capability: handle idmapped mounts

In order to determine whether a caller holds privilege over a given
inode the capability framework exposes the two helpers
privileged_wrt_inode_uidgid() and capable_wrt_inode_uidgid(). The former
verifies that the inode has a mapping in the caller's user namespace and
the latter additionally verifies that the caller has the requested
capability in their current user namespace.
If the inode is accessed through an idmapped mount map it into the
mount's user namespace. Afterwards the checks are identical to
non-idmapped inodes. If the initial user namespace is passed all
operations are a nop so non-idmapped mounts will not see a change in
behavior.

Link: https://lore.kernel.org/r/20210121131959.646623-5-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: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJames Morris <jamorris@linux.microsoft.com>
Acked-by: NSerge Hallyn <serge@hallyn.com>
Signed-off-by: NChristian Brauner <christian.brauner@ubuntu.com>

fs/attr.c

@@ -23,7 +23,7 @@ static bool chown_ok(const struct inode *inode, kuid_t uid)
 	if (uid_eq(current_fsuid(), inode->i_uid) &&
 	    uid_eq(uid, inode->i_uid))
 		return true;
-	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
+	if (capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_CHOWN))
 		return true;
 	if (uid_eq(inode->i_uid, INVALID_UID) &&
 	    ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
@@ -36,7 +36,7 @@ static bool chgrp_ok(const struct inode *inode, kgid_t gid)
 	if (uid_eq(current_fsuid(), inode->i_uid) &&
 	    (in_group_p(gid) || gid_eq(gid, inode->i_gid)))
 		return true;
-	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
+	if (capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_CHOWN))
 		return true;
 	if (gid_eq(inode->i_gid, INVALID_GID) &&
 	    ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
@@ -92,7 +92,7 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
 		/* Also check the setgid bit! */
 		if (!in_group_p((ia_valid & ATTR_GID) ? attr->ia_gid :
 				inode->i_gid) &&
-		    !capable_wrt_inode_uidgid(inode, CAP_FSETID))
+		    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
 			attr->ia_mode &= ~S_ISGID;
 	}
 
@@ -193,7 +193,7 @@ void setattr_copy(struct inode *inode, const struct iattr *attr)
 		umode_t mode = attr->ia_mode;
 
 		if (!in_group_p(inode->i_gid) &&
-		    !capable_wrt_inode_uidgid(inode, CAP_FSETID))
+		    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
 			mode &= ~S_ISGID;
 		inode->i_mode = mode;
 	}

fs/exec.c

@@ -1411,7 +1411,8 @@ void would_dump(struct linux_binprm *bprm, struct file *file)
 		/* Ensure mm->user_ns contains the executable */
 		user_ns = old = bprm->mm->user_ns;
 		while ((user_ns != &init_user_ns) &&
-		       !privileged_wrt_inode_uidgid(user_ns, inode))
+		       !privileged_wrt_inode_uidgid(user_ns, &init_user_ns,
+						    inode))
 			user_ns = user_ns->parent;
 
 		if (old != user_ns) {

fs/inode.c

@@ -2146,7 +2146,8 @@ void inode_init_owner(struct inode *inode, const struct inode *dir,
 			mode |= S_ISGID;
 		else if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP) &&
 			 !in_group_p(inode->i_gid) &&
-			 !capable_wrt_inode_uidgid(dir, CAP_FSETID))
+			 !capable_wrt_inode_uidgid(&init_user_ns, dir,
+						   CAP_FSETID))
 			mode &= ~S_ISGID;
 	} else
 		inode->i_gid = current_fsgid();

fs/namei.c

@@ -357,10 +357,11 @@ int generic_permission(struct inode *inode, int mask)
 	if (S_ISDIR(inode->i_mode)) {
 		/* DACs are overridable for directories */
 		if (!(mask & MAY_WRITE))
-			if (capable_wrt_inode_uidgid(inode,
+			if (capable_wrt_inode_uidgid(&init_user_ns, inode,
 						     CAP_DAC_READ_SEARCH))
 				return 0;
-		if (capable_wrt_inode_uidgid(inode, CAP_DAC_OVERRIDE))
+		if (capable_wrt_inode_uidgid(&init_user_ns, inode,
+					     CAP_DAC_OVERRIDE))
 			return 0;
 		return -EACCES;
 	}
@@ -370,7 +371,8 @@ int generic_permission(struct inode *inode, int mask)
 	 */
 	mask &= MAY_READ | MAY_WRITE | MAY_EXEC;
 	if (mask == MAY_READ)
-		if (capable_wrt_inode_uidgid(inode, CAP_DAC_READ_SEARCH))
+		if (capable_wrt_inode_uidgid(&init_user_ns, inode,
+					     CAP_DAC_READ_SEARCH))
 			return 0;
 	/*
 	 * Read/write DACs are always overridable.
@@ -378,7 +380,8 @@ int generic_permission(struct inode *inode, int mask)
 	 * at least one exec bit set.
 	 */
 	if (!(mask & MAY_EXEC) || (inode->i_mode & S_IXUGO))
-		if (capable_wrt_inode_uidgid(inode, CAP_DAC_OVERRIDE))
+		if (capable_wrt_inode_uidgid(&init_user_ns, inode,
+					     CAP_DAC_OVERRIDE))
 			return 0;
 
