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

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Date:   Tue, 6 Mar 2018 10:25:13 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Tobin C . Harding" <me@...in.cc>, Joe Perches <joe@...ches.com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH] vsprintf: Make "null" pointer dereference more robust

On Mon 2018-03-05 16:16:37, Rasmus Villemoes wrote:
> On 2 March 2018 at 13:53, Petr Mladek <pmladek@...e.com> 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.
> 
> Yeah, it's annoying that we can't reliably WARN for bogus vsprintf() uses.
> 
> > 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.
> >
> > Signed-off-by: Petr Mladek <pmladek@...e.com>
> > ---
> >  lib/vsprintf.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index d7a708f82559..5c2d1f44218a 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -1849,7 +1849,7 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
> >  {
> >         const int default_width = 2 * sizeof(void *);
> >
> > -       if (!ptr && *fmt != 'K' && *fmt != 'x') {
> > +       if ((unsigned long)ptr < PAGE_SIZE && *fmt != 'K' && *fmt != 'x') {
> 
> ISTM that accidentally passing an ERR_PTR would be just as likely as
> passing a NULL pointer (or some small offset from one), so if we do
> this, shouldn't the test also cover IS_ERR values?

It would make perfect sense to catch IS_ERR_PTR(). Derefenrecing
such pointer cause crash. But it might be pretty confusing to print
"(null)" in this case.

I would handle this in separate patch and print "(err)" or so.
Any volunteer to prepare the patch?

Best Regards,
Petr