[glibc/arm/gcs] aarch64: Add GCS support for makecontext

[Szabolcs Nagy <nsz@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=54e90582aabdb44f010ad5dfd64ce9c3e6d33914

commit 54e90582aabdb44f010ad5dfd64ce9c3e6d33914
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date:   Tue Apr 4 10:42:21 2023 +0100

    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:
---
 sysdeps/unix/sysv/linux/aarch64/makecontext.c | 61 ++++++++++++++++++++++++++-
 sysdeps/unix/sysv/linux/aarch64/setcontext.S  |  4 ++
 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 5aec182013..9e66b6761c 100644
--- 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 c08e83ee60..6aa7236693 100644
--- a/sysdeps/unix/sysv/linux/aarch64/setcontext.S
+++ b/sysdeps/unix/sysv/linux/aarch64/setcontext.S
@@ -181,7 +181,11 @@ 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)