Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 14 Nov 2019 10:41:06 -0800 | Subject | Re: [PATCH 2/3] x86/traps: Print non-canonical address on #GP |
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On Thu, Nov 14, 2019 at 10:20 AM Sean Christopherson <sean.j.christopherson@intel.com> 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@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).
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
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