aarch64: Add GCS support for makecontext

Changed the makecontext logic: previously the first setcontext jumped
straight to the user callback function and the return address is set
to __startcontext. This does not work when GCS is enabled as the
integrity of the return address is protected, so instead the context
is setup such that setcontext jumps to __startcontext which calls the
user callback (passed in x20).

The map_shadow_stack syscall is used to allocate a suitably sized GCS
(which includes some reserved area to account for altstack signal
handlers and otherwise supports maximum number of 16 byte aligned
stack frames on the given stack) however the GCS is never freed as
the lifetime of ucontext and related stack is user managed.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

2 files changed, 63 insertions, 2 deletions

diff --git a/sysdeps/unix/sysv/linux/aarch64/makecontext.c b/sysdeps/unix/sysv/linux/aarch64/makecontext.c
index 11516b7..09e5443 100644
--- a/sysdeps/unix/sysv/linux/aarch64/makecontext.c
+++ b/sysdeps/unix/sysv/linux/aarch64/makecontext.c
@@ -21,7 +21,51 @@
 #include <stdarg.h>
 #include <stdint.h>
 #include <ucontext.h>
+#include <sys/mman.h>
 
+#define GCS_MAGIC 0x47435300
+
+static struct _aarch64_ctx *extension (void *p)
+{
+  return p;
+}
+
+#ifndef SHADOW_STACK_SET_TOKEN
+# define SHADOW_STACK_SET_TOKEN (1UL << 0)
+# define SHADOW_STACK_SET_MARKER (1UL << 1)
+#endif
+
+static void *
+map_shadow_stack (void *addr, size_t size, unsigned long flags)
+{
+  return (void *) INLINE_SYSCALL_CALL (map_shadow_stack, addr, size, flags);
+}
+
+#define GCS_MAX_SIZE (1UL << 31)
+#define GCS_ALTSTACK_RESERVE 160
+
+static void *
+alloc_makecontext_gcs (size_t stack_size)
+{
+  size_t size = (stack_size / 2 + GCS_ALTSTACK_RESERVE) & -8UL;
+  if (size > GCS_MAX_SIZE)
+    size = GCS_MAX_SIZE;
+
+  unsigned long flags = SHADOW_STACK_SET_MARKER | SHADOW_STACK_SET_TOKEN;
+  void *base = map_shadow_stack (NULL, size, flags);
+  if (base == MAP_FAILED)
+    /* ENOSYS, bad size or OOM.  */
+    abort ();
+  uint64_t *gcsp = (uint64_t *) ((char *) base + size);
+  /* Skip end of GCS token.  */
+  gcsp--;
+  /* Verify GCS cap token.  */
+  gcsp--;
+  if (((uint64_t)gcsp & 0xfffffffffffff000) + 1 != *gcsp)
+    abort ();
+  /* Return the target GCS pointer for context switch.  */
+  return gcsp + 1;
+}
 
 /* makecontext sets up a stack and the registers for the
    user context.  The stack looks like this:
@@ -56,10 +100,23 @@ __makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
   sp = (uint64_t *) (((uintptr_t) sp) & -16L);
 
   ucp->uc_mcontext.regs[19] = (uintptr_t) ucp->uc_link;
+  ucp->uc_mcontext.regs[20] = (uintptr_t) func;
   ucp->uc_mcontext.sp = (uintptr_t) sp;
-  ucp->uc_mcontext.pc = (uintptr_t) func;
+  ucp->uc_mcontext.pc = (uintptr_t) __startcontext;
   ucp->uc_mcontext.regs[29] = (uintptr_t) 0;
-  ucp->uc_mcontext.regs[30] = (uintptr_t) &__startcontext;
+  ucp->uc_mcontext.regs[30] = (uintptr_t) 0;
+
+  void *p = ucp->uc_mcontext.__reserved;
+  if (extension (p)->magic == FPSIMD_MAGIC)
+    p = (char *)p + extension (p)->size;
+  if (extension (p)->magic == GCS_MAGIC)
+    {
+      /* Using the kernel struct gcs_context layout.  */
+      struct { uint64_t x, gcspr, y, z; } *q = p;
+      /* TODO: this allocation remains mapped even after thread
+         that uses it exits.  */
+      q->gcspr = (uint64_t) alloc_makecontext_gcs (ucp->uc_stack.ss_size);
+    }
 
   va_start (ap, argc);
   for (i = 0; i < argc; ++i)
diff --git a/sysdeps/unix/sysv/linux/aarch64/setcontext.S b/sysdeps/unix/sysv/linux/aarch64/setcontext.S
index 848229f..695fc5b 100644
--- a/sysdeps/unix/sysv/linux/aarch64/setcontext.S
+++ b/sysdeps/unix/sysv/linux/aarch64/setcontext.S
@@ -180,7 +180,11 @@ L(gcs_done):
 PSEUDO_END (__setcontext)
 weak_alias (__setcontext, setcontext)
 
+/* makecontext start function: receives uc_link in x19 and func in x20.
+   Arguments of func, x29, x30 and sp are set up by the caller.  */
 ENTRY (__startcontext)
+	cfi_undefined (x30)
+	blr	x20
 	mov	x0, x19
 	cbnz	x0, __setcontext
 1:	b       HIDDEN_JUMPTARGET (exit)