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Re: [PATCH v2 07/19] x86: introduce __uaccess_begin_nospec and ASM_IFENCE

On Fri, Jan 12, 2018 at 10:21:43AM -0800, Dan Williams wrote:
> > That just sounds wrong.  What if the speculation starts *after* the
> > access_ok() check?  Then the barrier has no purpose.
> >
> > Most access_ok/get_user/copy_from_user calls are like this:
> >
> >   if (copy_from_user(...uptr..))  /* or access_ok() or get_user() */
> >         return -EFAULT;
> >
> > So in other words, the usercopy function is called *before* the branch.
> >
> > But to halt speculation, the lfence needs to come *after* the branch.
> > So putting lfences *before* the branch doesn't solve anything.
> >
> > So what am I missing?
> We're trying to prevent a pointer under user control from being
> de-referenced inside the kernel, before we know it has been limited to
> something safe. In the following sequence the branch we are worried
> about speculating is the privilege check:
> if (access_ok(uptr))  /* <--- Privelege Check */
>     if (copy_from_user_(uptr))
> The cpu can speculatively skip that access_ok() check and cause a read
> of kernel memory.

Converting your example code to assembly:

	call	access_ok # no speculation which started before this point is allowed to continue past this point
	test	%rax, %rax
	jne	error_path
	(do nefarious things with the user pointer)

	mov -EINVAL, %rax

So the CPU is still free to speculately execute the dereference_uptr
block because the lfence was before the 'jne error_path' branch.