Re: [PATCH] kprobes, x86/alternatives: use text_mutex to protect smp_alt_modules

[zhouchengming]

On 2017/10/28 16:43, Masami Hiramatsu wrote:
> On Fri, 27 Oct 2017 21:30:24 +0800
> zhouchengming<zhouchengming1@xxxxxxxxxx>  wrote:
>
> > On 2017/10/27 20:33, Borislav Petkov wrote:
> > > On Fri, Oct 27, 2017 at 07:42:45PM +0800, zhouchengming wrote:
> > > > This is a real bug happened on one of our machines, below is the calltrace.
> > > > We can see the trigger is at alternatives_text_reserved+0x20/0x80, and
> > > > encounter a deleted (poisoned) list_head.
> > > Looks like some out-of-tree, old kernel thing. We don't have
> > > mlx4_stats_sysfs_create() upstream and looking at the boot timestamps,
> > > it could be that register_jprobe() is not ready yet.
> > Yes, it's an out-of-tree module, loaded when boot kernel. register_kprobe()
> > maybe not ready yet, but the bug is not caused by it obviously.
> >
> > > Looking at the Code, though:
> > >
> > >     20:   74 59                   je     0x7b
> > >     22:   66 0f 1f 84 00 00 00    nopw   0x0(%rax,%rax,1)
> > >     29:   00 00
> > >     2b:*  48 3b 71 20             cmp    0x20(%rcx),%rsi<-- trapping instruction
> > >     2f:   72 3a                   jb     0x6b
> > >     31:   48 3b 79 28             cmp    0x28(%rcx),%rdi
> > >     35:   77 34                   ja     0x6b
> > >
> > > %rcx is 0xdead0000000000d0 and that is POISON_POINTER_DELTA + 0xd0 so
> > > that looks more like smp_alt_modules is not initialized yet but I could
> > > could very well be wrong because this is an old kernel. So trigger that
> > > with the upstream kernel without out of tree modules.
> > The smp_alt_modules is defined by LIST_HEAD, so it's initialized at start.
> >
> > A deleted list_head->next = LIST_POISON1 = 0xdead000000000000 + 0x100, then
> > container_of() to get the struct smp_alt_module: -0x30 = 0xdead0000000000d0
> >
> > Obviously, it's a deleted list_head, and I have explained clearly how it happen in
> > the patch comment.
> Ah, I see. It looks alternatives_text_reserved() bug at a glance.
> But simply adding smp_alt mutex to alternatives_text_reserved() causes
> ABBA deadlock in the kprobe's path.
> So your solution is to replace the smp_alt with text_mutex, since
> alternatives_text_reserved is x86 specific function.
>
> Hmm, let me see... I agree that will be a simple way to solve, but
> it also means we have 2 resources protected by text_mutex.

Yes, the smp_alt mutex must be held outside the text_mutex, this is a simpler way
to solve, because we will need another x86 specific interface if we want to hold
the smp_alt mutex.

But like you said, it's not good to use one text_mutex to protect 2 resources...
I hope there is any better way.

Thanks.

> Thank you,