linux-kernel - Re: [PATCH v11 1/4] security: Allow all LSMs to provide xattrs for inode_init

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Date:   Fri, 9 Jun 2023 16:02:30 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Roberto Sassu <roberto.sassu@...weicloud.com>
Cc:     zohar@...ux.ibm.com, dmitry.kasatkin@...il.com, jmorris@...ei.org,
        serge@...lyn.com, stephen.smalley.work@...il.com,
        eparis@...isplace.org, casey@...aufler-ca.com,
        linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        bpf@...r.kernel.org, kpsingh@...nel.org, keescook@...omium.org,
        nicolas.bouchinet@...p-os.org,
        Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [PATCH v11 1/4] security: Allow all LSMs to provide xattrs for
 inode_init_security hook

On Sat, Jun 3, 2023 at 3:16 PM Roberto Sassu
<roberto.sassu@...weicloud.com> wrote:
> From: Roberto Sassu <roberto.sassu@...wei.com>
>
> Currently, the LSM infrastructure supports only one LSM providing an xattr
> and EVM calculating the HMAC on that xattr, plus other inode metadata.
>
> Allow all LSMs to provide one or multiple xattrs, by extending the security
> blob reservation mechanism. Introduce the new lbs_xattr_count field of the
> lsm_blob_sizes structure, so that each LSM can specify how many xattrs it
> needs, and the LSM infrastructure knows how many xattr slots it should
> allocate.
>
> Modify the inode_init_security hook definition, by passing the full
> xattr array allocated in security_inode_init_security(), and the current
> number of xattr slots in that array filled by LSMs. The first parameter
> would allow EVM to access and calculate the HMAC on xattrs supplied by
> other LSMs, the second to not leave gaps in the xattr array, when an LSM
> requested but did not provide xattrs (e.g. if it is not initialized).
>
> Introduce lsm_get_xattr_slot(), which LSMs can call as many times as the
> number specified in the lbs_xattr_count field of the lsm_blob_sizes
> structure. During each call, lsm_get_xattr_slot() increments the number of
> filled xattrs, so that at the next invocation it returns the next xattr
> slot to fill.
>
> Cleanup security_inode_init_security(). Unify the !initxattrs and
> initxattrs case by simply not allocating the new_xattrs array in the
> former. Update the documentation to reflect the changes, and fix the
> description of the xattr name, as it is not allocated anymore.
>
> Adapt both SELinux and Smack to use the new definition of the
> inode_init_security hook, and to call lsm_get_xattr_slot() to obtain and
> fill the reserved slots in the xattr array.
>
> Move the xattr->name assignment after the xattr->value one, so that it is
> done only in case of successful memory allocation.
>
> Finally, change the default return value of the inode_init_security hook
> from zero to -EOPNOTSUPP, so that BPF LSM correctly follows the hook
> conventions.
>
> Reported-by: Nicolas Bouchinet <nicolas.bouchinet@...p-os.org> (EVM crash)
> Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> ---
>  include/linux/lsm_hook_defs.h |  6 +--
>  include/linux/lsm_hooks.h     | 20 ++++++++++
>  security/security.c           | 71 +++++++++++++++++++++++------------
>  security/selinux/hooks.c      | 17 +++++----
>  security/smack/smack_lsm.c    | 25 ++++++------
>  5 files changed, 92 insertions(+), 47 deletions(-)

...

> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index ab2b2fafa4a..069ac73a84b 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -63,8 +64,27 @@ struct lsm_blob_sizes {
>         int     lbs_ipc;
>         int     lbs_msg_msg;
>         int     lbs_task;
> +       int     lbs_xattr_count; /* number of xattr slots in new_xattrs array */
>  };
>
> +/**
> + * lsm_get_xattr_slot - Return the next available slot and increment the index
> + * @xattrs: array storing LSM-provided xattrs
> + * @xattr_count: number of already stored xattrs (updated)
> + *
> + * Retrieve the first available slot in the @xattrs array to fill with an xattr,
> + * and increment @xattr_count.
> + *
> + * Return: The slot to fill in @xattrs if non-NULL, NULL otherwise.
> + */
> +static inline struct xattr *lsm_get_xattr_slot(struct xattr *xattrs,
> +                                              int *xattr_count)
> +{
> +       if (unlikely(!xattrs))
> +               return NULL;
> +       return xattrs + (*xattr_count)++;

I would have tried to avoid the pointer math by writing the above like
the line below:

  return &xattrs[(*xattr_count)++]

... but I wouldn't worry about that, what you have is fine; I only
mention this in case you need to respin this patchset for some other
reason.

-- 
paul-moore.com