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Date: Thu, 02 Apr 2020 09:14:38 -0500
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Jann Horn <jannh@...gle.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Adam Zabrocki <pi3@....com.pl>, kernel list <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Oleg Nesterov <oleg@...hat.com>, Andy Lutomirski <luto@...capital.net>, Bernd Edlinger <bernd.edlinger@...mail.de>, Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>, stable <stable@...r.kernel.org>
Subject: Re: [PATCH] signal: Extend exec_id to 64bits
Jann Horn <jannh@...gle.com> writes:
> On Wed, Apr 1, 2020 at 10:50 PM Eric W. Biederman <ebiederm@...ssion.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
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