Re: [PATCH 2/3] x86/traps: Print non-canonical address on #GP

[Andy Lutomirski]

On Thu, Nov 14, 2019 at 10:20 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Thu, Nov 14, 2019 at 10:00:35AM -0800, Andy Lutomirski wrote:
> > On Thu, Nov 14, 2019 at 9:46 AM Sean Christopherson
> > <sean.j.christopherson@xxxxxxxxx> wrote:
> > > > +     /*
> > > > +      * For the user half, check against TASK_SIZE_MAX; this way, if the
> > > > +      * access crosses the canonical address boundary, we don't miss it.
> > > > +      */
> > > > +     if (addr_ref <= TASK_SIZE_MAX)
> > >
> > > Any objection to open coding the upper bound instead of using
> > > TASK_SIZE_MASK to make the threshold more obvious?
> > >
> > > > +             return;
> > > > +
> > > > +     pr_alert("dereferencing non-canonical address 0x%016lx\n", addr_ref);
> > >
> > > Printing the raw address will confuse users in the case where the access
> > > straddles the lower canonical boundary.  Maybe combine this with open
> > > coding the straddle case?  With a rough heuristic to hedge a bit for
> > > instructions whose operand size isn't accurately reflected in opnd_bytes.
> > >
> > >         if (addr_ref > __VIRTUAL_MASK)
> > >                 pr_alert("dereferencing non-canonical address 0x%016lx\n", addr_ref);
> > >         else if ((addr_ref + insn->opnd_bytes - 1) > __VIRTUAL_MASK)
> > >                 pr_alert("straddling non-canonical boundary 0x%016lx - 0x%016lx\n",
> > >                          addr_ref, addr_ref + insn->opnd_bytes - 1);
> > >         else if ((addr_ref + PAGE_SIZE - 1) > __VIRTUAL_MASK)
> > >                 pr_alert("potentially straddling non-canonical boundary 0x%016lx - 0x%016lx\n",
> > >                          addr_ref, addr_ref + PAGE_SIZE - 1);
> >
> > This is unnecessarily complicated, and I suspect that Jann had the
> > right idea but just didn't quite explain it enough.  The secret here
> > is that TASK_SIZE_MAX is a full page below the canonical boundary
> > (thanks, Intel, for screwing up SYSRET), so, if we get #GP for an
> > address above TASK_SIZE_MAX,
>
> Ya, I followed all that.  My point is that if "addr_ref + insn->opnd_bytes"
> straddles the boundary then it's extremely likely the #GP is due to a
> non-canonical access, i.e. the pr_alert() doesn't have to hedge (as much).

I suppose.  But I don't think we have a real epidemic of failed
accesses to user memory between TASK_SIZE_MAX and the actual boundary
that get #GP instead of #PF but fail for a reason other than
non-canonicality :)

I think we should just go back in time and fix x86_64 to either give
#PF or at least give some useful page fault for a non-canonical
address. The only difficulties I'm aware of is that Intel CPUs would
either need to be redesigned better or would have slightly odd
semantics for jumps to non-canonical addresses -- #PF in Intel's model
of "RIP literally *can't* have a non-canonical value" would be a bit
strange.  Also, my time machine is out of commission.

--Andy