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From:   Bernd Edlinger <bernd.edlinger@hotmail.de>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
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Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace
 cred_guard_mutex
Thread-Topic: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace
 cred_guard_mutex
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Date:   Mon, 9 Mar 2020 19:24:58 +0000
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On 3/9/20 8:02 PM, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
> 
>> On 3/9/20 7:36 PM, Eric W. Biederman wrote:
>>>
>>>
>>> Does that sound better?
>>>
>>
>> almost done.
> 
> I think this text is finally clean.
> 
>     exec: Add exec_update_mutex to replace cred_guard_mutex
>     
>     The cred_guard_mutex is problematic as it is held over possibly
>     indefinite waits for userspace.  The possilbe indefinite waits for
>     userspace that I have identified are: The cred_guard_mutex is held in
>     PTRACE_EVENT_EXIT waiting for the tracer.  The cred_guard_mutex is
>     held over "put_user(0, tsk->clear_child_tid)" in exit_mm().  The
>     cred_guard_mutex is held over "get_user(futex_offset, ...")  in
>     exit_robust_list.  The cred_guard_mutex held over copy_strings.
>     
>     The functions get_user and put_user can trigger a page fault which can
>     potentially wait indefinitely in the case of userfaultfd or if
>     userspace implements part of the page fault path.
>     
>     In any of those cases the userspace process that the kernel is waiting
>     for might make a different system call that winds up taking the
>     cred_guard_mutex and result in deadlock.
>     
>     Holding a mutex over any of those possibly indefinite waits for
>     userspace does not appear necessary.  Add exec_update_mutex that will
>     just cover updating the process during exec where the permissions and
>     the objects pointed to by the task struct may be out of sync.
>     
>     The plan is to switch the users of cred_guard_mutex to
>     exec_update_mutex one by one.  This lets us move forward while still
>     being careful and not introducing any regressions.
>     
>     Link: https://lore.kernel.org/lkml/20160921152946.GA24210@dhcp22.suse.cz/
>     Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@AM6PR03MB5170.eurprd03.prod.outlook.com/
>     Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@redhat.com/
>     Link: https://lore.kernel.org/lkml/20160923095031.GA14923@redhat.com/
>     Link: https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/
>     Ref: 45c1a159b85b ("Add PTRACE_O_TRACEVFORKDONE and PTRACE_O_TRACEEXIT facilities.")
>     Ref: 456f17cd1a28 ("[PATCH] user-vm-unlock-2.5.31-A2")

I checked the urls they all work.
Just one last question, are these git references?
I can't find them in my linux git tree (cloned from linus' git)?

Sorry for being pedantically.


>     Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> 
> 
> Bernd do you want to give me your Reviewed-by for this part of the
> series?
> 

Sure also the other parts of course.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>

> After that do you think you can write the obvious patch for mm_access?
> 

Yes, I can do that.
I also have some typos in comments, will make them extra patches as well.

I wonder if the test case is okay to include the ptrace_attach altough
that is not yet passing?


Thanks
Bernd.