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

[Peter Zijlstra]

On Fri, Oct 27, 2017 at 02:33:48PM +0200, 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.
> 
> 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.

Not to mention that we're about (or just have) yanked jprobes out of the
kernel entirely.