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Subject: Re: [RFC v5 1/1] ns: add binfmt_misc to the user namespace
From:   Laurent Vivier <laurent@vivier.eu>
To:     Tycho Andersen <tycho@tycho.ws>
Cc:     linux-kernel@vger.kernel.org, Dmitry Safonov <dima@arista.com>,
        linux-api@vger.kernel.org, containers@lists.linux-foundation.org,
        Jann Horn <jannh@google.com>,
        James Bottomley <James.Bottomley@HansenPartnership.com>,
        Eric Biederman <ebiederm@xmission.com>,
        linux-fsdevel@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>
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Message-ID: <485d1157-9d3e-b4e0-4ece-4e80b2f252c4@vivier.eu>
Date:   Wed, 10 Oct 2018 12:11:34 +0200
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Sender: linux-kernel-owner@vger.kernel.org
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On 09/10/2018 17:19, Laurent Vivier wrote:
> Le 09/10/2018 à 17:16, Tycho Andersen a écrit :
>> On Tue, Oct 09, 2018 at 12:37:52PM +0200, Laurent Vivier wrote:
>>> @@ -80,18 +74,32 @@ static int entry_count;
>>>   */
>>>  #define MAX_REGISTER_LENGTH 1920
>>>  
>>> +static struct binfmt_namespace *binfmt_ns(struct user_namespace *ns)
>>> +{
>>> +	struct binfmt_namespace *b_ns;
>>> +
>>> +	while (ns) {
>>> +		b_ns = READ_ONCE(ns->binfmt_ns);
>>> +		if (b_ns)
>>> +			return b_ns;
>>> +		ns = ns->parent;
>>> +	}
>>> +	WARN_ON_ONCE(1);
>>
>> It looks like we warn here,
>>
>>> @@ -133,17 +141,18 @@ static int load_misc_binary(struct linux_binprm *bprm)
>>>  	struct file *interp_file = NULL;
>>>  	int retval;
>>>  	int fd_binary = -1;
>>> +	struct binfmt_namespace *ns = binfmt_ns(current_user_ns());
>>>  
>>>  	retval = -ENOEXEC;
>>> -	if (!enabled)
>>> +	if (!ns->enabled)
>>
>> ...but then in cases like this we immediately dereference the pointer
>> anyways and crash. Can we return some other error code here in the !ns
>> case so we don't crash?
> 
> My concern here is I don't want to add code to check an error case that
> cannot happen. The first namespace binfmt_ns pointer is initialized with
> &init_binfmt_ns, so the return value cannot be NULL.

Perhaps it could be reasonable to return &init_binfmt_ns rather than
NULL in this case?

Thanks,
Laurent