Date:   Mon, 23 May 2022 13:06:58 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Frederick Lawler <fred@...udflare.com>
Cc:     linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-cachefs@...hat.com,
        linux-cifs@...r.kernel.org, samba-technical@...ts.samba.org,
        linux-mm@...ck.org, linux-nfs@...r.kernel.org,
        linux-unionfs@...r.kernel.org,
        linux-security-module@...r.kernel.org, netdev@...r.kernel.org,
        keyrings@...r.kernel.org, selinux@...r.kernel.org,
        kernel-team@...udflare.com
Subject: Re: [PATCH] cred: Propagate security_prepare_creds() error code

On Fri, May 20, 2022 at 04:27:46PM -0500, Frederick Lawler wrote:
> While experimenting with the security_prepare_creds() LSM hook, we
> noticed that our EPERM error code was not propagated up the callstack.
> Instead ENOMEM is always returned.  As a result, some tools may send a
> confusing error message to the user:
> 
> $ unshare -rU
> unshare: unshare failed: Cannot allocate memory
> 
> A user would think that the system didn't have enough memory, when
> instead the action was denied.
> 
> This problem occurs because prepare_creds() and prepare_kernel_cred()
> return NULL when security_prepare_creds() returns an error code. Later,
> functions calling prepare_creds() and prepare_kernel_cred() return
> ENOMEM because they assume that a NULL meant there was no memory
> allocated.
> 
> Fix this by propagating an error code from security_prepare_creds() up
> the callstack.
> 
> Signed-off-by: Frederick Lawler <fred@...udflare.com>

This looks good.  I do have one fiddly request below, but

Reviewed-by: Serge Hallyn <serge@...lyn.com>

thanks,
-serge

> ---
>  fs/aio.c                               |  4 +--
>  fs/cachefiles/security.c               |  8 ++---
>  fs/cifs/cifs_spnego.c                  |  4 +--
>  fs/cifs/cifsacl.c                      |  4 +--
>  fs/coredump.c                          |  2 +-
>  fs/exec.c                              | 14 ++++++---
>  fs/ksmbd/smb_common.c                  |  4 +--
>  fs/nfs/flexfilelayout/flexfilelayout.c |  7 +++--
>  fs/nfs/nfs4idmap.c                     |  4 +--
>  fs/nfsd/auth.c                         |  4 +--
>  fs/nfsd/nfs4callback.c                 | 10 +++---
>  fs/nfsd/nfs4recover.c                  |  4 +--
>  fs/nfsd/nfsfh.c                        |  4 +--
>  fs/open.c                              | 10 +++---
>  fs/overlayfs/dir.c                     | 42 ++++++++++++++------------
>  fs/overlayfs/super.c                   |  5 +--
>  kernel/capability.c                    |  4 +--
>  kernel/cred.c                          | 28 +++++++++--------
>  kernel/groups.c                        |  4 +--
>  kernel/nsproxy.c                       | 18 ++++++++---
>  kernel/sys.c                           | 28 ++++++++---------
>  kernel/trace/trace_events_user.c       |  5 ++-
>  kernel/umh.c                           |  5 +--
>  kernel/user_namespace.c                | 17 ++++++-----
>  net/dns_resolver/dns_key.c             |  4 +--
>  security/apparmor/task.c               | 14 ++++-----
>  security/commoncap.c                   | 20 ++++++------
>  security/keys/keyctl.c                 |  8 ++---
>  security/keys/process_keys.c           | 16 +++++-----
