Messages in this thread Patch in this message | | | Date | Thu, 18 Jun 2020 21:02:07 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/7] x86/entry: Fix #UD vs WARN more |
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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; }
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