Re: [PATCH v4 12/30] integrity: implement get and set acl hook

[Paul Moore <paul@paul-moore.com>]

On Thu, Sep 29, 2022 at 11:33 AM Christian Brauner <brauner@kernel.org> 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. 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.
>
> 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>
> ---
>
> Notes:
>     /* v2 */
>     unchanged
>
>     /* v3 */
>     Paul Moore <paul@paul-moore.com>:
>     - Add get, and remove acl hook
>
>     /* v4 */
>     unchanged
>
>  include/linux/evm.h                   | 23 +++++++++
>  include/linux/ima.h                   | 21 ++++++++
>  security/integrity/evm/evm_main.c     | 70 ++++++++++++++++++++++++++-
>  security/integrity/ima/ima_appraise.c |  9 ++++
>  security/security.c                   | 21 +++++++-
>  5 files changed, 141 insertions(+), 3 deletions(-)

...

> 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))
> +               return 1;
> +#endif
> +       return 0;
> +}

I'm not too bothered by it either way, but one might consider pulling
the #ifdef outside the function definition, for example:

#ifdef CONFIG_FS_POSIX_ACL
static int evm_inode_foo(...)
{
  /* ... stuff ... */
}
#else
static int evm_inode_foo(...)
{
  return 0;
}
#endif /* CONFIG_FS_POSIX_ACL */

> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index bde74fcecee3..698a8ae2fe3e 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -770,6 +770,15 @@ int ima_inode_setxattr(struct dentry *dentry, const char *xattr_name,
>         return result;
>  }
>
> +int ima_inode_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
> +                     const char *acl_name, struct posix_acl *kacl)
> +{
> +       if (evm_revalidate_status(acl_name))
> +               ima_reset_appraise_flags(d_backing_inode(dentry), 0);
> +
> +       return 0;
> +}

While the ima_inode_set_acl() implementation above looks okay for the
remove case, I do see that the ima_inode_setxattr() function has a
call to validate_hash_algo() before calling
ima_reset_appraise_flags().  IANAIE (I Am Not An Ima Expert), but it
seems like we would still want that check in the ACL case.

-- 
paul-moore.com