Re: [PATCH 1/7] x86/entry: Fix #UD vs WARN more

Peter Zijlstra
2020-06-18T19:02:25Z

On Thu, Jun 18, 2020 at 11:36:53AM -0700, Andy Lutomirski wrote:

> I wasn't imagining going far down the rabbit hole at all -- I think
> that, at most, we should cover the path for when the fault wasn't a
> BUG/WARN in the first place. I admit that, for #UD in particular,
> this isn't a big deal, but if it were a different vector, this could
> matter.

Right, so there's 3 cases for ud2:

- WARN; ud2, bug_entry, recovers
- BUG; ud2, bug_entry, dies
- UBSAN; ud2, !bug_entry, dies

Nothing else should be generating ud2 instructions, any other #UD goes
into handle_invalid_op() -> do_error_trap() -> ... -> die().

[ while there, we should probably restructure do_trap() to have
cond_local_irq_enable() _after_ do_trap_no_signal(). ]

We could probably change is_valid_bugaddr() to not use
probe_kernel_address(), because if it couldn't read the instruction,
we'd not be getting #UD in the first place.

If we've gotten rid of probe_kernel_address() we can noinstr/inline that
and then we can only call into report_bug() IFF ud2.

Does that make things 'better' ? This can only go realy bad if there's a
1 byte instruction that triggers #UD, but I think that was ruled out.

---
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index c26751e303f1..275a621f1aff 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -91,10 +91,7 @@ int is_valid_bugaddr(unsigned long addr)
if (addr < TASK_SIZE_MAX)
return 0;

- if (probe_kernel_address((unsigned short *)addr, ud))
- return 0;
-
- return ud == INSN_UD0 || ud == INSN_UD2;
+ return *(unsigned short *)addr == INSN_UD2;
}

static nokprobe_inline int
@@ -220,15 +217,17 @@ static noinstr bool handle_bug(struct pt_regs *regs)
{
bool handled = false;

- /*
- * All lies, just get the WARN/BUG out.
- */
- instrumentation_begin();
- if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
- regs->ip += LEN_UD2;
- handled = true;
+ if (is_valid_bugaddr(regs->ip)) {
+ /*
+ * All lies, just get the WARN/BUG out.
+ */
+ instrumentation_begin();
+ if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
+ regs->ip += LEN_UD2;
+ handled = true;
+ }
+ instrumentation_end();
}
- instrumentation_end();

return handled;
}