[PATCH 1/2] riscv: Use pointer masking to limit uaccess speculation

[Lukas Gerlach]

Thanks for the review. You're right - we should only clear the sign bit
(b38/b47/b56 depending on mode), not b63. Clearing upper bits would
interfere with pointer masking.

Here's a fix that computes the sign bit position arithmetically to avoid
branches, this ensures the mitigation cannot be bypassed under speculation.
This is basically the VA_BITS macro but computed in a branch-free way.

In arch/riscv/include/asm/uaccess.h:

  #define UACCESS_SIGN_BIT \
      (VA_BITS_SV39 - 1 + 9*((unsigned long)pgtable_l4_enabled) + \
       9*((unsigned long)pgtable_l5_enabled))

  #define uaccess_mask_ptr(ptr) ((__typeof__(ptr))__uaccess_mask_ptr(ptr))
  static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
  {
      return (void __user *)((unsigned long)ptr & ~BIT_ULL(UACCESS_SIGN_BIT));
  }

This evaluates to bit 38 for Sv39, bit 47 for Sv48, and bit 56 for Sv57.