[PATCH v2 12/30] smack: implement set acl hook

[Casey Schaufler]

On 9/26/2022 7:08 AM, Christian Brauner wrote:
> The current way of setting and getting posix acls through the generic
> xattr interface is error prone and type unsafe. The vfs needs to
> interpret and fixup posix acls before storing or reporting it to
> userspace. Various hacks exist to make this work. The code is hard to
> understand and difficult to maintain in it's current form. Instead of
> making this work by hacking posix acls through xattr handlers we are
> building a dedicated posix acl api around the get and set inode
> operations. This removes a lot of hackiness and makes the codepaths
> easier to maintain. A lot of background can be found in [1].
>
> So far posix acls were passed as a void blob to the security and
> integrity modules. Some of them like evm then proceed to interpret the
> void pointer and convert it into the kernel internal struct posix acl
> representation to perform their integrity checking magic. This is
> obviously pretty problematic as that requires knowledge that only the
> vfs is guaranteed to have and has lead to various bugs. Add a proper
> security hook for setting posix acls and pass down the posix acls in
> their appropriate vfs format instead of hacking it through a void
> pointer stored in the uapi format.
>
> I spent considerate time in the security module infrastructure and
> audited all codepaths. Smack has no restrictions based on the posix
> acl values passed through it. The capability hook doesn't need to be
> called either because it only has restrictions on security.* xattrs. So
> this all becomes a very simple hook for smack.
>
> Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org [1]
> Signed-off-by: Christian Brauner (Microsoft) <brauner at kernel.org>

The Smack hook looks fine.
Acked-by: Casey Schaufler <casey at schaufler-ca.com>

> ---
>
> Notes:
>     /* v2 */
>     unchanged
>
>  security/smack/smack_lsm.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 001831458fa2..ec6d55632b4f 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -1393,6 +1393,29 @@ static int smack_inode_removexattr(struct user_namespace *mnt_userns,
>  	return 0;
>  }
>  
> +/**
> + * smack_inode_set_acl - Smack check for setting posix acls
> + * @mnt_userns: the userns attached to the mnt this request came from
> + * @dentry: the object
> + * @acl_name: name of the posix acl
> + * @kacl: the posix acls
> + *
> + * Returns 0 if access is permitted, an error code otherwise
> + */
> +static int smack_inode_set_acl(struct user_namespace *mnt_userns,
> +			       struct dentry *dentry, const char *acl_name,
> +			       struct posix_acl *kacl)
> +{
> +	struct smk_audit_info ad;
> +	int rc;
> +
> +	smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_DENTRY);
> +	smk_ad_setfield_u_fs_path_dentry(&ad, dentry);
> +	rc = smk_curacc(smk_of_inode(d_backing_inode(dentry)), MAY_WRITE, &ad);
> +	rc = smk_bu_inode(d_backing_inode(dentry), MAY_WRITE, rc);
> +	return rc;
> +}
> +
>  /**
>   * smack_inode_getsecurity - get smack xattrs
>   * @mnt_userns: active user namespace
> @@ -4772,6 +4795,7 @@ static struct security_hook_list smack_hooks[] __lsm_ro_after_init = {
>  	LSM_HOOK_INIT(inode_post_setxattr, smack_inode_post_setxattr),
>  	LSM_HOOK_INIT(inode_getxattr, smack_inode_getxattr),
>  	LSM_HOOK_INIT(inode_removexattr, smack_inode_removexattr),
> +	LSM_HOOK_INIT(inode_set_acl, smack_inode_set_acl),
>  	LSM_HOOK_INIT(inode_getsecurity, smack_inode_getsecurity),
>  	LSM_HOOK_INIT(inode_setsecurity, smack_inode_setsecurity),
>  	LSM_HOOK_INIT(inode_listsecurity, smack_inode_listsecurity),