Messages in this thread | | | Date | Sat, 28 Oct 2017 17:51:46 +0800 | From | zhouchengming <> | Subject | Re: [PATCH] kprobes, x86/alternatives: use text_mutex to protect smp_alt_modules |
| |
On 2017/10/28 16:43, Masami Hiramatsu wrote: > On Fri, 27 Oct 2017 21:30:24 +0800 > zhouchengming<zhouchengming1@huawei.com> 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, > >
| |