Note that the ABI is not yet part of Linux, so this patch is kinda
experimental.
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
---
configure | 14 ++++++++
util/Makefile.objs | 1 -
util/coroutine-x86.c | 80 ++++++++++++++++++++++++++++++++++++++++++--
3 files changed, 92 insertions(+), 3 deletions(-)
diff --git a/configure b/configure
index 4e553e521b..43e2321d3d 100755
--- a/configure
+++ b/configure
@@ -5090,6 +5090,20 @@ if test "$cet" != no; then
feature_not_found "cet" 'CET is not supported by your toolchain'
fi
cet=no
+ else
+ if test $cpu = x86_64; then
+ # only needed by coroutine-x86.c, however it should be rare to have
+ # CET support in the compiler but not in binutils
+ cat > $TMPC << EOF
+int main(void) { asm("rdsspq %%rax" : : : "rax"); }
+EOF
+ if ! compile_prog "" "" ; then
+ if test "$cet" = yes; then
+ feature_not_found "cet" 'CET is not supported by your toolchain'
+ fi
+ cet=no
+ fi
+ fi
fi
fi
if test "$cet" = ""; then
diff --git a/util/Makefile.objs b/util/Makefile.objs
index 93a8397aae..ce1ea310af 100644
--- a/util/Makefile.objs
+++ b/util/Makefile.objs
@@ -42,7 +42,6 @@ coroutine-x86.o-cflags := -mno-red-zone
ifeq ($(CONFIG_CET),y)
coroutine-sigaltstack.o-cflags := -fcf-protection=branch
coroutine-ucontext.o-cflags := -fcf-protection=branch
-coroutine-x86.o-cflags += -fcf-protection=branch
endif
util-obj-y += buffer.o
util-obj-y += timed-average.o
diff --git a/util/coroutine-x86.c b/util/coroutine-x86.c
index bcb9666700..d55f07dc92 100644
--- a/util/coroutine-x86.c
+++ b/util/coroutine-x86.c
@@ -22,6 +22,16 @@
#include "qemu/osdep.h"
#include "qemu-common.h"
#include "qemu/coroutine_int.h"
+#include "qemu/error-report.h"
+
+#ifdef CONFIG_CET
+#include <asm/prctl.h>
+#include <sys/prctl.h>
+int arch_prctl(int code, unsigned long addr);
+#ifndef ARCH_X86_CET_ALLOC_SHSTK
+#define ARCH_X86_CET_ALLOC_SHSTK 0x3004
+#endif
+#endif
#ifdef CONFIG_VALGRIND_H
#include <valgrind/valgrind.h>
@@ -39,10 +49,14 @@
typedef struct {
Coroutine base;
void *sp;
+ void *ssp;
void *stack;
size_t stack_size;
+ /* CET shadow stack */
+ void *sstack;
+ size_t sstack_size;
#ifdef CONFIG_VALGRIND_H
unsigned int valgrind_stack_id;
#endif
@@ -77,6 +91,30 @@ static void start_switch_fiber(void **fake_stack_save,
#endif
}
+static bool have_cet(void)
+{
+#if defined CONFIG_CET
+ uint64_t ssp;
+ asm ("xor %0, %0; rdsspq %0\n" : "=rm" (ssp));
+ return !!ssp;
+#else
+ return 0;
+#endif
+}
+
+static void *cet_alloc_sstack(size_t sz)
+{
+#if defined CONFIG_CET
+ uint64_t arg = sz;
+ if (arch_prctl(ARCH_X86_CET_ALLOC_SHSTK, (unsigned long) &arg) < 0) {
+ abort();
+ }
+
+ return (void *)arg;
+#else
+ abort();
+#endif
+}
/*
* We hardcode all operands to specific registers so that we can write down
all the
* others in the clobber list. Note that action also needs to be hardcoded so
that
@@ -87,6 +125,26 @@ 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;
\
@@ -101,7 +159,15 @@ 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 on... */
\
+ "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 */
\
\
@@ -110,7 +176,8 @@ static void start_switch_fiber(void **fake_stack_save,
".cfi_adjust_cfa_offset -8\n"
\
".cfi_restore_state\n"
\
: "+a" (action_), [FROM] "+b" (from_), [TO] "+D" (to_)
\
- : [SP] "i" (offsetof(CoroutineX86, sp))
\
+ : [SP] "i" (offsetof(CoroutineX86, sp)),
\
+ [SSP] "i" (offsetof(CoroutineX86, ssp))
\
: "rcx", "rdx", "rsi", "r8", "r9", "r10", "r11", "r12", "r13", "r14",
"r15", \
"memory");
\
action_;
\
@@ -139,6 +206,12 @@ Coroutine *qemu_coroutine_new(void)
co->stack = qemu_alloc_stack(&co->stack_size);
co->sp = co->stack + co->stack_size;
+ if (have_cet()) {
+ co->sstack_size = COROUTINE_SHADOW_STACK_SIZE;
+ co->sstack = cet_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);
@@ -178,6 +251,9 @@ 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);
}
--
2.20.1
