Re: [PATCH v12 11/26] ima: Define mac_admin_ns_capable() as a wrapper for ns_capable()

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On 5/22/22 13:31, Serge E. Hallyn wrote:
On Wed, Apr 20, 2022 at 10:06:18AM -0400, Stefan Berger wrote:
Define mac_admin_ns_capable() as a wrapper for the combined ns_capable()
checks on CAP_MAC_ADMIN and CAP_SYS_ADMIN in a user namespace. Return
true on the check if either capability or both are available.

Use mac_admin_ns_capable() in place of capable(SYS_ADMIN). This will allow
an IMA namespace to read the policy with only CAP_MAC_ADMIN, which has
less privileges than CAP_SYS_ADMIN.

Since CAP_MAC_ADMIN is an additional capability added to an existing gate
avoid auditing in case it is not set.

It would probably be best to become a user of
https://lore.kernel.org/all/20220217145003.78982-2-cgzones@xxxxxxxxxxxxxx/

when that lands.

Hm,I'll try to monitor to see when this gets merged. Though the function proposed there uses a hard-coded init_user_ns, which may not work for everyone.


Signed-off-by: Denis Semakin <denis.semakin@xxxxxxxxxx>
Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>

---
v11:
   - use ns_capable_noaudit for CAP_MAC_ADMIN to avoid auditing in this case
---
  include/linux/capability.h      | 6 ++++++
  security/integrity/ima/ima.h    | 6 ++++++
  security/integrity/ima/ima_fs.c | 5 ++++-
  3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/include/linux/capability.h b/include/linux/capability.h
index 65efb74c3585..dc3e1230b365 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -270,6 +270,12 @@ static inline bool checkpoint_restore_ns_capable(struct user_namespace *ns)
  		ns_capable(ns, CAP_SYS_ADMIN);
  }
+static inline bool mac_admin_ns_capable(struct user_namespace *ns)
+{
+	return ns_capable_noaudit(ns, CAP_MAC_ADMIN) ||
+		ns_capable(ns, CAP_SYS_ADMIN);
+}
+
  /* audit system wants to get cap info from files as well */
  int get_vfs_caps_from_disk(struct user_namespace *mnt_userns,
  			   const struct dentry *dentry,
diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index 5bf7f080c2be..626a6ce2453c 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -491,4 +491,10 @@ static inline int ima_filter_rule_match(u32 secid, u32 field, u32 op,
  #define	POLICY_FILE_FLAGS	S_IWUSR
  #endif /* CONFIG_IMA_READ_POLICY */
+static inline
+struct user_namespace *ima_user_ns_from_file(const struct file *filp)
+{
+	return file_inode(filp)->i_sb->s_user_ns;
+}

include/linux/fs.h has file_mnt_user_ns().  Maybe you should add a
file_sb_user_ns() next to that?

I did that now:


+static inline
+struct user_namespace *ima_user_ns_from_file(struct file *filp)
+{
+       return file_sb_user_ns(filp);
+}
+


+static inline struct user_namespace *file_sb_user_ns(struct file *file)
+{
+       return i_user_ns(file_inode(file));
+}
+




+
  #endif /* __LINUX_IMA_H */
diff --git a/security/integrity/ima/ima_fs.c b/security/integrity/ima/ima_fs.c
index 89d3113ceda1..c41aa61b7393 100644
--- a/security/integrity/ima/ima_fs.c
+++ b/security/integrity/ima/ima_fs.c
@@ -377,6 +377,9 @@ static const struct seq_operations ima_policy_seqops = {
   */
  static int ima_open_policy(struct inode *inode, struct file *filp)
  {
+#ifdef CONFIG_IMA_READ_POLICY
+	struct user_namespace *user_ns = ima_user_ns_from_file(filp);
+#endif
  	struct ima_namespace *ns = &init_ima_ns;
if (!(filp->f_flags & O_WRONLY)) {
@@ -385,7 +388,7 @@ static int ima_open_policy(struct inode *inode, struct file *filp)
  #else
  		if ((filp->f_flags & O_ACCMODE) != O_RDONLY)
  			return -EACCES;
-		if (!capable(CAP_SYS_ADMIN))
+		if (!mac_admin_ns_capable(user_ns))
  			return -EPERM;
  		return seq_open(filp, &ima_policy_seqops);
  #endif
--
2.34.1



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