Re: [RFC][PATCH] exec: Move cred computation under exec_update_lock

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

On 11/21/25 08:18, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
> 
>> Hi Eric,
>>
>> thanks for you valuable input on the topic.
>>
>> On 11/21/25 00:50, Eric W. Biederman wrote:
>>> "Eric W. Biederman" <ebiederm@xmission.com> writes:
>>>
>>>> Instead of computing the new cred before we pass the point of no
>>>> return compute the new cred just before we use it.
>>>>
>>>> This allows the removal of fs_struct->in_exec and cred_guard_mutex.
>>>>
>>>> I am not certain why we wanted to compute the cred for the new
>>>> executable so early.  Perhaps I missed something but I did not see any
>>>> common errors being signaled.   So I don't think we loose anything by
>>>> computing the new cred later.
>>>
>>> I should add that the permission checks happen in open_exec,
>>> everything that follows credential wise is just about representing in
>>> struct cred the credentials the new executable will have.
>>>
>>> So I am really at a loss why we have had this complicated way of
>>> computing of computed the credentials all of these years full of
>>> time of check to time of use problems.
>>>
>>
>> Well, I think I see a problem with your patch:
>>
>> When the security engine gets the LSM_UNSAFE_PTRACE flag, it might
>> e.g. return -EPERM in bprm_creds_for_exec in the apparmor, selinux
>> or the smack security engines at least.  Previously that callback
>> was called before the point of no return, and the return code should
>> be returned as a return code the the caller of execve.  But if we move
>> that check after the point of no return, the caller will get killed
>> due to the failed security check.
>>
>> Or did I miss something?
> 
> I think we definitely need to document this change in behavior.  I would
> call ending the exec with SIGSEGV vs -EPERM a quality of implementation
> issue.  The exec is failing one way or the other so I don't see it as a
> correctness issue.
> 
> In the case of ptrace in general I think it is a bug if the mere act of
> debugging a program changes it's behavior.  So which buggy behavior
> should we prefer?  SIGSEGV where it is totally clear that the behavior
> has changed or -EPERM and ask the debugged program to handle it.
> I lean towards SIGSEGV because then it is clear the code should not
> handle it.
> 
> In the case of LSM_UNSAFE_NO_NEW_PRIVS I believe the preferred way to
> handle unexpected things happening is to terminate the application.
> 
> In the case of LSM_UNSAFE_SHARE -EPERM might be better.  I don't know
> of any good uses of any good uses of sys_clone(CLONE_FS ...) outside
> of CLONE_THREAD.
> 
> 
> Plus all of these things are only considerations if we are exec'ing a
> program that transitions to a different set of credentials.  Something
> that happens but is quite rare itself.
> 
> In practice I don't expect there is anything that depends on the exact
> behavior of what happens when exec'ing a suid executable to gain
> privileges when ptraced.   The closes I can imagine is upstart and
> I think upstart ran as root when ptracing other programs so there is no
> gaining of privilege and thus no reason for a security module to
> complain.
> 
> Who knows I could be wrong, and someone could actually care.  Which is> hy I think we should document it.
> 


Well, I dont know for sure, but the security engine could deny the execution
for any reason, not only because of being ptraced.
Maybe there can be a policy which denies user X to execute e.g. any suid programs.


Bernd.