linux-kernel - Re: [PATCH] vsprintf: Do not break early boot with probing addresses

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Message-Id: <20190510184548.1eda01a9@mschwideX1>
Date:   Fri, 10 May 2019 18:45:48 +0200
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Tobin C . Harding" <me@...in.cc>, Michal Hocko <mhocko@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, Russell Currey <ruscur@...sell.cc>,
        Christophe Leroy <christophe.leroy@....fr>,
        Stephen Rothwell <sfr@...abs.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-arch@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH] vsprintf: Do not break early boot with probing
 addresses

On Fri, 10 May 2019 12:40:58 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 10 May 2019 18:32:58 +0200
> Martin Schwidefsky <schwidefsky@...ibm.com> wrote:
> 
> > On Fri, 10 May 2019 12:24:01 -0400
> > Steven Rostedt <rostedt@...dmis.org> wrote:
> >   
> > > On Fri, 10 May 2019 10:42:13 +0200
> > > Petr Mladek <pmladek@...e.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.