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Date: Fri, 10 May 2019 18:45:48 +0200
From: Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [PATCH] vsprintf: Do not break early boot with probing
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To: Steven Rostedt <rostedt@goodmis.org>
Cc: Petr Mladek <pmladek@suse.com>, Linus Torvalds <torvalds@linux-foundation.org>, Andy Shevchenko <andriy.shevchenko@linux.intel.com>, Rasmus Villemoes <linux@rasmusvillemoes.dk>, "Tobin C . Harding" <me@tobin.cc>, Michal Hocko <mhocko@suse.cz>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>, linux-kernel@vger.kernel.org, Michael Ellerman <mpe@ellerman.id.au>, linuxppc-dev@lists.ozlabs.org, Russell Currey <ruscur@russell.cc>, Christophe Leroy <christophe.leroy@c-s.fr>, Stephen Rothwell <sfr@ozlabs.org>, Heiko Carstens <heiko.carstens@de.ibm.com>, linux-arch@vger.kernel.org, linux-s390@vger.kernel.org
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On Fri, 10 May 2019 12:40:58 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 10 May 2019 18:32:58 +0200
> Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
> 
> > On Fri, 10 May 2019 12:24:01 -0400
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> >   
> > > On Fri, 10 May 2019 10:42:13 +0200
> > > Petr Mladek <pmladek@suse.com> wrote:
> > >     
> > > >  static const char *check_pointer_msg(const void *ptr)
> > > >  {
> > > > -	char byte;
> > > > -
> > > >  	if (!ptr)
> > > >  		return "(null)";
> > > >  
> > > > -	if (probe_kernel_address(ptr, byte))
> > > > +	if ((unsigned long)ptr < PAGE_SIZE || IS_ERR_VALUE(ptr))
> > > >  		return "(efault)";
> > > >        
> > > 
> > > 
> > > 	< PAGE_SIZE ?
> > > 
> > > do you mean: < TASK_SIZE ?    
> > 
> > The check with < TASK_SIZE would break on s390. The 'ptr' is
> > in the kernel address space, *not* in the user address space.
> > Remember s390 has two separate address spaces for kernel/user
> > the check < TASK_SIZE only makes sense with a __user pointer.
> >   
> 
> So we allow this to read user addresses? Can't that cause a fault?
> 
> If the condition is true, we return "(efault)".

On x86 this would allow a user space access as kernel and user live
in the same address space, on s390 it would not.
h
-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.