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

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Date:   Fri, 16 Mar 2018 10:18:52 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        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>, 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>
Subject: Re: [PATCH v3] vsprintf: Prevent crash when dereferencing invalid
 pointers

On (03/15/18 13:01), Steven Rostedt wrote:
> > > > +static const char *check_pointer_access(const void *ptr)
> > > > +{
> > > > +	unsigned char byte;
> > > > +
> > > > +	if (!ptr)
> > > > +		return "(null)";
> > > > +
> > > > +	if (probe_kernel_read(&byte, ptr, 1))  
> > >                                         ^^^^^
> > > Why one byte? 			     sizeof(ptr)?  
> > 
> > I think there is a shorter version - probe_kernel_address(),
> > which, seems, was designed for this particular kind of stuff.
> > 
> > 	void *p;
> > 
> > 	if (probe_kernel_address(ptr, p))
> > 		....
> >
> 
> Agreed.

Hm, may be sizeof(ptr) still won't suffice. It would be great if we
could always look at spec.field_width, which can be up to 2 * sizeof(void *),
and then just probe_kernel_read(spec.field_width). E.g., %b/%bl prints out a
bitmap, accessing max_t(int, spec.field_width, 0) bits, which is good. But,
for instance, %U (uuid printout) doesn't look at spec.field_width, and reads
in 16 bytes from the given memory address. Then we have ipv4/ipv6, mac, etc.
So I think that checking just 1 byte or sizeof(ptr) is not really enough if
we want to fix vsprintf. What do you think?

	-ss