>  security/landlock/syscalls.c           |  4 +--
>  security/selinux/hooks.c               |  8 ++---
>  security/smack/smack_lsm.c             |  8 ++---
>  security/smack/smackfs.c               |  4 +--
>  33 files changed, 175 insertions(+), 150 deletions(-)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 3c249b938632..84f52b8b5aae 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1628,8 +1628,8 @@ 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))
> +		return PTR_ERR(req->creds);
>  
>  	req->datasync = datasync;
>  	INIT_WORK(&req->work, aio_fsync_work);
> diff --git a/fs/cachefiles/security.c b/fs/cachefiles/security.c
> index fe777164f1d8..8dc256b18312 100644
> --- a/fs/cachefiles/security.c
> +++ b/fs/cachefiles/security.c
> @@ -21,8 +21,8 @@ int cachefiles_get_security_ID(struct cachefiles_cache *cache)
>  	_enter("{%s}", cache->secctx);
>  
>  	new = prepare_kernel_cred(current);
> -	if (!new) {
> -		ret = -ENOMEM;
> +	if (IS_ERR(new)) {
> +		ret = PTR_ERR(new);
>  		goto error;
>  	}
>  
> @@ -84,8 +84,8 @@ int cachefiles_determine_cache_security(struct cachefiles_cache *cache,
>  	/* duplicate the cache creds for COW (the override is currently in
>  	 * force, so we can use prepare_creds() to do this) */
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	cachefiles_end_secure(cache, *_saved_cred);
>  
> diff --git a/fs/cifs/cifs_spnego.c b/fs/cifs/cifs_spnego.c
> index 342717bf1dc2..0a5b8157387a 100644
> --- a/fs/cifs/cifs_spnego.c
> +++ b/fs/cifs/cifs_spnego.c
> @@ -190,8 +190,8 @@ init_cifs_spnego(void)
>  	 */
>  
>  	cred = prepare_kernel_cred(NULL);
> -	if (!cred)
> -		return -ENOMEM;
> +	if (IS_ERR(cred))
> +		return PTR_ERR(cred);
>  
>  	keyring = keyring_alloc(".cifs_spnego",
>  				GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, cred,
> diff --git a/fs/cifs/cifsacl.c b/fs/cifs/cifsacl.c
> index bf861fef2f0c..1debcfa927d1 100644
> --- a/fs/cifs/cifsacl.c
> +++ b/fs/cifs/cifsacl.c
> @@ -466,8 +466,8 @@ init_cifs_idmap(void)
>  	 * with add_key().
>  	 */
>  	cred = prepare_kernel_cred(NULL);
> -	if (!cred)
> -		return -ENOMEM;
> +	if (IS_ERR(cred))
> +		return PTR_ERR(cred);
>  
>  	keyring = keyring_alloc(".cifs_idmap",
>  				GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, cred,
> diff --git a/fs/coredump.c b/fs/coredump.c
> index ebc43f960b64..ea4ccae6368a 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -546,7 +546,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
>  		goto fail;
>  
>  	cred = prepare_creds();
> -	if (!cred)
> +	if (IS_ERR(cred))
>  		goto fail;
>  	/*
>  	 * We cannot trust fsuid as being the "true" uid of the process
> diff --git a/fs/exec.c b/fs/exec.c
> index e3e55d5e0be1..a9139f31eb8f 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1466,15 +1466,19 @@ 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;
>  }
>  
>  static void free_bprm(struct linux_binprm *bprm)
> diff --git a/fs/ksmbd/smb_common.c b/fs/ksmbd/smb_common.c
> index 9a7e211dbf4f..285006184afb 100644
> --- a/fs/ksmbd/smb_common.c
> +++ b/fs/ksmbd/smb_common.c
> @@ -620,8 +620,8 @@ int ksmbd_override_fsids(struct ksmbd_work *work)
>  		gid = share->force_gid;
>  
>  	cred = prepare_kernel_cred(NULL);
> -	if (!cred)
> -		return -ENOMEM;
> +	if (IS_ERR(cred))
> +		return PTR_ERR(cred);
