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

[Sean Christopherson <sean.j.christopherson@intel.com>]

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@intel.com> 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).

> then it's either a #GP for a different
> reason or it's a genuine non-canonical access.

Heh, "canonical || !canonical" would be the options :-D

> 
> So I think that just a comment about this would be enough.
> 
> *However*, the printout should at least hedge a bit and say something
> like "probably dereferencing non-canonical address", since there are
> plenty of ways to get #GP with an operand that is nominally
> non-canonical but where the actual cause of #GP is different.  And I
> think this code should be skipped entirely if error_code != 0.
> 
> --Andy