RE: [PATCH v3] x86/uaccess: Use pointer masking to limit uaccess speculation

[David Laight <David.Laight@ACULAB.COM>]

From: Borislav Petkov
> Sent: 14 September 2020 18:56
> 
> On Thu, Sep 10, 2020 at 12:22:53PM -0500, Josh Poimboeuf wrote:
> > +/*
> > + * Sanitize a user pointer such that it becomes NULL if it's not a valid user
> > + * pointer.  This prevents speculative dereferences of user-controlled pointers
> > + * to kernel space when access_ok() speculatively returns true.  This should be
> > + * done *after* access_ok(), to avoid affecting error handling behavior.
> 
> Err, stupid question: can this macro then be folded into access_ok() so
> that you don't have to touch so many places and the check can happen
> automatically?

My thoughts are that access_ok() could return 0 for fail and ~0u
for success.
You could then do (with a few casts):
	mask = access_ok(ptr, size);
	/* Stop gcc tracking the value of mask. */
	asm volatile( "" : "+r" (mask));
	addr = ptr & mask;
	if (!addr && ptr)  // Let NULL through??
		return -EFAULT;

I think there are other changes in the pipeline to remove
most of the access_ok() apart from those inside put/get_user()
and copy_to/from_user().
So the changes should be more limited than you might think.

	David

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