RE: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation

[David Laight]

From: Josh Poimboeuf
> Sent: 05 May 2021 04:55
> 
> The x86 uaccess code uses barrier_nospec() in various places to prevent
> speculative dereferencing of user-controlled pointers (which might be
> combined with further gadgets or CPU bugs to leak data).
...
> Remove existing barrier_nospec() usage, and instead do user pointer
> masking, throughout the x86 uaccess code.  This is similar to what arm64
> is already doing with uaccess_mask_ptr().
...
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index fb75657b5e56..ebe9ab46b183 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -66,12 +66,35 @@ static inline bool pagefault_disabled(void);
>   * Return: true (nonzero) if the memory block may be valid, false (zero)
>   * if it is definitely invalid.
>   */
> -#define access_ok(addr, size)					\
> +#define access_ok(addr, size)						\
>  ({									\
>  	WARN_ON_IN_IRQ();						\
>  	likely(!__range_not_ok(addr, size, TASK_SIZE_MAX));		\
>  })
> 
> +/*
> + * Sanitize a user pointer such that it becomes NULL if it's not a valid user
> + * pointer.  This prevents speculatively dereferencing a user-controlled
> + * pointer to kernel space if access_ok() speculatively returns true.  This
> + * should be done *after* access_ok(), to avoid affecting error handling
> + * behavior.
> + */
> +#define mask_user_ptr(ptr)						\
> +({									\
> +	unsigned long _ptr = (__force unsigned long)ptr;		\
> +	unsigned long mask;						\
> +									\
> +	asm volatile("cmp %[max], %[_ptr]\n\t"				\
> +		     "sbb %[mask], %[mask]\n\t"				\
> +		     : [mask] "=r" (mask)				\
> +		     : [_ptr] "r" (_ptr),				\
> +		       [max] "r" (TASK_SIZE_MAX)			\
> +		     : "cc");						\
> +									\
> +	mask &= _ptr;							\
> +	((typeof(ptr)) mask);						\
> +})
> +

access_ok() and mask_user_ptr() are doing much the same check.
Is there scope for making access_ok() return the masked pointer?

So the canonical calling code would be:
	uptr = access_ok(uptr, size);
	if (!uptr)
		return -EFAULT;

This would error requests for address 0 earlier - but I don't
believe they are ever valid in Linux.
(Some historic x86 a.out formats did load to address 0.)

Clearly for a follow up patch.

	David

-
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