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

[zhouchengming]

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.

Thanks.

> Thx.