>  
>  	cred->fsuid = make_kuid(current_user_ns(), uid);
>  	cred->fsgid = make_kgid(current_user_ns(), gid);
> diff --git a/fs/nfs/flexfilelayout/flexfilelayout.c b/fs/nfs/flexfilelayout/flexfilelayout.c
> index 604be402ae13..74d950a6dd55 100644
> --- a/fs/nfs/flexfilelayout/flexfilelayout.c
> +++ b/fs/nfs/flexfilelayout/flexfilelayout.c
> @@ -493,9 +493,12 @@ ff_layout_alloc_lseg(struct pnfs_layout_hdr *lh,
>  			kcred = prepare_kernel_cred(NULL);
>  			memalloc_nofs_restore(nofs_flags);
>  		}
> -		rc = -ENOMEM;
> -		if (!kcred)
> +
> +		if (IS_ERR(kcred)) {
> +			rc = PTR_ERR(kcred);
>  			goto out_err_free;
> +		}
> +
>  		kcred->fsuid = uid;
>  		kcred->fsgid = gid;
>  		cred = RCU_INITIALIZER(kcred);
> diff --git a/fs/nfs/nfs4idmap.c b/fs/nfs/nfs4idmap.c
> index f331866dd418..6ddceff5fbe0 100644
> --- a/fs/nfs/nfs4idmap.c
> +++ b/fs/nfs/nfs4idmap.c
> @@ -204,8 +204,8 @@ int nfs_idmap_init(void)
>  		key_type_id_resolver.name);
>  
>  	cred = prepare_kernel_cred(NULL);
> -	if (!cred)
> -		return -ENOMEM;
> +	if (IS_ERR(cred))
> +		return PTR_ERR(cred);
>  
>  	keyring = keyring_alloc(".id_resolver",
>  				GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, cred,
> diff --git a/fs/nfsd/auth.c b/fs/nfsd/auth.c
> index fdf2aad73470..9206ec3ed0f1 100644
> --- a/fs/nfsd/auth.c
> +++ b/fs/nfsd/auth.c
> @@ -31,8 +31,8 @@ int nfsd_setuser(struct svc_rqst *rqstp, struct svc_export *exp)
>  	/* discard any old override before preparing the new set */
>  	revert_creds(get_cred(current_real_cred()));
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	new->fsuid = rqstp->rq_cred.cr_uid;
>  	new->fsgid = rqstp->rq_cred.cr_gid;
> diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
> index 11f8715d92d6..630c2af0ec74 100644
> --- a/fs/nfsd/nfs4callback.c
> +++ b/fs/nfsd/nfs4callback.c
> @@ -872,8 +872,8 @@ static const struct cred *get_backchannel_cred(struct nfs4_client *clp, struct r
>  		struct cred *kcred;
>  
>  		kcred = prepare_kernel_cred(NULL);
> -		if (!kcred)
> -			return NULL;
> +		if (IS_ERR(kcred))
> +			return ERR_CAST(kcred);
>  
>  		kcred->uid = ses->se_cb_sec.uid;
>  		kcred->gid = ses->se_cb_sec.gid;
> @@ -932,10 +932,10 @@ static int setup_callback_client(struct nfs4_client *clp, struct nfs4_cb_conn *c
>  		return PTR_ERR(client);
>  	}
>  	cred = get_backchannel_cred(clp, client, ses);
> -	if (!cred) {
> -		trace_nfsd_cb_setup_err(clp, -ENOMEM);
> +	if (IS_ERR(cred)) {
> +		trace_nfsd_cb_setup_err(clp, PTR_ERR(cred));
>  		rpc_shutdown_client(client);
> -		return -ENOMEM;
> +		return PTR_ERR(cred);
>  	}
>  	clp->cl_cb_client = client;
>  	clp->cl_cb_cred = cred;
> diff --git a/fs/nfsd/nfs4recover.c b/fs/nfsd/nfs4recover.c
> index c634483d85d2..8e1b196928c1 100644
> --- a/fs/nfsd/nfs4recover.c
> +++ b/fs/nfsd/nfs4recover.c
> @@ -75,8 +75,8 @@ nfs4_save_creds(const struct cred **original_creds)
>  	struct cred *new;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	new->fsuid = GLOBAL_ROOT_UID;
>  	new->fsgid = GLOBAL_ROOT_GID;
> diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
> index c29baa03dfaf..e09e614117bd 100644
> --- a/fs/nfsd/nfsfh.c
> +++ b/fs/nfsd/nfsfh.c
> @@ -219,8 +219,8 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst *rqstp, struct svc_fh *fhp)
>  		 * fix that case easily.
