acl: handle idmapped mounts

The posix acl permission checking helpers determine whether a caller is
privileged over an inode according to the acls associated with the
inode. Add helpers that make it possible to handle acls on idmapped
mounts.

The vfs and the filesystems targeted by this first iteration make use of
posix_acl_fix_xattr_from_user() and posix_acl_fix_xattr_to_user() to
translate basic posix access and default permissions such as the
ACL_USER and ACL_GROUP type according to the initial user namespace (or
the superblock's user namespace) to and from the caller's current user
namespace. Adapt these two helpers to handle idmapped mounts whereby we
either map from or into the mount's user namespace depending on in which
direction we're translating.
Similarly, cap_convert_nscap() is used by the vfs to translate user
namespace and non-user namespace aware filesystem capabilities from the
superblock's user namespace to the caller's user namespace. Enable it to
handle idmapped mounts by accounting for the mount's user namespace.

In addition the fileystems targeted in the first iteration of this patch
series make use of the posix_acl_chmod() and, posix_acl_update_mode()
helpers. Both helpers perform permission checks on the target inode. Let
them handle idmapped mounts. These two helpers are called when posix
acls are set by the respective filesystems to handle this case we extend
the ->set() method to take an additional user namespace argument to pass
the mount's user namespace down.

Link: https://lore.kernel.org/r/20210121131959.646623-9-christian.brauner@ubuntu.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

4 files changed, 19 insertions, 10 deletions

diff --git a/include/linux/capability.h b/include/linux/capability.h
index 62bfa3ed84d7..da21ef118b04 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -273,6 +273,7 @@ static inline bool checkpoint_restore_ns_capable(struct user_namespace *ns)
 /* audit system wants to get cap info from files as well */
 extern int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data *cpu_caps);
 
-extern int cap_convert_nscap(struct dentry *dentry, const void **ivalue, size_t size);
+int cap_convert_nscap(struct user_namespace *mnt_userns, struct dentry *dentry,
+		      const void **ivalue, size_t size);
 
 #endif /* !_LINUX_CAPABILITY_H */
diff --git a/include/linux/posix_acl.h b/include/linux/posix_acl.h
index 85fb4c0c650a..6dcd8b8f6ab5 100644
--- a/include/linux/posix_acl.h
+++ b/include/linux/posix_acl.h
@@ -69,13 +69,15 @@ extern int __posix_acl_create(struct posix_acl **, gfp_t, umode_t *);
 extern int __posix_acl_chmod(struct posix_acl **, gfp_t, umode_t);
 
 extern struct posix_acl *get_posix_acl(struct inode *, int);
-extern int set_posix_acl(struct inode *, int, struct posix_acl *);
+extern int set_posix_acl(struct user_namespace *, struct inode *, int,
+			 struct posix_acl *);
 
 #ifdef CONFIG_FS_POSIX_ACL
-extern int posix_acl_chmod(struct inode *, umode_t);
+int posix_acl_chmod(struct user_namespace *, struct inode *, umode_t);
 extern int posix_acl_create(struct inode *, umode_t *, struct posix_acl **,
 		struct posix_acl **);
-extern int posix_acl_update_mode(struct inode *, umode_t *, struct posix_acl **);
+int posix_acl_update_mode(struct user_namespace *, struct inode *, umode_t *,
+			  struct posix_acl **);
 
 extern int simple_set_acl(struct inode *, struct posix_acl *, int);
 extern int simple_acl_create(struct inode *, struct inode *);
@@ -95,7 +97,8 @@ static inline void cache_no_acl(struct inode *inode)
 	inode->i_default_acl = NULL;
 }
 #else
-static inline int posix_acl_chmod(struct inode *inode, umode_t mode)
+static inline int posix_acl_chmod(struct user_namespace *mnt_userns,
+				  struct inode *inode, umode_t mode)
 {
 	return 0;
 }
diff --git a/include/linux/posix_acl_xattr.h b/include/linux/posix_acl_xattr.h
index 2387709991b5..060e8d203181 100644
--- a/include/linux/posix_acl_xattr.h
+++ b/include/linux/posix_acl_xattr.h
@@ -33,13 +33,17 @@ posix_acl_xattr_count(size_t size)
 }
 
 #ifdef CONFIG_FS_POSIX_ACL
-void posix_acl_fix_xattr_from_user(void *value, size_t size);
-void posix_acl_fix_xattr_to_user(void *value, size_t size);
+void posix_acl_fix_xattr_from_user(struct user_namespace *mnt_userns,
+				   void *value, size_t size);
+void posix_acl_fix_xattr_to_user(struct user_namespace *mnt_userns,
+				 void *value, size_t size);
 #else
-static inline void posix_acl_fix_xattr_from_user(void *value, size_t size)
+static inline void posix_acl_fix_xattr_from_user(struct user_namespace *mnt_userns,
+						 void *value, size_t size)
 {
 }
-static inline void posix_acl_fix_xattr_to_user(void *value, size_t size)
+static inline void posix_acl_fix_xattr_to_user(struct user_namespace *mnt_userns,
+					       void *value, size_t size)
 {
 }
 #endif
diff --git a/include/linux/xattr.h b/include/linux/xattr.h
index 10b4dc2709f0..260c9bcb0edb 100644
--- a/include/linux/xattr.h
+++ b/include/linux/xattr.h
@@ -34,7 +34,8 @@ struct xattr_handler {
 	int (*get)(const struct xattr_handler *, struct dentry *dentry,
 		   struct inode *inode, const char *name, void *buffer,
 		   size_t size);
-	int (*set)(const struct xattr_handler *, struct dentry *dentry,
+	int (*set)(const struct xattr_handler *,
+		   struct user_namespace *mnt_userns, struct dentry *dentry,
 		   struct inode *inode, const char *name, const void *buffer,
 		   size_t size, int flags);
 };