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

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Date:   Wed, 14 Nov 2018 04:46:18 -0700
From:   William Kucharski <william.kucharski@...cle.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "Isaac J. Manjarres" <isaacm@...eaurora.org>,
        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



> On Nov 14, 2018, at 4:09 AM, David Laight <David.Laight@...LAB.COM> wrote:
> 
> 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.

The extra test is only a few extra instructions, but I understand the concern. (Though I don't
know how you'd access the carry flag from C in a machine-independent way. Also, for the
calculation to be correct you still need to check 'ptr + (len - 1)' for the wrap.)

You could also theoretically call gcc's __builtin_uadd_overflow() if you want to get carried away.

As I mentioned, I was just being paranoid, but the passed zero length issue stood out to me.

    William Kucharski