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From:   David Laight <David.Laight@ACULAB.COM>
To:     'William Kucharski' <william.kucharski@oracle.com>,
        "Isaac J. Manjarres" <isaacm@codeaurora.org>
CC:     Kees Cook <keescook@chromium.org>,
        "crecklin@redhat.com" <crecklin@redhat.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "psodagud@codeaurora.org" <psodagud@codeaurora.org>,
        "tsoni@codeaurora.org" <tsoni@codeaurora.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: RE: [PATCH] mm/usercopy: Use memory range to be accessed for
 wraparound check
Thread-Topic: [PATCH] mm/usercopy: Use memory range to be accessed for
 wraparound check
Thread-Index: AQHUfAXMm0KhA+w52Ui13XJlKrLpAaVPG7lg
Date:   Wed, 14 Nov 2018 11:09:25 +0000
Message-ID: <5dcd06a0f84a4824bb9bab2b437e190d@AcuMS.aculab.com>
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 <FFE931C2-DE41-4AD8-866B-FD37C1493590@oracle.com>
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From: William Kucharski
> Sent: 14 November 2018 10:35
> 
> > On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres <isaacm@codeaurora.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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