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