>  		 */
>  		struct cred *new = prepare_creds();
> -		if (!new) {
> -			error =  nfserrno(-ENOMEM);
> +		if (IS_ERR(new)) {
> +			error =  nfserrno(PTR_ERR(new));
>  			goto out;
>  		}
>  		new->cap_effective =
> diff --git a/fs/open.c b/fs/open.c
> index 1315253e0247..d4601a557df0 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -350,8 +350,8 @@ static const struct cred *access_override_creds(void)
>  	struct cred *override_cred;
>  
>  	override_cred = prepare_creds();
> -	if (!override_cred)
> -		return NULL;
> +	if (IS_ERR(override_cred))
> +		return ERR_CAST(override_cred);
>  
>  	override_cred->fsuid = override_cred->uid;
>  	override_cred->fsgid = override_cred->gid;
> @@ -414,8 +414,8 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
>  
>  	if (!(flags & AT_EACCESS)) {
>  		old_cred = access_override_creds();
> -		if (!old_cred)
> -			return -ENOMEM;
> +		if (IS_ERR(old_cred))
> +			return PTR_ERR(old_cred);
>  	}
>  
>  retry:
> @@ -1173,7 +1173,7 @@ struct file *filp_open(const char *filename, int flags, umode_t mode)
>  {
>  	struct filename *name = getname_kernel(filename);
>  	struct file *file = ERR_CAST(name);
> -	
> +
>  	if (!IS_ERR(name)) {
>  		file = file_open_name(name, flags, mode);
>  		putname(name);
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index f18490813170..905eb8f69d64 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -589,28 +589,32 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
>  			goto out_revert_creds;
>  	}
>  
> -	err = -ENOMEM;
>  	override_cred = prepare_creds();
> -	if (override_cred) {
> -		override_cred->fsuid = inode->i_uid;
> -		override_cred->fsgid = inode->i_gid;
> -		if (!attr->hardlink) {
> -			err = security_dentry_create_files_as(dentry,
> -					attr->mode, &dentry->d_name, old_cred,
> -					override_cred);
> -			if (err) {
> -				put_cred(override_cred);
> -				goto out_revert_creds;
> -			}
> -		}
> -		put_cred(override_creds(override_cred));
> -		put_cred(override_cred);
> +	if (IS_ERR(override_cred)) {
> +		err = PTR_ERR(override_cred);
> +		goto out_revert_creds;
> +	}
>  
> -		if (!ovl_dentry_is_whiteout(dentry))
> -			err = ovl_create_upper(dentry, inode, attr);
> -		else
> -			err = ovl_create_over_whiteout(dentry, inode, attr);
> +	override_cred->fsuid = inode->i_uid;
> +	override_cred->fsgid = inode->i_gid;
> +	if (!attr->hardlink) {
> +		err = security_dentry_create_files_as(dentry, attr->mode,
> +						      &dentry->d_name,
> +						      old_cred,
> +						      override_cred);
> +		if (err) {
> +			put_cred(override_cred);
> +			goto out_revert_creds;
> +		}
>  	}
> +	put_cred(override_creds(override_cred));
> +	put_cred(override_cred);
> +
> +	if (!ovl_dentry_is_whiteout(dentry))
> +		err = ovl_create_upper(dentry, inode, attr);
> +	else
> +		err = ovl_create_over_whiteout(dentry, inode, attr);
> +
>  out_revert_creds:
>  	revert_creds(old_cred);
>  	return err;
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 001cdbb8f015..b29b62670e10 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1984,10 +1984,11 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>  	if (!ofs)
>  		goto out;
>  
> -	err = -ENOMEM;
>  	ofs->creator_cred = cred = prepare_creds();
> -	if (!cred)
> +	if (IS_ERR(ofs->creator_cred)) {
> +		err = PTR_ERR(ofs->creator_cred);
>  		goto out_err;
> +	}
>  
>  	/* Is there a reason anyone would want not to share whiteouts? */
>  	ofs->share_whiteout = true;
> diff --git a/kernel/capability.c b/kernel/capability.c
> index 765194f5d678..7a722754f571 100644
> --- a/kernel/capability.c
> +++ b/kernel/capability.c
> @@ -263,8 +263,8 @@ SYSCALL_DEFINE2(capset, cap_user_header_t, header, const cap_user_data_t, data)
>  	inheritable.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	ret = security_capset(new, current_cred(),
>  			      &effective, &inheritable, &permitted);
> diff --git a/kernel/cred.c b/kernel/cred.c
> index e10c15f51c1f..dba33c9fa869 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -245,12 +245,13 @@ struct cred *cred_alloc_blank(void)
>   *
>   * Preparation involves making a copy of the objective creds for modification.
