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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Jann Horn <jannh@google.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Adam Zabrocki <pi3@pi3.com.pl>,
        kernel list <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Bernd Edlinger <bernd.edlinger@hotmail.de>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        stable <stable@vger.kernel.org>
References: <20200324215049.GA3710@pi3.com.pl> <202003291528.730A329@keescook>
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        <CAG48ez1dCPw9Dep-+GWn=SnHv1nVv4Npv1FpFxmomk6tmazB-g@mail.gmail.com>
Date:   Thu, 02 Apr 2020 09:14:38 -0500
In-Reply-To: <CAG48ez1dCPw9Dep-+GWn=SnHv1nVv4Npv1FpFxmomk6tmazB-g@mail.gmail.com>
        (Jann Horn's message of "Thu, 2 Apr 2020 06:46:49 +0200")
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Subject: Re: [PATCH] signal: Extend exec_id to 64bits
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Jann Horn <jannh@google.com> writes:

> On Wed, Apr 1, 2020 at 10:50 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>> Replace the 32bit exec_id with a 64bit exec_id to make it impossible
>> to wrap the exec_id counter.  With care an attacker can cause exec_id
>> wrap and send arbitrary signals to a newly exec'd parent.  This
>> bypasses the signal sending checks if the parent changes their
>> credentials during exec.
>>
>> The severity of this problem can been seen that in my limited testing
>> of a 32bit exec_id it can take as little as 19s to exec 65536 times.
>> Which means that it can take as little as 14 days to wrap a 32bit
>> exec_id.  Adam Zabrocki has succeeded wrapping the self_exe_id in 7
>> days.  Even my slower timing is in the uptime of a typical server.
>
> FYI, if you actually optimize this, it's more like 12s to exec 1048576
> times according to my test, which means ~14 hours for 2^32 executions
> (on a single core). That's on an i7-4790 (a Haswell desktop processor
> that was launched about six years ago, in 2014).

Half a day.  I am not at all surprised, but it is good to know it can
take so little time.

Eric