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Subject: Re: [PATCH v1 3/7] kernel/fork: always deny write access to current
 MM exe_file
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Kees Cook <keescook@chromium.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Greg Ungerer <gerg@linux-m68k.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Mike Rapoport <rppt@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Chinwen Chang <chinwen.chang@mediatek.com>,
        Michel Lespinasse <walken@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Huang Ying <ying.huang@intel.com>,
        Jann Horn <jannh@google.com>, Feng Tang <feng.tang@intel.com>,
        Kevin Brodsky <Kevin.Brodsky@arm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Shawn Anastasio <shawn@anastas.io>,
        Steven Price <steven.price@arm.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Jens Axboe <axboe@kernel.dk>,
        Gabriel Krisman Bertazi <krisman@collabora.com>,
        Peter Xu <peterx@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Marco Elver <elver@google.com>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        Nicolas Viennot <Nicolas.Viennot@twosigma.com>,
        Thomas Cedeno <thomascedeno@google.com>,
        Collin Fijalkovich <cfijalkovich@google.com>,
        Michal Hocko <mhocko@suse.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Chengguang Xu <cgxu519@mykernel.net>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>,
        linux-unionfs@vger.kernel.org, linux-api@vger.kernel.org,
        x86@kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        Andrei Vagin <avagin@gmail.com>
References: <20210812084348.6521-1-david@redhat.com>
 <20210812084348.6521-4-david@redhat.com>
 <20210812100544.uhsfp75b4jcrv3qx@wittgenstein>
 <1b6d27cf-2238-0c1c-c563-b38728fbabc2@redhat.com>
 <20210812123239.trksnm57owzwzokj@wittgenstein>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <59b6be94-c7d0-0535-1348-4e9c7f188d20@redhat.com>
Date:   Thu, 12 Aug 2021 14:38:59 +0200
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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@redhat.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