>   *
> - * Returns a pointer to the new creds-to-be if successful, NULL otherwise.
> + * Returns a pointer to the new creds-to-be if successful, < 0 on error.
>   *
>   * Call commit_creds() or abort_creds() to clean up.
>   */
>  struct cred *prepare_creds(void)
>  {
> +	int err = -ENOMEM;
>  	struct task_struct *task = current;
>  	const struct cred *old;
>  	struct cred *new;
> @@ -259,7 +260,7 @@ struct cred *prepare_creds(void)
>  
>  	new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
>  	if (!new)
> -		return NULL;
> +		return ERR_PTR(err);
>  
>  	kdebug("prepare_creds() alloc %p", new);
>  
> @@ -288,7 +289,8 @@ struct cred *prepare_creds(void)
>  	if (!new->ucounts)
>  		goto error;
>  
> -	if (security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT) < 0)
> +	err = security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT);
> +	if (err < 0)
>  		goto error;
>  
>  	validate_creds(new);
> @@ -296,7 +298,7 @@ struct cred *prepare_creds(void)
>  
>  error:
>  	abort_creds(new);
> -	return NULL;
> +	return ERR_PTR(err);
>  }
>  EXPORT_SYMBOL(prepare_creds);
>  
> @@ -309,8 +311,8 @@ struct cred *prepare_exec_creds(void)
>  	struct cred *new;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return new;
> +	if (IS_ERR(new))
> +		return ERR_CAST(new);
>  
>  #ifdef CONFIG_KEYS
>  	/* newly exec'd tasks don't get a thread keyring */
> @@ -363,8 +365,8 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
>  	}
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	if (clone_flags & CLONE_NEWUSER) {
>  		ret = create_user_ns(new);
> @@ -707,16 +709,17 @@ void __init cred_init(void)
>   *
>   * The caller may change these controls afterwards if desired.
>   *
> - * Returns the new credentials or NULL if out of memory.
> + * Returns the new credentials or < 0 on error
>   */
>  struct cred *prepare_kernel_cred(struct task_struct *daemon)
>  {
> +	int err = -ENOMEM;
>  	const struct cred *old;
>  	struct cred *new;
>  
>  	new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
>  	if (!new)
> -		return NULL;
> +		return ERR_PTR(err);
>  
>  	kdebug("prepare_kernel_cred() alloc %p", new);
>  
> @@ -750,7 +753,8 @@ struct cred *prepare_kernel_cred(struct task_struct *daemon)
>  	if (!new->ucounts)
>  		goto error;
>  
> -	if (security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT) < 0)
> +	err = security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT);
> +	if (err < 0)
>  		goto error;
>  
>  	put_cred(old);
> @@ -760,7 +764,7 @@ struct cred *prepare_kernel_cred(struct task_struct *daemon)
>  error:
>  	put_cred(new);
>  	put_cred(old);
> -	return NULL;
> +	return ERR_PTR(err);
>  }
>  EXPORT_SYMBOL(prepare_kernel_cred);
>  
> diff --git a/kernel/groups.c b/kernel/groups.c
> index 787b381c7c00..140915fbb31f 100644
> --- a/kernel/groups.c
> +++ b/kernel/groups.c
> @@ -136,8 +136,8 @@ int set_current_groups(struct group_info *group_info)
>  	struct cred *new;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	set_groups(new, group_info);
>  	return commit_creds(new);
> diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
> index eec72ca962e2..2c937e62a83d 100644
> --- a/kernel/nsproxy.c
> +++ b/kernel/nsproxy.c
> @@ -311,18 +311,26 @@ 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);
>  	if (IS_ERR(nsset->nsproxy))
>  		return PTR_ERR(nsset->nsproxy);
>  
> -	if (flags & CLONE_NEWUSER)
> +	if (flags & CLONE_NEWUSER) {
>  		nsset->cred = prepare_creds();
> -	else
> +		if (IS_ERR(nsset->cred)) {
> +			err = PTR_ERR(nsset->cred);
> +			nsset->cred = NULL;
> +			goto out;
> +		}
> +	} else {
>  		nsset->cred = current_cred();
> -	if (!nsset->cred)
> -		goto out;
> +		if (!nsset->cred) {
> +			goto out;
> +		}
> +	}

I think this will be easier to read if you keep just the
different actions in the if/else to make clear what's being
done differently, so something like:

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

	if (!nsset->cred)
		goto out;
	if (IS_ERR(nsset->cred)) {
		err = PTR_ERR(nsset->cred);
		nsset->cred = NULL;
		goto out;
	}

It's not beautiful, but it makes clear what CLONE_NEWUSER is
doing.

