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Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace
 cred_guard_mutex
From:   Bernd Edlinger <bernd.edlinger@hotmail.de>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Yuyang Du <duyuyang@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
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        Shakeel Butt <shakeelb@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Christian Kellner <christian@kellner.me>,
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On 3/14/20 10:57 AM, Bernd Edlinger wrote:
> On 3/13/20 10:13 AM, Kirill Tkhai wrote:
>>
>> Despite this should fix the problem, this looks like a broken puzzle.
>>
>> We can't use bprm->cred as an identifier whether the mutex was locked or not.
>> We can check for bprm->cred in regard to cred_guard_mutex, because of there is
>> strong rule: "cred_guard_mutex is becomes locked together with bprm->cred assignment
>> (see prepare_bprm_creds()), and it becomes unlocked together with bprm->cred zeroing".
>> Take attention on modularity of all this: there is no dependencies between anything else.
>>
>> In regard to newly introduced exec_update_mutex, your fix and source patch way look like
>> an obfuscation. The mutex becomes deadly glued to unrelated bprm->cred and bprm->mm,
>> and this introduces the problems in the future modifications and support of all involved
>> entities. If someone wants to move some functions in relation to each other, there will
>> be a pain, and this person will have to go again the same dependencies and bug way,
>> Eric stepped on in the original patch.
>>
> 
> Okay, yes, valid points you make, thanks.
> I just wanted to understand what was exactly wrong with this patch,
> since the failure mode looked a lot like it was failing because of
> something clobbering the data unexpectedly.
> 
> 
> So I have posted a few updated patch for the failed one here:
> 
> [PATCH v3 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex
> [PATCH] pidfd: Use new infrastructure to fix deadlocks in execve
> 
> which replaces these:
> [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex
> https://lore.kernel.org/lkml/87zhcq4jdj.fsf_-_@x220.int.ebiederm.org/
> 
> [PATCH] pidfd: Stop taking cred_guard_mutex 
> https://lore.kernel.org/lkml/87wo7svy96.fsf_-_@x220.int.ebiederm.org/
> 
> 
> and a new patch series to fix deadlock in ptrace_attach and update doc:
> [PATCH 0/2] exec: Fix dead-lock in de_thread with ptrace_attach
> [PATCH 1/2] exec: Fix dead-lock in de_thread with ptrace_attach
> [PATCH 2/2] doc: Update documentation of ->exec_*_mutex
> 
> 
> Other patches needed, still valid:
> 
> [PATCH v2 1/5] exec: Only compute current once in flush_old_exec
> https://lore.kernel.org/lkml/87pndm5y3l.fsf_-_@x220.int.ebiederm.org/
> 
> [PATCH v2 2/5] exec: Factor unshare_sighand out of de_thread and call it separately
> https://lore.kernel.org/lkml/87k13u5y26.fsf_-_@x220.int.ebiederm.org/
> 

Ah, sorry, forgot this one:
[PATCH v2 3/5] exec: Move cleanup of posix timers on exec out of de_thread
https://lore.kernel.org/lkml/87eeu25y14.fsf_-_@x220.int.ebiederm.org/

> [PATCH v2 4/5] exec: Move exec_mmap right after de_thread in flush_old_exec
> https://lore.kernel.org/lkml/875zfe5xzb.fsf_-_@x220.int.ebiederm.org/
> 
> [PATCH 1/4] exec: Fix a deadlock in ptrace
> https://lore.kernel.org/lkml/AM6PR03MB517033EAD25BED15CC84E17DE4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/
> 
> [PATCH 2/4] selftests/ptrace: add test cases for dead-locks
> https://lore.kernel.org/lkml/AM6PR03MB51703199741A2C27A78980FFE4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/
> 
> [PATCH 3/4] mm: docs: Fix a comment in process_vm_rw_core
> https://lore.kernel.org/lkml/AM6PR03MB5170ED6D4D216EEEEF400136E4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/
> 
> [PATCH 4/4] kernel: doc: remove outdated comment cred.c
> https://lore.kernel.org/lkml/AM6PR03MB517039DB07AB641C194FEA57E4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/
> 
> [PATCH 1/4] kernel/kcmp.c: Use new infrastructure to fix deadlocks in execve
> https://lore.kernel.org/lkml/AM6PR03MB517057A2269C3A4FB287B76EE4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/
> 
> [PATCH 2/4] proc: Use new infrastructure to fix deadlocks in execve
> https://lore.kernel.org/lkml/AM6PR03MB51705D211EC8E7EA270627B1E4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/
> 
> [PATCH 3/4] proc: io_accounting: Use new infrastructure to fix deadlocks in execve
> https://lore.kernel.org/lkml/AM6PR03MB5170BD2476E35068E182EFA4E4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/
> 
> [PATCH 4/4] perf: Use new infrastructure to fix deadlocks in execve
> https://lore.kernel.org/lkml/AM6PR03MB517035DEEDB9C8699CB6B34EE4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com/
> 
> 
> I think most of the existing patches are already approved, but if
> there are still change requests, please let me know.
> 
> 
> Thanks
> Bernd.
> 

Hope it is correct now.
I haven't seen the new patches on the kernel archives yet,
so I cannot add URLs for them.

Bernd.