linux-kernel - Re: [PATCH] vsprintf: Make "null" pointer dereference more robust

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Message-ID: <20180329151309.omdoxcdku5xoshgx@pathway.suse.cz>
Date:   Thu, 29 Mar 2018 17:13:09 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     "Tobin C . Harding" <me@...in.cc>, linux@...musvillemoes.dk,
        Joe Perches <joe@...ches.com>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] vsprintf: Make "null" pointer dereference more robust

On Fri 2018-03-02 16:17:34, Andy Shevchenko wrote:
> On Fri, 2018-03-02 at 13:53 +0100, Petr Mladek wrote:
> > %p has many modifiers where the pointer is dereferenced. An invalid
> > pointer might cause kernel to crash silently.
> > 
> > Note that printk() formats the string under logbuf_lock. Any recursive
> > printks are redirected to the printk_safe implementation and the
> > messages
> > are stored into per-CPU buffers. These buffers might be eventually
> > flushed
> > in printk_safe_flush_on_panic() but it is not guaranteed.
> > 
> > In general, we should do our best to get useful message from printk().
> > All pointers to the first memory page must be invalid. Let's prevent
> > the dereference and print "(null)" in this case. This is already done
> > in many other situations, including "%s" format handling and many
> > page fault handlers.
> > 
> 
> 
> With such explanation it makes at least clear for the reader why it's
> done.
> 
> Thanks!
> 
> Would you be okay if I take this one as a first in my series and
> resubmit the series based on it?

The original simple patch grew into something bigger. I have it
almost ready. It looks the following way at the moment:

  vsprintf: Shuffle ptr_to_id() code
  vsprintf: Consistent %pK handling for kptr_restrict == 0
  vsprintf: Do not check address of well-known strings
  vsprintf: Consolidate handling of unknown pointer specifiers
  vsprintf: Factor out %p[iI] handler as ip_addr_string()
  vsprintf: Factor out %pV handler as va_format()
  vsprintf: Factor out %pO handler as kobject_string()
  vsprintf: Prevent crash when dereferencing invalid pointers
  vsprintf: Avoid confusion between invalid address and value

 Documentation/core-api/printk-formats.rst |   8 +
 lib/test_printf.c                         |  37 ++-
 lib/vsprintf.c                            | 445 ++++++++++++++++++------------
 3 files changed, 307 insertions(+), 183 deletions(-)

I still would like to do some final review and check with a fresh
head after Easter holidays.


Now, I am unsure how to merge it with your patchset. Most of your
patches are independent and should be applied. But there will be
some merge conflicts.

On possibility would be that I take your still valid changes via
printk.git. Then you would even need not send anything. I could
resolve the conflicts when applying the patches.

Or would you prefer to resend your patchset without the controversal
check removal? And wait for my patchset?

Best Regards,
Petr