Re: [PATCH v3] cred: Propagate security_prepare_creds() error code

From: Frederick Lawler <fred@cloudflare.com>
To: Eric Biggers <ebiggers@kernel.org>
Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-aio@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-cachefs@redhat.com, linux-cifs@vger.kernel.org,
	samba-technical@lists.samba.org, linux-mm@kvack.org,
	linux-nfs@vger.kernel.org, linux-unionfs@vger.kernel.org,
	linux-security-module@vger.kernel.org, netdev@vger.kernel.org,
	keyrings@vger.kernel.org, selinux@vger.kernel.org,
	serge@hallyn.com, amir73il@gmail.com, kernel-team@cloudflare.com,
	Jeff Moyer <jmoyer@redhat.com>, Paul Moore <paul@paul-moore.com>
Subject: Re: [PATCH v3] cred: Propagate security_prepare_creds() error code
Date: Mon, 13 Jun 2022 08:46:49 -0500	[thread overview]
Message-ID: <b4113083-73de-3ab6-e23f-32c6627d177e@cloudflare.com> (raw)
In-Reply-To: <YqJ/0W3wxPThWqgC@sol.localdomain>

Hi Eric,

On 6/9/22 6:18 PM, Eric Biggers wrote:
> On Wed, Jun 08, 2022 at 10:09:42AM -0500, Frederick Lawler wrote:
>> diff --git a/fs/aio.c b/fs/aio.c
>> index 3c249b938632..5abbe88c3ca7 100644
>> --- a/fs/aio.c
>> +++ b/fs/aio.c
>> @@ -1620,6 +1620,8 @@ static void aio_fsync_work(struct work_struct *work)
>>   static int aio_fsync(struct fsync_iocb *req, const struct iocb *iocb,
>>   		     bool datasync)
>>   {
>> +	int err;
>> +
>>   	if (unlikely(iocb->aio_buf || iocb->aio_offset || iocb->aio_nbytes ||
>>   			iocb->aio_rw_flags))
>>   		return -EINVAL;
>> @@ -1628,8 +1630,11 @@ static int aio_fsync(struct fsync_iocb *req, const struct iocb *iocb,
>>   		return -EINVAL;
>>   
>>   	req->creds = prepare_creds();
>> -	if (!req->creds)
>> -		return -ENOMEM;
>> +	if (IS_ERR(req->creds)) {
>> +		err = PTR_ERR(req->creds);
>> +		req->creds = NULL;
>> +		return err;
>> +	}
> 
> This part is a little ugly.  How about doing:
> 
> 	creds = prepare_creds();
> 	if (IS_ERR(creds))
> 		return PTR_ERR(creds);
> 	req->creds = creds;
> 

I can do that, and same for below.

>> diff --git a/fs/exec.c b/fs/exec.c
>> index 0989fb8472a1..02624783e40e 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1468,15 +1468,19 @@ EXPORT_SYMBOL(finalize_exec);
>>    */
>>   static int prepare_bprm_creds(struct linux_binprm *bprm)
>>   {
>> +	int err = -ERESTARTNOINTR;
>>   	if (mutex_lock_interruptible(&current->signal->cred_guard_mutex))
>> -		return -ERESTARTNOINTR;
>> +		return err;
>>   
>>   	bprm->cred = prepare_exec_creds();
>> -	if (likely(bprm->cred))
>> -		return 0;
>> +	if (IS_ERR(bprm->cred)) {
>> +		err = PTR_ERR(bprm->cred);
>> +		bprm->cred = NULL;
>> +		mutex_unlock(&current->signal->cred_guard_mutex);
>> +		return err;
>> +	}
>>   
>> -	mutex_unlock(&current->signal->cred_guard_mutex);
>> -	return -ENOMEM;
>> +	return 0;
>>   }
> 
> Similarly:
> 
> static int prepare_bprm_creds(struct linux_binprm *bprm)
> {
> 	struct cred *cred;
> 
> 	if (mutex_lock_interruptible(&current->signal->cred_guard_mutex))
> 		return -ERESTARTNOINTR;
> 
> 	cred = prepare_exec_creds();
> 	if (IS_ERR(cred)) {
> 		mutex_unlock(&current->signal->cred_guard_mutex);
> 		return PTR_ERR(cred);
> 	}
> 	bprm->cred = cred;
> 	return 0;
> }
> 
>> diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
>> index eec72ca962e2..6cf75aa83b6c 100644
>> --- a/kernel/nsproxy.c
>> +++ b/kernel/nsproxy.c
>> @@ -311,6 +311,7 @@ static void put_nsset(struct nsset *nsset)
>>   
>>   static int prepare_nsset(unsigned flags, struct nsset *nsset)
>>   {
>> +	int err = -ENOMEM;
>>   	struct task_struct *me = current;
>>   
>>   	nsset->nsproxy = create_new_namespaces(0, me, current_user_ns(), me->fs);
>> @@ -324,6 +325,12 @@ static int prepare_nsset(unsigned flags, struct nsset *nsset)
>>   	if (!nsset->cred)
>>   		goto out;
>>   
>> +	if (IS_ERR(nsset->cred)) {
>> +		err = PTR_ERR(nsset->cred);
>> +		nsset->cred = NULL;
>> +		goto out;
>> +	}
> 
> Why is the NULL check above being kept?
> 

In the branch prior:

	if (flags & CLONE_NEWUSER) {
		nsset->cred = prepare_creds();
	else
		nsset->cred = current_cred();

I don't see cases where others are checking for null after 
current_cred(), therefore I can remove that check.

> Also, drivers/crypto/ccp/sev-dev.c needs to be updated.
> 

Nice catch! I clearly missed addition after the merge window.

> - Eric