Re: [PATCH 3/6] arm64: untag user addresses in copy_from_user and others

On Wed, Apr 18, 2018 at 08:53:12PM +0200, Andrey Konovalov wrote:

quoted

@@ -238,12 +239,15 @@ static inline void uaccess_enable_not_uao(void)
 /*
  * Sanitise a uaccess pointer such that it becomes NULL if above the
  * current addr_limit.
+ * Also untag user pointers that have the top byte tag set.
  */
 #define uaccess_mask_ptr(ptr) (__typeof__(ptr))__uaccess_mask_ptr(ptr)
 static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
 {
      void __user *safe_ptr;

+     ptr = untagged_addr(ptr);
+
      asm volatile(
      "       bics    xzr, %1, %2\n"
      "       csel    %0, %1, xzr, eq\n"

First of all, passing a tagged user pointer throughout the kernel is
safe with uaccess routines but not suitable for find_vma() etc.

With this change, we may have an inconsistent behaviour on the tag
masking, depending on whether the entry code uses __uaccess_mask_ptr()
or not. We could preserve the tag with something like:

diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index e66b0fca99c2..ed15bfcbd797 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h

@@ -244,10 +244,11 @@ static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
        void __user *safe_ptr;

        asm volatile(
-       "       bics    xzr, %1, %2\n"
+       "       bics    xzr, %3, %2\n"
        "       csel    %0, %1, xzr, eq\n"
        : "=&r" (safe_ptr)
-       : "r" (ptr), "r" (current_thread_info()->addr_limit)
+       : "r" (ptr), "r" (current_thread_info()->addr_limit),
+         "r" (untagged_addr(ptr))
        : "cc");

        csdb();