linux-kernel - Re: [PATCH v3] fs/proc, core/debug: Don't expose absolute kernel addresses via wchan

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Date:	Thu, 1 Oct 2015 13:16:28 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andy Lutomirski <luto@...capital.net>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Andi Kleen <ak@...ux.intel.com>,
	kasan-dev <kasan-dev@...glegroups.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v3] fs/proc, core/debug: Don't expose absolute kernel
 addresses via wchan

On 10/01/2015 12:29 PM, Ingo Molnar wrote:
> 
> * Andrey Ryabinin <ryabinin.a.a@...il.com> wrote:
> 
>> 2015-10-01 10:57 GMT+03:00 Ingo Molnar <mingo@...nel.org>:
>>> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
>>> index d411ca63c8b6..db64f7d6492d 100644
>>> --- a/Documentation/filesystems/proc.txt
>>> +++ b/Documentation/filesystems/proc.txt
>>> @@ -140,7 +140,8 @@ Table 1-1: Process specific entries in /proc
>>>   stat          Process status
>>>   statm         Process memory status information
>>>   status                Process status in human readable form
>>> - wchan         If CONFIG_KALLSYMS is set, a pre-decoded wchan
>>> + wchan         If CONFIG_KALLSYMS=y, wchan (the kernel function the process is
>>> +               blocked in) symbol string. "0" if not blocked or !KALLSYMS.
>>
>> /proc/PID/wchan is under #ifdef CONFIG_KALLSYMS.
> 
> Yeah, indeed, so I clarified that text to now read:
> 
> + wchan         Present with CONFIG_KALLSYMS=y: it shows the kernel function
> +               symbol the task is blocked in - or "0" if not blocked.
> 
>>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>>> index b25eee4cead5..6f05aabce3aa 100644
>>> --- a/fs/proc/base.c
>>> +++ b/fs/proc/base.c
>>> @@ -430,13 +430,10 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
>>>
>>>         wchan = get_wchan(task);
>>>
>>> -       if (lookup_symbol_name(wchan, symname) < 0) {
>>> -               if (!ptrace_may_access(task, PTRACE_MODE_READ))
>>> -                       return 0;
>>> -               seq_printf(m, "%lu", wchan);
>>> -       } else {
>>> +       if (!lookup_symbol_name(wchan, symname))
>>>                 seq_printf(m, "%s", symname);
>>> -       }
>>> +       else
>>> +               seq_putc(m, '0');
>>
>> Maybe we should respect 'kptr_restrict' sysctl when we use '%ps', '%pB' etc. 
>> printk formats (AFAIK %ps just prints address if KALLSYMS=n, or lookup failed). 
>> In that case you could just do 'seq_printf(m, "%ps", wchan)'.
>>
>> OTOH, %ps, %pS are used mostly in debugging, so investigating some crash in 
>> production kernel with no !KALLSYMS and with kptr_restrict != 0 will be a 
>> nightmare.
> 
> So this code does not use %pX, it prints the symbol. 

I think you misunderstood me.
Yes, this code currently doesn't use %pX, but it could:

diff --git a/fs/proc/base.c b/fs/proc/base.c
index b25eee4..f58f66e 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -425,18 +425,7 @@ static int proc_pid_auxv(struct seq_file *m, struct pid_namespace *ns,
 static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
                          struct pid *pid, struct task_struct *task)
 {
-       unsigned long wchan;
-       char symname[KSYM_NAME_LEN];
-
-       wchan = get_wchan(task);
-
-       if (lookup_symbol_name(wchan, symname) < 0) {
-               if (!ptrace_may_access(task, PTRACE_MODE_READ))
-                       return 0;
-               seq_printf(m, "%lu", wchan);
-       } else {
-               seq_printf(m, "%s", symname);
-       }
+       seq_printf(m, "%ps", get_wchan(task));
 
        return 0;
 }


There is a problem here, though. %ps will print absolute kernel address instead of symbol name
if KALLSYMS=n or if resolution of address failed.
So I was wondering, may be should just fix %ps ?
i.e. print 0 instead of absolute address if KALLSYMS=n or lookup failure?


> Yes, the symbol in itself is 
> 'information' about the execution of the task in itself - but /proc per se is all 
> about providing information about tasks in the system (including to unprivileged 
> users), so there's IMHO little point in restricting this output any further ...
> 
> I think ktrp_restrict is mostly about not exposing absolute addresses.
> 

Right, and '%ps' may expose absolute address if KALLSYMS=n or address lookup failed for some reason.

> Thanks,
> 
> 	Ingo
> 
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