linux-kernel - Re: [PATCH v3 2/2] Protected O_CREAT open in sticky directories

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Date:   Fri, 24 Nov 2017 12:43:47 +0100
From:   Salvatore Mesoraca <s.mesoraca16@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Solar Designer <solar@...nwall.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH v3 2/2] Protected O_CREAT open in sticky directories

2017-11-24 11:53 GMT+01:00 David Laight <David.Laight@...lab.com>:
> From: Alan Cox
>> Sent: 22 November 2017 16:52
>>
>> On Wed, 22 Nov 2017 09:01:46 +0100
>> Salvatore Mesoraca <s.mesoraca16@...il.com> wrote:
>>
>> > Disallows O_CREAT open missing the O_EXCL flag, in world or
>> > group writable directories, even if the file doesn't exist yet.
>> > With few exceptions (e.g. shared lock files based on flock())
>>
>> Enough exceptions to make it a bad idea.
>>
>> Firstly if you care this much *stop* having shared writable directories.
>> We have namespaces, you don't need them. You can give every user their
>> own /tmp etc.
>
> Looks like a very bad idea to me as well.
>
> Doesn't this stop all shell redirects into a shared /tmp ?
> I'm pretty sure most programs use O_CREAT | O_TRUNC for output
> files - they'll all stop working.

If some program does such a thing, that's a potential vulnerability.
With "protected_hardlinks" you are, in most cases, safe.
But, still, that program has a bug and having this feature enabled will
help you notice it soon.
For that matter, I'm using this patch on my system and I don't have any
program behaving like this.

> If there are some directories where you need to force O_EXCL you
> need to make it a property of the directory, not the kernel.
>
>         David
>