>  	/* Only create a temporary copy of fs_struct if we really need to. */
>  	if (flags == CLONE_NEWNS) {
> @@ -338,7 +346,7 @@ static int prepare_nsset(unsigned flags, struct nsset *nsset)
>  
>  out:
>  	put_nsset(nsset);
> -	return -ENOMEM;
> +	return err;
>  }
>  
>  static inline int validate_ns(struct nsset *nsset, struct ns_common *ns)
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 374f83e95239..53a12bc4da70 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -372,8 +372,8 @@ long __sys_setregid(gid_t rgid, gid_t egid)
>  		return -EINVAL;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  	old = current_cred();
>  
>  	retval = -EPERM;
> @@ -434,8 +434,8 @@ long __sys_setgid(gid_t gid)
>  		return -EINVAL;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  	old = current_cred();
>  
>  	retval = -EPERM;
> @@ -529,8 +529,8 @@ long __sys_setreuid(uid_t ruid, uid_t euid)
>  		return -EINVAL;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  	old = current_cred();
>  
>  	retval = -EPERM;
> @@ -606,8 +606,8 @@ long __sys_setuid(uid_t uid)
>  		return -EINVAL;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  	old = current_cred();
>  
>  	retval = -EPERM;
> @@ -672,8 +672,8 @@ long __sys_setresuid(uid_t ruid, uid_t euid, uid_t suid)
>  		return -EINVAL;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	old = current_cred();
>  
> @@ -767,8 +767,8 @@ long __sys_setresgid(gid_t rgid, gid_t egid, gid_t sgid)
>  		return -EINVAL;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  	old = current_cred();
>  
>  	retval = -EPERM;
> @@ -850,7 +850,7 @@ long __sys_setfsuid(uid_t uid)
>  		return old_fsuid;
>  
>  	new = prepare_creds();
> -	if (!new)
> +	if (IS_ERR(new))
>  		return old_fsuid;
>  
>  	if (uid_eq(kuid, old->uid)  || uid_eq(kuid, old->euid)  ||
> @@ -894,7 +894,7 @@ long __sys_setfsgid(gid_t gid)
>  		return old_fsgid;
>  
>  	new = prepare_creds();
> -	if (!new)
> +	if (IS_ERR(new))
>  		return old_fsgid;
>  
>  	if (gid_eq(kgid, old->gid)  || gid_eq(kgid, old->egid)  ||
> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> index 706e1686b5eb..1ff2e5fab8d8 100644
> --- a/kernel/trace/trace_events_user.c
> +++ b/kernel/trace/trace_events_user.c
> @@ -558,9 +558,8 @@ static int user_event_set_call_visible(struct user_event *user, bool visible)
>  	struct cred *cred;
>  
>  	cred = prepare_creds();
> -
> -	if (!cred)
> -		return -ENOMEM;
> +	if (IS_ERR(cred))
> +		return PTR_ERR(cred);
>  
>  	/*
>  	 * While by default tracefs is locked down, systems can be configured
> diff --git a/kernel/umh.c b/kernel/umh.c
> index 36c123360ab8..2bf9b402083a 100644
> --- a/kernel/umh.c
> +++ b/kernel/umh.c
> @@ -87,10 +87,11 @@ static int call_usermodehelper_exec_async(void *data)
>  	 */
>  	set_user_nice(current, 0);
>  
> -	retval = -ENOMEM;
>  	new = prepare_kernel_cred(current);
> -	if (!new)
> +	if (IS_ERR(new)) {
> +		retval = PTR_ERR(new);
>  		goto out;
> +	}
>  
>  	spin_lock(&umh_sysctl_lock);
>  	new->cap_bset = cap_intersect(usermodehelper_bset, new->cap_bset);
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 5481ba44a8d6..3287666f942c 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -171,19 +171,20 @@ int create_user_ns(struct cred *new)
>  int unshare_userns(unsigned long unshare_flags, struct cred **new_cred)
>  {
>  	struct cred *cred;
> -	int err = -ENOMEM;
> +	int err;
>  
>  	if (!(unshare_flags & CLONE_NEWUSER))
>  		return 0;
>  
>  	cred = prepare_creds();
> -	if (cred) {
> -		err = create_user_ns(cred);
> -		if (err)
> -			put_cred(cred);
> -		else
> -			*new_cred = cred;
> -	}
> +	if (IS_ERR(cred))
> +		return PTR_ERR(cred);
> +
> +	err = create_user_ns(cred);
> +	if (err)
> +		put_cred(cred);
> +	else
> +		*new_cred = cred;
>  
>  	return err;
>  }
> diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
> index 3aced951d5ab..dbfb2b17491e 100644
> --- a/net/dns_resolver/dns_key.c
> +++ b/net/dns_resolver/dns_key.c
> @@ -338,8 +338,8 @@ static int __init init_dns_resolver(void)
>  	 * with add_key().
