Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1

[Bernd Edlinger]

On 4/30/20 6:40 PM, Linus Torvalds wrote:
> On Thu, Apr 30, 2020 at 7:29 AM Bernd Edlinger
> <bernd.edlinger@xxxxxxxxxx> wrote:
>>
>> Ah, now I see, that was of course not the intended effect,
>> but that is not where the pseudo-deadlock happens at all,
>> would returning -RESTARTNOINTR in this function make this
>> patch acceptable, it will not have an effect on the test case?
> 
> So that was why I suggested doing it all with a helper function, and
> also doing that
> 
>         set_thread_flag(TIF_SIGPENDING);
> 
> because without going through the "check-for-signals" code at return
> to user space, -ERESTARTNOINTR doesn't actually _do_ any restart.
> 
> However, the more I looked at it, the less I actually liked that hack.
> 
> Part of it is simply because it can cause the exact same problem that
> ptrace() does (at least in theory). And even if you don't get the
> livelock thing, you can get the "use 100% CPU time" thing, because if
> that case ever triggers, and we re-try, it will generally just _keep_
> on triggering (think "execve is waiting for a zombie, nobody is
> reaping it").
> 
> IOW, restarting doesn't really fix the problem, or guarantee any
> forward progress.
> 

Right, if it is a real time process it will result in priority-inversion.
Correct.

If it is a virus checker it will be real time priority and it will be
very aggressive ;-) I can feel its aggressiveness already :-) shiver...

And this little zombie-process will paralyze it immediately, nice try.

You see what I mean?

> So I'd have been ok with your "unsafe_exec_flag" if
> 
>  (a) it had been done in one place with a helper function.
> 
>  (b) it would _only_ trigger for ptrace (and perhaps seccomp).
> 
> but I don't think it works for that write() case.
> 
> That said, I'm not 100% convinced that that write() case really even
> needs that cred_guard_mutex (renamed or not).
> 
> Maybe we can introduce a new mutex just against concurrent ptrace
> (which is what at least the _comment_ says_ that
> security_setprocattr() wants - I didn't check the actual low-level
> security code).
> 
> So maybe that proc_pid_attr_write() case could be done some other way entirely.
> 
> Th emore we go through all this, the more I really think that Oleg's
> patch to just delay the waiting for things until after dropping the
> mutex in execve() is the way to go.
> 
> Is it a "simple" and small patch? No. But it really addresses the core
> issue, without introducing new odd rules or special cases, or making a
> lock that doesn't reliably work as a lock.
> 

Hmm.  I think I can agree, that this problem deserves to be solved
really slowly.

Oleg where was your last patch, does it still work or does it
need to be re-based?

And I almost forgot about Eric, are you still with us?


Thanks
Bernd.