On Mon, Sep 14, 2020 at 09:23:59PM +0000, David Laight wrote: > 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.
Maybe, but I believe that's still going to end up a treewide change. And, if we're going to the trouble of changing the access_ok() interface, we should change it enough to make sure that accidental uses of the old interface (after years of muscle memory) will fail to build. We could either add a 3rd argument, or rename it to access_ok_mask() or something. -- Josh
