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From: Vasiliy Kulikov <segoon@openwall.com>
To: kernel-hardening@lists.openwall.com
Cc: Kees Cook <keescook@chromium.org>,
	Ubuntu security discussion <ubuntu-hardened@lists.ubuntu.com>,
	linux-kernel@vger.kernel.org, David Windsor <dwindsor@gmail.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	pageexec@freemail.hu, spender@grsecurity.net
Subject: Re: [kernel-hardening] Re: Add overflow protection to kref
Date: Fri, 17 Feb 2012 11:59:45 +0400	[thread overview]
Message-ID: <20120217075945.GA2831@albatros> (raw)
In-Reply-To: <20120217002405.GB7746@kroah.com>

Hi,

> And in all that time, I've never seen an instance where you can overflow
> the reference count,

Do you mean that the overflow is theoretically impossible or that this
type of programmer error is rare?

If the former, it is only 2**32 incs - if you can find open() implementation
with a missing atomic_dec() in error path and you can call open() faster than
10000 times per second, you can overflow the counter in ~4 days.

If the latter, it is just a question of finding missing put() in some triggerable
error path.  Kees has already posted a link to a bug with a missing fput().


BTW, moving from atomic_t to 64 bit refcounter would kill the possibility of
overflow.  Unfortunately, AFAIU, 64 bit operations are not atomic on some 64 bit
archs.

Thanks,

-- 
Vasiliy

WARNING: multiple messages have this Message-ID (diff)
From: Vasiliy Kulikov <segoon@openwall.com>
To: kernel-hardening@lists.openwall.com
Cc: Kees Cook <keescook@chromium.org>,
	Ubuntu security discussion <ubuntu-hardened@lists.ubuntu.com>,
	linux-kernel@vger.kernel.org, David Windsor <dwindsor@gmail.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	pageexec@freemail.hu, spender@grsecurity.net,
	kernel-hardening@lists.openwall.com
Subject: Re: [kernel-hardening] Re: Add overflow protection to kref
Date: Fri, 17 Feb 2012 11:59:45 +0400	[thread overview]
Message-ID: <20120217075945.GA2831@albatros> (raw)
In-Reply-To: <20120217002405.GB7746@kroah.com>

Hi,

> And in all that time, I've never seen an instance where you can overflow
> the reference count,

Do you mean that the overflow is theoretically impossible or that this
type of programmer error is rare?

If the former, it is only 2**32 incs - if you can find open() implementation
with a missing atomic_dec() in error path and you can call open() faster than
10000 times per second, you can overflow the counter in ~4 days.

If the latter, it is just a question of finding missing put() in some triggerable
error path.  Kees has already posted a link to a bug with a missing fput().


BTW, moving from atomic_t to 64 bit refcounter would kill the possibility of
overflow.  Unfortunately, AFAIU, 64 bit operations are not atomic on some 64 bit
archs.

Thanks,

-- 
Vasiliy

  parent reply	other threads:[~2012-02-17  7:59 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-02-16 14:02 [kernel-hardening] Add overflow protection to kref David Windsor
2012-02-16 20:45 ` [kernel-hardening] " Kees Cook
2012-02-16 20:45   ` Kees Cook
2012-02-17  0:24   ` [kernel-hardening] " Greg Kroah-Hartman
2012-02-17  0:24     ` Greg Kroah-Hartman
2012-02-17  1:06     ` [kernel-hardening] " Kees Cook
2012-02-17  1:06       ` Kees Cook
2012-02-17  1:40       ` [kernel-hardening] " Greg KH
2012-02-17  1:40         ` Greg KH
2012-02-17  2:11         ` [kernel-hardening] Re: [ubuntu-hardened] " Kees Cook
2012-02-17  2:11           ` Kees Cook
2012-02-17  2:48           ` [kernel-hardening] " David Windsor
2012-02-17  2:48             ` David Windsor
2012-02-17  3:32             ` [kernel-hardening] " Greg KH
2012-02-17  3:32               ` Greg KH
2012-02-17  6:33             ` [kernel-hardening] " Alexey Dobriyan
2012-02-17  6:33               ` Alexey Dobriyan
2012-02-17 13:23         ` [kernel-hardening] " pageexec
2012-02-17 13:23           ` pageexec
2012-02-17  7:59     ` Vasiliy Kulikov [this message]
2012-02-17  7:59       ` [kernel-hardening] " Vasiliy Kulikov
2012-02-17 17:53       ` Greg KH
2012-02-17 17:54       ` Greg KH
2012-02-17 19:37         ` Vasiliy Kulikov
2012-02-17 23:39           ` Djalal Harouni
2012-02-18  1:44             ` Roland Dreier
2012-02-18 16:15               ` David Windsor
2012-02-18 16:35                 ` Vasiliy Kulikov
2012-02-18 16:18               ` Greg KH
2012-02-24 17:58                 ` David Windsor
2012-02-24 18:37                   ` Greg KH
2012-02-24 18:52                     ` Kees Cook
2012-02-24 19:05                       ` Nick Bowler
2012-02-24 19:13                         ` Vasiliy Kulikov
2012-02-24 19:35                           ` Nick Bowler
2012-02-24 21:59                           ` PaX Team
2012-02-24 18:58                     ` Vasiliy Kulikov
2012-02-24 19:41                       ` Greg KH
2012-02-24 20:04                         ` Kees Cook
2012-02-24 19:04                     ` David Windsor
2012-02-24 22:14                       ` PaX Team

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