 	return -EACCES;
@@ -2659,7 +2662,7 @@ int __check_sticky(struct inode *dir, struct inode *inode)
 		return 0;
 	if (uid_eq(dir->i_uid, fsuid))
 		return 0;
-	return !capable_wrt_inode_uidgid(inode, CAP_FOWNER);
+	return !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FOWNER);
 }
 EXPORT_SYMBOL(__check_sticky);
 

fs/overlayfs/super.c

@@ -1017,7 +1017,7 @@ ovl_posix_acl_xattr_set(const struct xattr_handler *handler,
 	if (unlikely(inode->i_mode & S_ISGID) &&
 	    handler->flags == ACL_TYPE_ACCESS &&
 	    !in_group_p(inode->i_gid) &&
-	    !capable_wrt_inode_uidgid(inode, CAP_FSETID)) {
+	    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID)) {
 		struct iattr iattr = { .ia_valid = ATTR_KILL_SGID };
 
 		err = ovl_setattr(dentry, &iattr);

fs/posix_acl.c

@@ -656,7 +656,7 @@ int posix_acl_update_mode(struct inode *inode, umode_t *mode_p,
 	if (error == 0)
 		*acl = NULL;
 	if (!in_group_p(inode->i_gid) &&
-	    !capable_wrt_inode_uidgid(inode, CAP_FSETID))
+	    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
 		mode &= ~S_ISGID;
 	*mode_p = mode;
 	return 0;

fs/xfs/xfs_ioctl.c

@@ -1502,7 +1502,7 @@ xfs_ioctl_setattr(
 	 */
 
 	if ((VFS_I(ip)->i_mode & (S_ISUID|S_ISGID)) &&
-	    !capable_wrt_inode_uidgid(VFS_I(ip), CAP_FSETID))
+	    !capable_wrt_inode_uidgid(&init_user_ns, VFS_I(ip), CAP_FSETID))
 		VFS_I(ip)->i_mode &= ~(S_ISUID|S_ISGID);
 
 	/* Change the ownerships and register project quota modifications */

include/linux/capability.h

@@ -247,8 +247,11 @@ static inline bool ns_capable_setid(struct user_namespace *ns, int cap)
 	return true;
 }
 #endif /* CONFIG_MULTIUSER */
-extern bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct inode *inode);
-extern bool capable_wrt_inode_uidgid(const struct inode *inode, int cap);
+bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
+				 struct user_namespace *mnt_userns,
+				 const struct inode *inode);
+bool capable_wrt_inode_uidgid(struct user_namespace *mnt_userns,
+			      const struct inode *inode, int cap);
 extern bool file_ns_capable(const struct file *file, struct user_namespace *ns, int cap);
 extern bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns);
 static inline bool perfmon_capable(void)

kernel/capability.c

@@ -484,10 +484,12 @@ EXPORT_SYMBOL(file_ns_capable);
  *
  * Return true if the inode uid and gid are within the namespace.
  */
-bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct inode *inode)
+bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
+				 struct user_namespace *mnt_userns,
+				 const struct inode *inode)
 {
-	return kuid_has_mapping(ns, inode->i_uid) &&
-		kgid_has_mapping(ns, inode->i_gid);
+	return kuid_has_mapping(ns, i_uid_into_mnt(mnt_userns, inode)) &&
+	       kgid_has_mapping(ns, i_gid_into_mnt(mnt_userns, inode));
 }
 
 /**
@@ -499,11 +501,13 @@ bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct inode *
  * its own user namespace and that the given inode's uid and gid are
  * mapped into the current user namespace.
  */
-bool capable_wrt_inode_uidgid(const struct inode *inode, int cap)
+bool capable_wrt_inode_uidgid(struct user_namespace *mnt_userns,
+			      const struct inode *inode, int cap)
 {
 	struct user_namespace *ns = current_user_ns();
 
-	return ns_capable(ns, cap) && privileged_wrt_inode_uidgid(ns, inode);
+	return ns_capable(ns, cap) &&
+	       privileged_wrt_inode_uidgid(ns, mnt_userns, inode);
 }
 EXPORT_SYMBOL(capable_wrt_inode_uidgid);
 

security/commoncap.c

@@ -489,7 +489,7 @@ int cap_convert_nscap(struct dentry *dentry, const void **ivalue, size_t size)
 		return -EINVAL;
 	if (!validheader(size, cap))
 		return -EINVAL;
-	if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
+	if (!capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_SETFCAP))
 		return -EPERM;
 	if (size == XATTR_CAPS_SZ_2)
 		if (ns_capable(inode->i_sb->s_user_ns, CAP_SETFCAP))
@@ -956,7 +956,8 @@ int cap_inode_removexattr(struct dentry *dentry, const char *name)
 		struct inode *inode = d_backing_inode(dentry);
 		if (!inode)
 			return -EINVAL;
-		if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
+		if (!capable_wrt_inode_uidgid(&init_user_ns, inode,
+					      CAP_SETFCAP))
 			return -EPERM;
 		return 0;
 	}