linux-kernel - Re: [PATCH v6 8/9] vsprintf: Prevent crash when dereferencing invalid pointers

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Message-ID: <20190219141516.GA15836@tigerII.localdomain>
Date:   Tue, 19 Feb 2019 23:15:16 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Tobin C . Harding" <me@...in.cc>, Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.cz>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 8/9] vsprintf: Prevent crash when dereferencing
 invalid pointers

On (02/19/19 15:49), Andy Shevchenko wrote:
> > On (02/19/19 13:02), Andy Shevchenko wrote:
> > [..]
> > > And if it's not? You will get in either case incomplete information,
> > > but at least with "(e" (or even "(") you might get a clue that it
> > > errornous conditions.
> > 
> > The thing I'm signaling here is that in some cases we still can
> > crash the kernel; with the difference that invalid dereference
> > can now be a memory corruption. Just saying.
> 
> Wouldn't that mean that the culprit in the caller, not in the callee?
> 
> (As far as I got your another example with badly called sprintf() which may
>  overwrite stack, etc).

ipv4 printout case does not look like a caller bug to me: we expect a 15
bytes ipv4 address, allocate a 16 bytes buffer, sprintf() voluntarily
writes 18 bytes. This error reporting is a bit of a dangerous practice;
next year someone might add another specifier and another

	return string(buf, end, "(this data does not look right)", spec);

We probably would want to do something about it. For instance, mandating
that "(error)" string cannot be larger than 8 bytes can be a good starting
point.

	-ss