linux-kernel - Re: [PATCH v3] vsprintf: Prevent crash when dereferencing invalid pointers

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Date:   Tue, 03 Apr 2018 16:40:58 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Tobin C . Harding" <me@...in.cc>, Joe Perches <joe@...ches.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH v3] vsprintf: Prevent crash when dereferencing invalid
 pointers

On Tue, 2018-04-03 at 15:13 +0200, Petr Mladek wrote:
> On Tue 2018-04-03 14:54:18, Andy Shevchenko wrote:
> > On Tue, 2018-04-03 at 13:46 +0200, Petr Mladek wrote:
> > > On Mon 2018-04-02 17:15:23, Andy Shevchenko wrote:
> > > > On Thu, 2018-03-29 at 16:53 +0200, Petr Mladek wrote:
> > > > > On Fri 2018-03-16 20:19:35, Andy Shevchenko wrote:
> > > > > > On Thu, 2018-03-15 at 16:26 +0100, Petr Mladek wrote:
> > > > > > > On Thu 2018-03-15 15:09:03, Andy Shevchenko wrote:
> > > > > > > > I still think that printing a hex value of the error
> > > > > > > > code is
> > > > > > > > much
> > > > > > > > better
> > > > > > > > than some odd "(efault)".
> > > > > > > 
> > > > > > > Do you mean (err:0e)? Google gives rather confusing
> > > > > > > answers
> > > > > > > for
> > > > > > > this.
> > > > > > 
> > > > > > More like "(0xHHHH)" (we have already more than 512 error
> > > > > > code
> > > > > > numbers.
> > > > > 
> > > > > Hmm, I have never seen the error code in this form.
> > > > 
> > > > We have limited space to print it and error numbers currently
> > > > can be
> > > > up
> > > > to 0xfff (4095). So, I have no better idea how to squeeze them
> > > > while
> > > > thinking that "(efault)" is much harder to parse in case of
> > > > error
> > > > pointer.
> > > 
> > > But this will not be used instead of address value. It is used in
> > > situations
> > > where we print the information that is stored at the address, for
> > > example,
> > > string, IP address, dentry name.
> > 
> > We have a lot of API functions which returns:
> > -ERR_PTR
> > NULL
> > struct foo *
> > 
> > There is no guarantee that one of that API won't be used as a
> > supplier
> > for printf().
> 
> OK, I think that I have finally understood it. You would like to
> detect ERR_PTR values and handle them specially? I mean to show
> the value?
> 
> But then we would need to distinguish three types of errors,
> something like:
> 
>  + (null)       for pure NULL address
>  + (e:XXXX)     for address in IS_ERR_VALUE() range

// Just IS_ERR(). IS_ERR_VALUE() is not meant to be used widely

>  + (efault)     for any other invalid address
> 
> Then people might want to see values also from the first 4096 bytes.
> This is getting too complicated.

No, it's not. (null) case is already in kernel, you came with (efault),
but IS_ERR() case or any other case like it is just printing of standard
pointer value. See in the code where special_hex_number() is called.

>  I am not sure if it is worth it.

Your patch will hide values for error codes. Not good for debugging.

> 
> 
> > You can't dereference ERR_PTR value, but anything else except the
> > actual
> > error value is worse than value itself...
> 
> Yes and no, see below.

Yes, there is no "no".

> 
> > > 
> > > > >  Also google gives
> > > > > rather confusing results when searching, for example for
> > > > > "(0x000E)".
> > > > 
> > > > It's not primarily for google, though yeah, people would google
> > > > for
> > > > error messages...
> > > > 
> > > > Another question is what the format: decimal versus hex for
> > > > errors.
> > > > Maybe just "(-DDDDD)"?
> > > 
> > > This still looks confusing and google does not help.
> > 
> > ...then we have a last option just to print a value as a pointer
> > address.
> 
> We could not print the real address from security reasons. The hashed
> pointer value is not much helpful. IMHO, a common error string is
> easier to spot or search for.

Did you read what I'm writing? How on the earth the pointer in the range
of -1...-4095 would be a security issue?!

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy