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

Waiman Long
2021-12-21T16:41:36Z

On 12/21/21 10:55, Eric W. Biederman wrote:
> Waiman Long <longman@redhat.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