Date:   Thu, 12 Aug 2021 14:38:59 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
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        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Kees Cook <keescook@...omium.org>,
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        Greg Ungerer <gerg@...ux-m68k.org>,
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        Marco Elver <elver@...gle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Nicolas Viennot <Nicolas.Viennot@...sigma.com>,
        Thomas Cedeno <thomascedeno@...gle.com>,
        Collin Fijalkovich <cfijalkovich@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Chengguang Xu <cgxu519@...ernel.net>,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        linux-unionfs@...r.kernel.org, linux-api@...r.kernel.org,
        x86@...nel.org, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Andrei Vagin <avagin@...il.com>
Subject: Re: [PATCH v1 3/7] kernel/fork: always deny write access to current
 MM exe_file

On 12.08.21 14:32, Christian Brauner wrote:
> On Thu, Aug 12, 2021 at 12:13:44PM +0200, David Hildenbrand wrote:
>> On 12.08.21 12:05, Christian Brauner wrote:
>>> [+Cc Andrei]
>>>
>>> On Thu, Aug 12, 2021 at 10:43:44AM +0200, David Hildenbrand wrote:
>>>> We want to remove VM_DENYWRITE only currently only used when mapping the
>>>> executable during exec. During exec, we already deny_write_access() the
>>>> executable, however, after exec completes the VMAs mapped
>>>> with VM_DENYWRITE effectively keeps write access denied via
>>>> deny_write_access().
>>>>
>>>> Let's deny write access when setting the MM exe_file. With this change, we
>>>> can remove VM_DENYWRITE for mapping executables.
>>>>
>>>> This represents a minor user space visible change:
>>>> sys_prctl(PR_SET_MM_EXE_FILE) can now fail if the file is already
>>>> opened writable. Also, after sys_prctl(PR_SET_MM_EXE_FILE), the file
>>>
>>> Just for completeness, this also affects PR_SET_MM_MAP when exe_fd is
>>> set.
>>
>> Correct.
>>
>>>
>>>> cannot be opened writable. Note that we can already fail with -EACCES if
>>>> the file doesn't have execute permissions.
>>>>
>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>>> ---
>>>
>>> The biggest user I know and that I'm involved in is CRIU which heavily
>>> uses PR_SET_MM_MAP (with a fallback to PR_SET_MM_EXE_FILE on older
>>> kernels) during restore. Afair, criu opens the exe fd as an O_PATH
>>> during dump and thus will use the same flag during restore when
>>> opening it. So that should be fine.
>>
>> Yes.
>>
>>>
>>> However, if I understand the consequences of this change correctly, a
>>> problem could be restoring workloads that hold a writable fd open to
>>> their exe file at dump time which would mean that during restore that fd
>>> would be reopened writable causing CRIU to fail when setting the exe
>>> file for the task to be restored.
>>
>> If it's their exe file, then the existing VM_DENYWRITE handling would have
>> forbidden these workloads to open the fd of their exe file writable, right?
> 
> Yes.
> 
>> At least before doing any PR_SET_MM_MAP/PR_SET_MM_EXE_FILE. But that should
>> rule out quite a lot of cases we might be worried about, right?
> 
> Yes, it rules out the most obvious cases. The problem is really just
> that we don't know how common weirder cases are. But that doesn't mean
> we shouldn't try and risk it. This is a nice cleanup and playing
> /proc/self/exe games isn't super common.
> 

Right, and having the file your executing opened writable isn't 
something very common as well.

If we really run into problems, we could not protect the new file when 
issuing PR_SET_MM_MAP/PR_SET_MM_EXE_FILE. But I'd like to avoid that, if 
possible, because it feels like working around something that never 
should have worked that way and is quite inconsistent.

-- 
Thanks,

David / dhildenb

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