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.

diff --git a/sysdeps/unix/sysv/linux/aarch64/makecontext.c b/sysdeps/unix/sysv/linux/aarch64/makecontext.c
index 5aec182013580e9e09318775ab0d6d992dd52ad3..9e66b6761cf7cd4719aec6174be30ebb8665df62 100644 (file)
--- a/sysdeps/unix/sysv/linux/aarch64/makecontext.c
+++ b/sysdeps/unix/sysv/linux/aarch64/makecontext.c
@@ -22,6 +22,52 @@
 #include <stdint.h>
 #include <ucontext.h>
 
+#define GCS_MAGIC 0x47435300
+
+static struct _aarch64_ctx *extension (void *p)
+{
+  return p;
+}
+
+#ifndef __NR_map_shadow_stack
+# define __NR_map_shadow_stack 453
+#endif
+#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 == (void *) -1)
+    /* 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 +102,21 @@ __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;
+      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 bdfd4580ec3ff7876716a1cf4155ba07d90118c3..17b0a48ce116a0926e5a0dc28a76d840f9550eca 100644 (file)
--- 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)