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

[Frederick Lawler <fred@xxxxxxxxxxxxxx>]

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