Re: [PATCH v3 12/29] integrity: implement get and set acl hook

[Mimi Zohar <zohar@linux.ibm.com>]

Hi Christian,

On Wed, 2022-09-28 at 18:08 +0200, 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 and integrity
> infrastructure and audited all codepaths. EVM is the only part that
> really has restrictions based on the actual posix acl values passed
> through it

(e.g. i_mode).

> Before this dedicated hook EVM used to translate from the
> uapi posix acl format sent to it in the form of a void pointer into the
> vfs format. This is not a good thing. Instead of hacking around in the
> uapi struct give EVM the posix acls in the appropriate vfs format and
> perform sane permissions checks that mirror what it used to to in the
> generic xattr hook.
> 
> IMA doesn't have any restrictions on posix acls. When posix acls are
> changed it just wants to update its appraisal status.

to trigger an EVM re-validation.

> The removal of posix acls is equivalent to passing NULL to the posix set
> acl hooks. This is the same as before through the generic xattr api.
> 
> Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org [1]
> Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>


> ---

> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 23d484e05e6f..7904786b610f 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -8,7 +8,7 @@
>   *
>   * File: evm_main.c
>   *	implements evm_inode_setxattr, evm_inode_post_setxattr,
> - *	evm_inode_removexattr, and evm_verifyxattr
> + *	evm_inode_removexattr, evm_verifyxattr, and evm_inode_set_acl.
>   */
>  
>  #define pr_fmt(fmt) "EVM: "fmt
> @@ -670,6 +670,74 @@ int evm_inode_removexattr(struct user_namespace *mnt_userns,
>  	return evm_protect_xattr(mnt_userns, dentry, xattr_name, NULL, 0);
>  }
>  
> +static int evm_inode_set_acl_change(struct user_namespace *mnt_userns,
> +				    struct dentry *dentry, const char *name,
> +				    struct posix_acl *kacl)
> +{
> +#ifdef CONFIG_FS_POSIX_ACL
> +	int rc;
> +
> +	umode_t mode;
> +	struct inode *inode = d_backing_inode(dentry);
> +
> +	if (!kacl)
> +		return 1;
> +
> +	rc = posix_acl_update_mode(mnt_userns, inode, &mode, &kacl);
> +	if (rc || (inode->i_mode != mode))

acl_res in the existing evm_xattr_acl_change() code is based on the
init_user_ns.  Is that the same here?   Is it guaranteed?

> +		return 1;
> +#endif
> +	return 0;
> +}
> +
> +/**
> + * evm_inode_set_acl - protect the EVM extended attribute for posix acls

^from posix acls


> + * @mnt_userns: user namespace of the idmapped mount
> + * @dentry: pointer to the affected dentry
> + * @acl_name: name of the posix acl
> + * @kacl: pointer to the posix acls

Prevent modifying posix acls causing the EVM HMAC to be re-calculated
and 'security.evm' xattr updated, unless the existing 'security.evm' is
valid.

-- 
thanks,

Mimi