linux-kernel - Re: [PATCH 1/2] lib/vsprintf.c: Fix potential NULL deref in hex

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Date:	Wed, 28 Jan 2015 18:43:11 +0200
From:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Randy Dunlap <rdunlap@...radead.org>,
	Fabian Frederick <fabf@...net.be>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Ryan Mallon <rmallon@...il.com>,
	Masanari Iida <standby24x7@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] lib/vsprintf.c: Fix potential NULL deref in
 hex_string

On Wed, 2015-01-28 at 16:49 +0100, Rasmus Villemoes wrote:
> On Wed, Jan 28 2015, Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> 
> > On Wed, 2015-01-28 at 14:25 +0100, Rasmus Villemoes wrote:
> >> The helper hex_string() is broken in two ways. First, it doesn't
> >> increment buf regardless of whether there is room to print, so callers
> >> such as kasprintf() that try to probe the correct storage to allocate
> >> will get a too small return value. But even worse, kasprintf() (and
> >> likely anyone else trying to find the size of the result) pass NULL
> >> for buf and 0 for size, so we also have end == NULL. But this means
> >> that the end-1 in hex_string() is (char*)-1, so buf < end-1 is true
> >> and we get a NULL pointer deref. I double-checked this with a trivial
> >> kernel module that just did a kasprintf(GFP_KERNEL, "%14ph",
> >> "CrashBoomBang").
> >
> > Good catch, though I don't like the implementation of fix.
> >
> > What about the following?
> >
> >
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index 8690798..47b36ddd 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -783,11 +783,20 @@ char *hex_string(char *buf, char *end, u8 *addr,
> > struct printf_spec spec,
> >         if (spec.field_width > 0)
> >                 len = min_t(int, spec.field_width, 64);
> >  
> > -       for (i = 0; i < len && buf < end - 1; i++) {
> > -               buf = hex_byte_pack(buf, addr[i]);
> > +       for (i = 0; i < len; i++) {
> > +               if (buf < end)
> > +                       *buf = hex_asc_hi(addr[i]);
> > +               ++buf;
> > +
> > +               if (buf < end)
> > +                       *buf = hex_asc_lo(addr[i]);
> > +               ++buf;
> >  
> > -               if (buf < end && separator && i != len - 1)
> > -                       *buf++ = separator;
> > +               if (separator && i != len - 1) {
> > +                       if (buf < end)
> > +                               *buf = separator;
> > +                       ++buf;
> > +               }
> >         }
> >  
> >         return buf;
> 
> I had exactly that at one point. I think the only reason I ended up
> doing it the other way was that I wanted to introduce the write_bytes
> helper, and then use that to do some simplifications and optimizations
> for other %p helpers. Many of those write their output to a temporary
> buffer, knowing exactly how much there is, then delegate to string(),
> which then recomputes the string length before printing. But all that
> can be done another time, so I'm fine with this also.
> 

I ACK the version w/o write_bytes() helper. Meanwhile you may submit
your helper with usage of it as another patch.


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

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