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

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Date:   Thu, 18 Jun 2020 21:02:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Mark Rutland <Mark.Rutland@....com>,
        Matthew Helsley <mhelsley@...are.com>,
        Steven Rostedt <rostedt@...dmis.org>, jthierry@...hat.com,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: [PATCH 1/7] x86/entry: Fix #UD vs WARN more

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;
 }