>  	 */
>  	cred = prepare_kernel_cred(NULL);
> -	if (!cred)
> -		return -ENOMEM;
> +	if (IS_ERR(cred))
> +		return PTR_ERR(cred);
>  
>  	keyring = keyring_alloc(".dns_resolver",
>  				GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, cred,
> diff --git a/security/apparmor/task.c b/security/apparmor/task.c
> index d17130ee6795..2c1940c26cb2 100644
> --- a/security/apparmor/task.c
> +++ b/security/apparmor/task.c
> @@ -52,9 +52,9 @@ int aa_replace_current_label(struct aa_label *label)
>  	if (current_cred() != current_real_cred())
>  		return -EBUSY;
>  
> -	new  = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	new = prepare_creds();
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	if (ctx->nnp && label_is_stale(ctx->nnp)) {
>  		struct aa_label *tmp = ctx->nnp;
> @@ -118,8 +118,8 @@ int aa_set_current_hat(struct aa_label *label, u64 token)
>  	struct cred *new;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  	AA_BUG(!label);
>  
>  	if (!ctx->previous) {
> @@ -164,8 +164,8 @@ int aa_restore_previous_label(u64 token)
>  		return 0;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	aa_put_label(cred_label(new));
>  	set_cred_label(new, aa_get_newest_label(ctx->previous));
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 5fc8986c3c77..906d1bf4a226 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -1247,8 +1247,8 @@ static int cap_prctl_drop(unsigned long cap)
>  		return -EINVAL;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  	cap_lower(new->cap_bset, cap);
>  	return commit_creds(new);
>  }
> @@ -1323,8 +1323,8 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>  			return -EPERM;
>  
>  		new = prepare_creds();
> -		if (!new)
> -			return -ENOMEM;
> +		if (IS_ERR(new))
> +			return PTR_ERR(new);
>  		new->securebits = arg2;
>  		return commit_creds(new);
>  
> @@ -1341,8 +1341,8 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>  			return -EPERM;
>  
>  		new = prepare_creds();
> -		if (!new)
> -			return -ENOMEM;
> +		if (IS_ERR(new))
> +			return PTR_ERR(new);
>  		if (arg2)
>  			new->securebits |= issecure_mask(SECURE_KEEP_CAPS);
>  		else
> @@ -1355,8 +1355,8 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>  				return -EINVAL;
>  
>  			new = prepare_creds();
> -			if (!new)
> -				return -ENOMEM;
> +			if (IS_ERR(new))
> +				return PTR_ERR(new);
>  			cap_clear(new->cap_ambient);
>  			return commit_creds(new);
>  		}
> @@ -1378,8 +1378,8 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>  				return -EPERM;
>  
>  			new = prepare_creds();
> -			if (!new)
> -				return -ENOMEM;
> +			if (IS_ERR(new))
> +				return PTR_ERR(new);
>  			if (arg2 == PR_CAP_AMBIENT_RAISE)
>  				cap_raise(new->cap_ambient, arg3);
>  			else
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index 96a92a645216..cb3be208bc7d 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -1146,8 +1146,8 @@ static int keyctl_change_reqkey_auth(struct key *key)
>  	struct cred *new;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	key_put(new->request_key_auth);
>  	new->request_key_auth = key_get(key);
> @@ -1396,8 +1396,8 @@ long keyctl_set_reqkey_keyring(int reqkey_defl)
>  		return old_setting;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	switch (reqkey_defl) {
>  	case KEY_REQKEY_DEFL_THREAD_KEYRING:
> diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
> index b5d5333ab330..8e7655d48319 100644
