x86-64/CET: Extend ucontext_t to save shadow stack

This patch adds a field to ucontext_t to save shadow stack:

1. getcontext and swapcontext are updated to save the caller's shadow
stack pointer and return addresses.
2. setcontext and swapcontext are updated to restore shadow stack and
jump to new context directly.
3. makecontext is updated to allocate a new shadow stack and set the
caller's return address to __start_context.

Since makecontext allocates a new shadow stack when making a new
context and kernel allocates a new shadow stack for clone/fork/vfork
syscalls, we track the current shadow stack base.  In setcontext and
swapcontext, if the target shadow stack base is the same as the current
shadow stack base, we unwind the shadow stack.  Otherwise it is a stack
switch and we look for a restore token.

We enable shadow stack at run-time only if program and all used shared
objects, including dlopened ones, are shadow stack enabled, which means
that they must be compiled with GCC 8 or above and glibc 2.28 or above.
We need to save and restore shadow stack only if shadow stack is enabled.
When caller of getcontext, setcontext, swapcontext and makecontext is
compiled with smaller ucontext_t, shadow stack won't be enabled at
run-time.  We check if shadow stack is enabled before accessing the
extended field in ucontext_t.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

	* sysdeps/unix/sysv/linux/x86/sys/ucontext.h (ucontext_t): Add
	__ssp.
	* sysdeps/unix/sysv/linux/x86_64/__start_context.S: Include
	<asm/prctl.h> and "ucontext_i.h" when shadow stack is enabled.
	(__push___start_context): New.
	* sysdeps/unix/sysv/linux/x86_64/getcontext.S: Include
	<asm/prctl.h>.
	(__getcontext): Record the current shadow stack base.  Save the
	caller's shadow stack pointer and base.
	* sysdeps/unix/sysv/linux/x86_64/makecontext.c: Include
	<pthread.h>, <libc-pointer-arith.h> and <sys/prctl.h>.
	(__push___start_context): New prototype.
	(__makecontext): Call __push___start_context to allocate a new
	shadow stack, push __start_context onto the new stack as well
	as the new shadow stack.
	* sysdeps/unix/sysv/linux/x86_64/setcontext.S: Include
	<asm/prctl.h>.
	(__setcontext): Restore the target shadow stack.
	* sysdeps/unix/sysv/linux/x86_64/swapcontext.S: Include
	<asm/prctl.h>.
	(__swapcontext): Record the current shadow stack base.  Save
	the caller's shadow stack pointer and base.  Restore the target
	shadow stack.
	* sysdeps/unix/sysv/linux/x86_64/sysdep.h
	(STACK_SIZE_TO_SHADOW_STACK_SIZE_SHIFT): New.
	* sysdeps/unix/sysv/linux/x86_64/ucontext_i.sym (oSSP): New.

1 files changed, 37 insertions, 1 deletions

diff --git a/sysdeps/unix/sysv/linux/x86_64/makecontext.c b/sysdeps/unix/sysv/linux/x86_64/makecontext.c
index 0d0802b..3eb4c59 100644
--- a/sysdeps/unix/sysv/linux/x86_64/makecontext.c
+++ b/sysdeps/unix/sysv/linux/x86_64/makecontext.c
@@ -21,6 +21,11 @@
 #include <stdarg.h>
 #include <stdint.h>
 #include <ucontext.h>
+#if SHSTK_ENABLED
+# include <pthread.h>
+# include <libc-pointer-arith.h>
+# include <sys/prctl.h>
+#endif
 
 #include "ucontext_i.h"
 
@@ -52,6 +57,8 @@ void
 __makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
 {
   extern void __start_context (void) attribute_hidden;
+  extern void __push___start_context (ucontext_t *)
+    attribute_hidden;
   greg_t *sp;
   unsigned int idx_uc_link;
   va_list ap;
@@ -74,7 +81,36 @@ __makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
   ucp->uc_mcontext.gregs[REG_RSP] = (uintptr_t) sp;
 
   /* Setup stack.  */
-  sp[0] = (uintptr_t) &__start_context;
+#if SHSTK_ENABLED
+  struct pthread *self = THREAD_SELF;
+  unsigned int feature_1 = THREAD_GETMEM (self, header.feature_1);
+  /* NB: We must check feature_1 before accessing __ssp since caller
+	 may be compiled against ucontext_t without __ssp.  */
+  if ((feature_1 & X86_FEATURE_1_SHSTK) != 0)
+    {
+      /* Shadow stack is enabled.  We need to allocate a new shadow
+         stack.  */
+      unsigned long ssp_size = (((uintptr_t) sp
+				 - (uintptr_t) ucp->uc_stack.ss_sp)
+				>> STACK_SIZE_TO_SHADOW_STACK_SIZE_SHIFT);
+      /* Align shadow stack to 8 bytes.  */
+      ssp_size = ALIGN_UP (ssp_size, 8);
+
+      ucp->__ssp[1] = ssp_size;
+      ucp->__ssp[2] = ssp_size;
+
+      /* Call __push___start_context to allocate a new shadow stack,
+	 push __start_context onto the new stack as well as the new
+	 shadow stack.  NB: After __push___start_context returns,
+	   ucp->__ssp[0]: The new shadow stack pointer.
+	   ucp->__ssp[1]: The base address of the new shadow stack.
+	   ucp->__ssp[2]: The size of the new shadow stack.
+       */
+      __push___start_context (ucp);
+    }
+  else
+#endif
+    sp[0] = (uintptr_t) &__start_context;
   sp[idx_uc_link] = (uintptr_t) ucp->uc_link;
 
   va_start (ap, argc);