From 58ebc2c31337734a8a79b0566b31b19040deb2ea Mon Sep 17 00:00:00 2001 From: Daniele Buono Date: Fri, 29 May 2020 16:51:19 -0400 Subject: [PATCH] coroutine: support SafeStack in ucontext backend LLVM's SafeStack instrumentation does not yet support programs that make use of the APIs in ucontext.h With the current implementation of coroutine-ucontext, the resulting binary is incorrect, with different coroutines sharing the same unsafe stack and producing undefined behavior at runtime. This fix allocates an additional unsafe stack area for each coroutine, and sets the new unsafe stack pointer before calling swapcontext() in qemu_coroutine_new. This is the only place where the pointer needs to be manually updated, since sigsetjmp/siglongjmp are already instrumented by LLVM to properly support SafeStack. The additional stack is then freed in qemu_coroutine_delete. Signed-off-by: Daniele Buono Message-id: 20200529205122.714-2-dbuono@linux.vnet.ibm.com Signed-off-by: Stefan Hajnoczi --- include/qemu/coroutine_int.h | 5 +++++ util/coroutine-ucontext.c | 28 ++++++++++++++++++++++++++++ 2 files changed, 33 insertions(+) diff --git a/include/qemu/coroutine_int.h b/include/qemu/coroutine_int.h index bd6b0468e1..1da148552f 100644 --- a/include/qemu/coroutine_int.h +++ b/include/qemu/coroutine_int.h @@ -28,6 +28,11 @@ #include "qemu/queue.h" #include "qemu/coroutine.h" +#ifdef CONFIG_SAFESTACK +/* Pointer to the unsafe stack, defined by the compiler */ +extern __thread void *__safestack_unsafe_stack_ptr; +#endif + #define COROUTINE_STACK_SIZE (1 << 20) typedef enum { diff --git a/util/coroutine-ucontext.c b/util/coroutine-ucontext.c index 613f4c118e..f0b66320e1 100644 --- a/util/coroutine-ucontext.c +++ b/util/coroutine-ucontext.c @@ -45,6 +45,11 @@ typedef struct { Coroutine base; void *stack; size_t stack_size; +#ifdef CONFIG_SAFESTACK + /* Need an unsafe stack for each coroutine */ + void *unsafe_stack; + size_t unsafe_stack_size; +#endif sigjmp_buf env; void *tsan_co_fiber; @@ -179,6 +184,10 @@ Coroutine *qemu_coroutine_new(void) co = g_malloc0(sizeof(*co)); co->stack_size = COROUTINE_STACK_SIZE; co->stack = qemu_alloc_stack(&co->stack_size); +#ifdef CONFIG_SAFESTACK + co->unsafe_stack_size = COROUTINE_STACK_SIZE; + co->unsafe_stack = qemu_alloc_stack(&co->unsafe_stack_size); +#endif co->base.entry_arg = &old_env; /* stash away our jmp_buf */ uc.uc_link = &old_uc; @@ -203,6 +212,22 @@ Coroutine *qemu_coroutine_new(void) COROUTINE_YIELD, &fake_stack_save, co->stack, co->stack_size, co->tsan_co_fiber); + +#ifdef CONFIG_SAFESTACK + /* + * Before we swap the context, set the new unsafe stack + * The unsafe stack grows just like the normal stack, so start from + * the last usable location of the memory area. + * NOTE: we don't have to re-set the usp afterwards because we are + * coming back to this context through a siglongjmp. + * The compiler already wrapped the corresponding sigsetjmp call with + * code that saves the usp on the (safe) stack before the call, and + * restores it right after (which is where we return with siglongjmp). + */ + void *usp = co->unsafe_stack + co->unsafe_stack_size; + __safestack_unsafe_stack_ptr = usp; +#endif + swapcontext(&old_uc, &uc); } @@ -235,6 +260,9 @@ void qemu_coroutine_delete(Coroutine *co_) #endif qemu_free_stack(co->stack, co->stack_size); +#ifdef CONFIG_SAFESTACK + qemu_free_stack(co->unsafe_stack, co->unsafe_stack_size); +#endif g_free(co); }