> --- a/security/keys/process_keys.c
> +++ b/security/keys/process_keys.c
> @@ -247,8 +247,8 @@ static int install_thread_keyring(void)
>  	int ret;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	ret = install_thread_keyring_to_cred(new);
>  	if (ret < 0) {
> @@ -294,8 +294,8 @@ static int install_process_keyring(void)
>  	int ret;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	ret = install_process_keyring_to_cred(new);
>  	if (ret < 0) {
> @@ -359,8 +359,8 @@ static int install_session_keyring(struct key *keyring)
>  	int ret;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	ret = install_session_keyring_to_cred(new, keyring);
>  	if (ret < 0) {
> @@ -842,8 +842,8 @@ long join_session_keyring(const char *name)
>  	long ret, serial;
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  	old = current_cred();
>  
>  	/* if no name is provided, install an anonymous keyring */
> diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
> index 7e27ce394020..905ae7f9717f 100644
> --- a/security/landlock/syscalls.c
> +++ b/security/landlock/syscalls.c
> @@ -419,8 +419,8 @@ SYSCALL_DEFINE2(landlock_restrict_self,
>  
>  	/* Prepares new credentials. */
>  	new_cred = prepare_creds();
> -	if (!new_cred) {
> -		err = -ENOMEM;
> +	if (IS_ERR(new_cred)) {
> +		err = PTR_ERR(new_cred);
>  		goto out_put_ruleset;
>  	}
>  	new_llcred = landlock_cred(new_cred);
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index e9e959343de9..a0b3671ee1f1 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3472,8 +3472,8 @@ static int selinux_inode_copy_up(struct dentry *src, struct cred **new)
>  
>  	if (new_creds == NULL) {
>  		new_creds = prepare_creds();
> -		if (!new_creds)
> -			return -ENOMEM;
> +		if (IS_ERR(new_creds))
> +			return PTR_ERR(new_creds);
>  	}
>  
>  	tsec = selinux_cred(new_creds);
> @@ -6457,8 +6457,8 @@ static int selinux_setprocattr(const char *name, void *value, size_t size)
>  	}
>  
>  	new = prepare_creds();
> -	if (!new)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	/* Permission checking based on the specified context is
>  	   performed during the actual operation (execve,
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 6207762dbdb1..ca4e2b906cce 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -3555,8 +3555,8 @@ static int smack_setprocattr(const char *name, void *value, size_t size)
>  	}
>  
>  	new = prepare_creds();
> -	if (new == NULL)
> -		return -ENOMEM;
> +	if (IS_ERR(new))
> +		return PTR_ERR(new);
>  
>  	tsp = smack_cred(new);
>  	tsp->smk_task = skp;
> @@ -4633,8 +4633,8 @@ static int smack_inode_copy_up(struct dentry *dentry, struct cred **new)
>  
>  	if (new_creds == NULL) {
>  		new_creds = prepare_creds();
> -		if (new_creds == NULL)
> -			return -ENOMEM;
> +		if (IS_ERR(new_creds))
> +			return PTR_ERR(new_creds);
>  	}
>  
>  	tsp = smack_cred(new_creds);
> diff --git a/security/smack/smackfs.c b/security/smack/smackfs.c
> index 658eab05599e..1e90b325c56b 100644
> --- a/security/smack/smackfs.c
> +++ b/security/smack/smackfs.c
> @@ -2777,8 +2777,8 @@ static ssize_t smk_write_relabel_self(struct file *file, const char __user *buf,
>  		struct task_smack *tsp;
>  
>  		new = prepare_creds();
> -		if (!new) {
> -			rc = -ENOMEM;
> +		if (IS_ERR(new)) {
> +			rc = PTR_ERR(new);
>  			goto out;
>  		}
>  		tsp = smack_cred(new);
> -- 
> 2.30.2
> 

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