linux-kernel - RE: [PATCH] mm/usercopy: Use memory range to be accessed for wraparound check

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Message-ID: <5dcd06a0f84a4824bb9bab2b437e190d@AcuMS.aculab.com>
Date:   Wed, 14 Nov 2018 11:09:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'William Kucharski' <william.kucharski@...cle.com>,
        "Isaac J. Manjarres" <isaacm@...eaurora.org>
CC:     Kees Cook <keescook@...omium.org>,
        "crecklin@...hat.com" <crecklin@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "psodagud@...eaurora.org" <psodagud@...eaurora.org>,
        "tsoni@...eaurora.org" <tsoni@...eaurora.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] mm/usercopy: Use memory range to be accessed for
 wraparound check

From: William Kucharski
> Sent: 14 November 2018 10:35
> 
> > On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres <isaacm@...eaurora.org> wrote:
> >
> > diff --git a/mm/usercopy.c b/mm/usercopy.c
> > index 852eb4e..0293645 100644
> > --- a/mm/usercopy.c
> > +++ b/mm/usercopy.c
> > @@ -151,7 +151,7 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
> > 				       bool to_user)
> > {
> > 	/* Reject if object wraps past end of memory. */
> > -	if (ptr + n < ptr)
> > +	if (ptr + (n - 1) < ptr)
> > 		usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
> 
> I'm being paranoid, but is it possible this routine could ever be passed "n" set to zero?
> 
> If so, it will erroneously abort indicating a wrapped address as (n - 1) wraps to ULONG_MAX.
> 
> Easily fixed via:
> 
> 	if ((n != 0) && (ptr + (n - 1) < ptr))

Ugg... you don't want a double test.

I'd guess that a length of zero is likely, but a usercopy that includes
the highest address is going to be invalid because it is a kernel address
(on most archs, and probably illegal on others).
What you really want to do is add 'ptr + len' and check the carry flag.

	David

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