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 11:29:14 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andrey Ryabinin <ryabinin.a.a@...il.com>
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


* 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. 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.

Thanks,

	Ingo
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