }
+
+ return (void *)arg;
+#else
+ abort();
+#endif
+}
+
#ifdef __x86_64__
/*
* We hardcode all operands to specific registers so that we can write down all the
@@ -88,6 +128,26 @@ static void start_switch_fiber(void **fake_stack_save,
* Note that push and call would clobber the red zone. Makefile.objs compiles this
* file with -mno-red-zone. The alternative is to subtract/add 128 bytes from rsp
* around the switch, with slightly lower cache performance.
+ *
+ * The RSTORSSP and SAVEPREVSSP instructions are intricate. In a nutshell they are:
+ *
+ * RSTORSSP(mem): oldSSP = SSP
+ * SSP = mem
+ * *SSP = oldSSP
+ *
+ * SAVEPREVSSP: oldSSP = shadow_stack_pop()
+ * *(oldSSP - 8) = oldSSP # "push" to old shadow stack
+ *
+ * Therefore, RSTORSSP(mem) followed by SAVEPREVSSP is the same as
+ *
+ * shadow_stack_push(SSP)
+ * SSP = mem
+ * shadow_stack_pop()
+ *
+ * From the simplified description you can see that co->ssp, being stored before
+ * the RSTORSSP+SAVEPREVSSP sequence, points to the top actual entry of the shadow
+ * stack, not to the restore token. Hence we use an offset of -8 in the operand
+ * of rstorssp.
*/
#define CO_SWITCH(from, to, action, jump) ({ \
int action_ = action; \
@@ -100,7 +160,15 @@ static void start_switch_fiber(void **fake_stack_save,
"jmp 2f\n" /* switch back continues at label 2 */ \
\
"1: .cfi_adjust_cfa_offset 8\n" \
- "movq %%rsp, %c[SP](%[FROM])\n" /* save source SP */ \
+ "xor %%rbp, %%rbp\n" /* use old frame pointer as scratch reg */ \
+ "rdsspq %%rbp\n" \
+ "test %%rbp, %%rbp\n" /* if CET is enabled... */ \
+ "jz 9f\n" \
+ "movq %%rbp, %c[SSP](%[FROM])\n" /* ... save source shadow SP, */ \
+ "movq %c[SSP](%[TO]), %%rbp\n" /* restore destination shadow stack, */ \
+ "rstorssp -8(%%rbp)\n" \
+ "saveprevssp\n" /* and save source shadow SP token */ \
+ "9: movq %%rsp, %c[SP](%[FROM])\n" /* save source SP */ \
"movq %c[SP](%[TO]), %%rsp\n" /* load destination SP */ \
jump "\n" /* coroutine switch */ \
\
@@ -108,7 +176,8 @@ static void start_switch_fiber(void **fake_stack_save,
"popq %%rbp\n" \
".cfi_adjust_cfa_offset -8\n" \
: "+a" (action_), [FROM] "+b" (from_), [TO] "+D" (to_) \
- : [SP] "i" (offsetof(CoroutineAsm, sp)) \
+ : [SP] "i" (offsetof(CoroutineAsm, sp)), \
+ [SSP] "i" (offsetof(CoroutineAsm, ssp)) \
: "rcx", "rdx", "rsi", "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15", \
"memory"); \
action_; \
@@ -141,6 +210,12 @@ Coroutine *qemu_coroutine_new(void)
co->stack = qemu_alloc_stack(&co->stack_size);
co->sp = co->stack + co->stack_size;
+ if (have_sstack()) {
+ co->sstack_size = COROUTINE_SHADOW_STACK_SIZE;
+ co->sstack = alloc_sstack(co->sstack_size);
+ co->ssp = co->sstack + co->sstack_size;
+ }
+
#ifdef CONFIG_VALGRIND_H
co->valgrind_stack_id =
VALGRIND_STACK_REGISTER(co->stack, co->stack + co->stack_size);
@@ -186,6 +261,9 @@ void qemu_coroutine_delete(Coroutine *co_)
#endif
qemu_free_stack(co->stack, co->stack_size);
+ if (co->sstack) {
+ munmap(co->sstack, co->sstack_size);
+ }
g_free(co);
}
--
2.40.1