linux-kernel - Re: [PATCH 1/2] task_struct cleanup: move binfmt field to mm

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 28 Jul 2009 15:21:16 +0900
From:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 1/2] task_struct cleanup: move binfmt field to mm_struct

Oleg Nesterov wrote:
> (add Rusty)
> 
> On 07/27, Hiroshi Shimamoto wrote:
>>  void set_binfmt(struct linux_binfmt *new)
>>  {
>> -	if (current->binfmt)
>> -		module_put(current->binfmt->module);
>> +	struct mm_struct *mm = current->mm;
>> +
>> +	BUG_ON(!mm);
>> +	if (mm->binfmt)
> 
> I am not sure we need this BUG_ON() above. If mm == NULL we will
> have the same debug info after the crash.

Ah, right. It will crash at accessing mm->binfmt and we'll get the
same information. I didn't think seriously.
BTW, now I noticed that it looks similar the security issue with
NULL page mapping. If the kernel has the page mapping to NULL,
it causes unstable behavior, no?

> 
>> @@ -619,6 +621,9 @@ struct mm_struct *dup_mm(struct task_struct *tsk)
>>  	mm->hiwater_rss = get_mm_rss(mm);
>>  	mm->hiwater_vm = mm->total_vm;
>>
>> +	if (mm->binfmt && !try_module_get(mm->binfmt->module))
>> +		goto free_pt;
> 
> free_pt does mmput() which calls module_put(mm->binfmt->module). This
> is not right when try_module_get() fails. Hmm, the same if dup_mmap()
> fails, we are doing an extra module_put().

Good catch, should avoid over putting binfmt->module.

> 
> Perhaps we can use __module_get() again, current->mm->binfmt holds a
> reference. But this is a user-visible change, currently fork() can't
> succeed if ->module->state == MODULE_STATE_GOING.
> 
> So I think we need another small change:
> 
> 	 free_pt:
> 	+	mm->binfmt = NULL;
> 	 	mmput(mm);

looks good for me.
Will send an update.

thanks,
Hiroshi

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/