linux-kernel - Re: [PATCH v17 08/10] PM: hibernate: disable when there are active secretmem users

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Date:   Mon, 8 Feb 2021 12:26:31 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, Will Deacon <will@...nel.org>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-riscv@...ts.infradead.org,
        x86@...nel.org, Hagen Paul Pfeifer <hagen@...u.net>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>
Subject: Re: [PATCH v17 08/10] PM: hibernate: disable when there are active
 secretmem users

On 08.02.21 12:14, David Hildenbrand wrote:
> On 08.02.21 12:13, David Hildenbrand wrote:
>> On 08.02.21 11:57, Michal Hocko wrote:
>>> On Mon 08-02-21 11:53:58, David Hildenbrand wrote:
>>>> On 08.02.21 11:51, Michal Hocko wrote:
>>>>> On Mon 08-02-21 11:32:11, David Hildenbrand wrote:
>>>>>> On 08.02.21 11:18, Michal Hocko wrote:
>>>>>>> On Mon 08-02-21 10:49:18, Mike Rapoport wrote:
>>>>>>>> From: Mike Rapoport <rppt@...ux.ibm.com>
>>>>>>>>
>>>>>>>> It is unsafe to allow saving of secretmem areas to the hibernation
>>>>>>>> snapshot as they would be visible after the resume and this essentially
>>>>>>>> will defeat the purpose of secret memory mappings.
>>>>>>>>
>>>>>>>> Prevent hibernation whenever there are active secret memory users.
>>>>>>>
>>>>>>> Does this feature need any special handling? As it is effectivelly
>>>>>>> unevictable memory then it should behave the same as other mlock, ramfs
>>>>>>> which should already disable hibernation as those cannot be swapped out,
>>>>>>> no?
>>>>>>>
>>>>>>
>>>>>> Why should unevictable memory not go to swap when hibernating? We're merely
>>>>>> dumping all of our system RAM (including any unmovable allocations) to swap
>>>>>> storage and the system is essentially completely halted.
>>>>>>
>>>>> My understanding is that mlock is never really made visible via swap
>>>>> storage.
>>>>
>>>> "Using swap storage for hibernation" and "swapping at runtime" are two
>>>> different things. I might be wrong, though.
>>>
>>> Well, mlock is certainly used to keep sensitive information, not only to
>>> protect from major/minor faults.
>>>
>>
>> I think you're right in theory, the man page mentions "Cryptographic
>> security software often handles critical bytes like passwords or secret
>> keys as data structures" ...
>>
>> however, I am not aware of any such swap handling and wasn't able to
>> spot it quickly. Let me take a closer look.
> 
> s/swap/hibernate/

My F33 system happily hibernates to disk, even with an application that 
succeeded in din doing an mlockall().

And it somewhat makes sense. Even my freshly-booted, idle F33 has

$ cat /proc/meminfo  | grep lock
Mlocked:            4860 kB

So, stopping to hibernate with mlocked memory would essentially prohibit 
any modern Linux distro to hibernate ever.

-- 
Thanks,

David / dhildenb