linux-kernel - Re: [PATCH] exec: Make suid_dumpable apply to SUID/SGID binaries irrespective of invoking users

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Message-ID: <4f67dc4c-7038-7dde-cad9-4feeaa6bc71b@redhat.com>
Date:   Tue, 21 Dec 2021 11:41:27 -0500
From:   Waiman Long <longman@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Laurent Vivier <laurent@...ier.eu>,
        YunQiang Su <ysu@...ecomp.com>, Helge Deller <deller@....de>
Subject: Re: [PATCH] exec: Make suid_dumpable apply to SUID/SGID binaries
 irrespective of invoking users


On 12/21/21 10:55, Eric W. Biederman wrote:
> Waiman Long <longman@...hat.com> writes:
>
>> The begin_new_exec() function checks for SUID or SGID binaries by
>> comparing effective uid and gid against real uid and gid and using
>> the suid_dumpable sysctl parameter setting only if either one of them
>> differs.
>>
>> In the special case that the uid and/or gid of the SUID/SGID binaries
>> matches the id's of the user invoking it, the suid_dumpable is not
>> used and SUID_DUMP_USER will be used instead. The documentation for the
>> suid_dumpable sysctl parameter does not include that exception and so
>> this will be an undocumented behavior.
>>
>> Eliminate this undocumented behavior by adding a flag in the linux_binprm
>> structure to designate a SUID/SGID binary and use it for determining
>> if the suid_dumpable setting should be applied or not.
> I see that you are making the code match the documentation.
> What harm/problems does this mismatch cause in practice?
> What is the motivation for this change?
>
> I am trying to see the motivation but all I can see is that
> in the case where suid and sgid do nothing in practice the code
> does not change dumpable.  The point of dumpable is to refuse to
> core dump when it is not safe.  In this case since nothing happened
> in practice it is safe.
>
> So how does this matter in practice.  If there isn't a good
> motivation my feel is that it is the documentation that needs to be
> updated rather than the code.
>
> There are a lot of warts to the suid/sgid handling during exec.  This
> just doesn't look like one of them

This patch is a minor mitigation in response to the security 
vulnerability as posted in 
https://www.openwall.com/lists/oss-security/2021/10/20/2 (aka 
CVE-2021-3864). In particular, the Su PoC (tested on CentOS 7) showing 
that the su invokes /usr/sbin/unix_chkpwd which is also a SUID binary. 
The initial su invocation won't generate a core dump because the real 
uid and euid differs, but the second unix_chkpwd invocation will. This 
patch eliminates this hole by making sure that all SUID binaries follow 
suid_dumpable setting.

Cheers,
Longman