Re: [PATCH] x86/uaccess: Use pointer masking to limit uaccess speculation

[Josh Poimboeuf]

On Wed, Aug 19, 2020 at 09:39:10AM -0700, Andy Lutomirski wrote:
> On Wed, Aug 19, 2020 at 7:50 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > +/*
> > + * Sanitize a uaccess pointer such that it becomes NULL if it's not a valid
> > + * user pointer.  This blocks speculative dereferences of user-controlled
> > + * pointers.
> > + */
> > +#define uaccess_mask_ptr(ptr) \
> > +       (__typeof__(ptr)) array_index_nospec((__force unsigned long)ptr, user_addr_max())
> > +
> 
> If I dug through all the macros correctly, this is generating a fairly
> complex pile of math to account for the fact that user_addr_max() is
> variable and that it's a nasty number.

The math is actually pretty simple.  It's identical to what getuser.S is
doing:

	cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
	sbb %_ASM_DX, %_ASM_DX
	and %_ASM_DX, %_ASM_AX

> But I don't think there's any particular need to use the real maximum
> user address here.  Allowing a mis-speculated user access to a
> non-canonical address or to the top guard page of the lower canonical
> region is harmless.  With current kernels, a sequence like:
> 
> if (likely((long)addr > 0) {
>   masked_addr = addr & 0x7FFFFFFFFFFFFFFFUL;
> } else {
>   if (kernel fs) {
>     masked_addr = addr;
>   } else {
>     EFAULT;
>   }
> }

The masking has to be done without conditional branches, otherwise it
defeats the point.

> could plausibly be better.  But Christoph's series fixes this whole
> mess, and I think that this should be:
> 
> #define uaccess_mask_ptr(ptr) ((__typeof___(ptr)) (__force unsigned
> long)ptr & USER_ADDR_MASK))
> 
> where USER_ADDR_MASK is the appropriate value for 32-bit or 64-bit.

Yeah, we could do that.  Though in the meantime, the simple merge
conflict resolution with Christoph's patches would be
s/user_addr_max/TASK_SIZE_MAX/ in my uaccess_mask_ptr() macro.

-- 
Josh