linux-kernel - Re: [PATCH v2] cred: Propagate security_prepare

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Date:   Fri, 27 May 2022 16:05:26 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Frederick Lawler <fred@...udflare.com>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-aio@...ck.org, linux-fsdevel@...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,
        serge@...lyn.com, amir73il@...il.com, kernel-team@...udflare.com
Subject: Re: [PATCH v2] cred: Propagate security_prepare_creds() error code

On Wed, May 25, 2022 at 2:37 PM Frederick Lawler <fred@...udflare.com> 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>
>
> ---
> Changes since v1:
> - Revert style churn in ovl_create_or_link() noted by Amir
> - Revert style churn in prepare_nsset() noted by Serge
> - Update documentation for prepare_creds()
> - Set ofs->creator_cred in ovl_fill_super() and req->creds in aio_fsync()
>   to NULL on error noted by Amir
> ---
>  Documentation/security/credentials.rst |  6 +++---
>  fs/aio.c                               |  9 +++++++--
>  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                              |  8 ++++----
>  fs/overlayfs/dir.c                     |  6 ++++--
>  fs/overlayfs/super.c                   |  6 ++++--
>  kernel/capability.c                    |  4 ++--
>  kernel/cred.c                          | 28 +++++++++++++++-----------
>  kernel/groups.c                        |  4 ++--
>  kernel/nsproxy.c                       |  9 ++++++++-
>  kernel/sys.c                           | 28 +++++++++++++-------------
>  kernel/trace/trace_events_user.c       |  4 ++--
>  kernel/umh.c                           |  5 +++--
>  kernel/user_namespace.c                |  6 ++++--
>  net/dns_resolver/dns_key.c             |  4 ++--
>  security/apparmor/task.c               | 12 +++++------
>  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 ++--
>  34 files changed, 153 insertions(+), 123 deletions(-)

The SELinux bits look fine to me.

Acked-by: Paul Moore <paul@...l-moore.com> (SELinux)

-- 
paul-moore.com