linux-kernel - Re: [PATCH] Netfilter: Fix integer overflow in net/ipv6/netfilter/ip6

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Date:	Tue, 23 Mar 2010 04:04:59 +0100 (CET)
From:	Jan Engelhardt <jengelh@...ozas.de>
To:	wzt wzt <wzt.wzt@...il.com>
cc:	Xiaotian Feng <xtfeng@...il.com>,
	Patrick McHardy <kaber@...sh.net>,
	linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH] Netfilter: Fix integer overflow in
 net/ipv6/netfilter/ip6_tables.c

On Tuesday 2010-03-23 03:37, wzt wzt wrote:
>> And, for the addition overflow, can it be caught by
>>
>> "if (*len != sizeof(struct ipt_get_entries) + get.size)"  ???
>
>sizeof(struct ipt_get_entries) + get.size can be overflow as *len,
>get.size is control by user space with copy_from_user().

The != should catch it.

For 64-bit environments:
* + invoked with size_t, unsigned int
  => right side promoted to size_t, result type is size_t
* != invoked with int and size_t
  => left-side promoted to ssize_t (probably; but something as large as size_t)
* get.size is 32-bit bounded, as is *len,
  so no overflow to worry about at all unless you make
  sizeof(X) hilariously big close to 2^64 which is rather unlikely.

For 32-bit environments:
* Let *len be a number of choice (e.g. 36)
* Find a sizeof(X)+get.size that equals 36 mod 2^32.
* Since sizeof(X) is const, get.size must be 0 mod 2^32.
* So get.size must be a multiple of 2^32 to fool the system.
* Since get.size itself is only a 32-bit quantity, you cannot
  represent any value larger than 4294967295.

What Was What Was Wanted.
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