[PATCH 03/11] fs: Allow superblock owner to change ownership of inodes

[Dongsu Park]

From: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>

Allow users with CAP_SYS_CHOWN over the superblock of a filesystem to
chown files.  Ordinarily the capable_wrt_inode_uidgid check is
sufficient to allow access to files but when the underlying filesystem
has uids or gids that don't map to the current user namespace it is
not enough, so the chown permission checks need to be extended to
allow this case.

Calling chown on filesystem nodes whose uid or gid don't map is
necessary if those nodes are going to be modified as writing back
inodes which contain uids or gids that don't map is likely to cause
filesystem corruption of the uid or gid fields.

Once chown has been called the existing capable_wrt_inode_uidgid
checks are sufficient, to allow the owner of a superblock to do anything
the global root user can do with an appropriate set of capabilities.

For the proc filesystem this relaxation of permissions is not safe, as
some files are owned by users (particularly GLOBAL_ROOT_UID) outside
of the control of the mounter of the proc and that would be unsafe to
grant chown access to.  So update setattr on proc to disallow changing
files whose uids or gids are outside of proc's s_user_ns.

The original version of this patch was written by: Seth Forshee.  I
have rewritten and rethought this patch enough so it's really not the
same thing (certainly it needs a different description), but he
deserves credit for getting out there and getting the conversation
started, and finding the potential gotcha's and putting up with my
semi-paranoid feedback.

Patch v4 is available: https://patchwork.kernel.org/patch/8944611/

Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: "Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Inspired-by: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
[saf: Resolve conflicts caused by s/inode_change_ok/setattr_prepare/]
Signed-off-by: Dongsu Park <dongsu@xxxxxxxxxx>
---
 fs/attr.c             | 34 ++++++++++++++++++++++++++--------
 fs/proc/base.c        |  7 +++++++
 fs/proc/generic.c     |  7 +++++++
 fs/proc/proc_sysctl.c |  7 +++++++
 4 files changed, 47 insertions(+), 8 deletions(-)

diff --git a/fs/attr.c b/fs/attr.c
index 12ffdb6f..bf8e94f3 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -18,6 +18,30 @@
 #include <linux/evm.h>
 #include <linux/ima.h>
 
+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))
+		return true;
+	if (ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
+		return true;
+	return false;
+}
+
+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))
+		return true;
+	if (ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
+		return true;
+	return false;
+}
+
 /**
  * setattr_prepare - check if attribute changes to a dentry are allowed
  * @dentry:	dentry to check
@@ -52,17 +76,11 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
 		goto kill_priv;
 
 	/* Make sure a caller can chown. */
-	if ((ia_valid & ATTR_UID) &&
-	    (!uid_eq(current_fsuid(), inode->i_uid) ||
-	     !uid_eq(attr->ia_uid, inode->i_uid)) &&
-	    !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
+	if ((ia_valid & ATTR_UID) && !chown_ok(inode, attr->ia_uid))
 		return -EPERM;
 
 	/* Make sure caller can chgrp. */
-	if ((ia_valid & ATTR_GID) &&
-	    (!uid_eq(current_fsuid(), inode->i_uid) ||
-	    (!in_group_p(attr->ia_gid) && !gid_eq(attr->ia_gid, inode->i_gid))) &&
-	    !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
+	if ((ia_valid & ATTR_GID) && !chgrp_ok(inode, attr->ia_gid))
 		return -EPERM;
 
 	/* Make sure a caller can chmod. */
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 31934cb9..9d50ec92 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -665,10 +665,17 @@ int proc_setattr(struct dentry *dentry, struct iattr *attr)
 {
 	int error;
 	struct inode *inode = d_inode(dentry);
+	struct user_namespace *s_user_ns;
 
 	if (attr->ia_valid & ATTR_MODE)
 		return -EPERM;
 
+	/* Don't let anyone mess with weird proc files */
+	s_user_ns = inode->i_sb->s_user_ns;
+	if (!kuid_has_mapping(s_user_ns, inode->i_uid) ||
+	    !kgid_has_mapping(s_user_ns, inode->i_gid))
+		return -EPERM;
+
 	error = setattr_prepare(dentry, attr);
 	if (error)
 		return error;
diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index 793a6757..527d46c8 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -106,8 +106,15 @@ static int proc_notify_change(struct dentry *dentry, struct iattr *iattr)
 {
 	struct inode *inode = d_inode(dentry);
 	struct proc_dir_entry *de = PDE(inode);
+	struct user_namespace *s_user_ns;
 	int error;
 
+	/* Don't let anyone mess with weird proc files */
+	s_user_ns = inode->i_sb->s_user_ns;
+	if (!kuid_has_mapping(s_user_ns, inode->i_uid) ||
+	    !kgid_has_mapping(s_user_ns, inode->i_gid))
+		return -EPERM;
+
 	error = setattr_prepare(dentry, iattr);
 	if (error)
 		return error;
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index c5cbbdff..0f9562d1 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -802,11 +802,18 @@ static int proc_sys_permission(struct inode *inode, int mask)
 static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr)
 {
 	struct inode *inode = d_inode(dentry);
+	struct user_namespace *s_user_ns;
 	int error;
 
 	if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
 		return -EPERM;
 
+	/* Don't let anyone mess with weird proc files */
+	s_user_ns = inode->i_sb->s_user_ns;
+	if (!kuid_has_mapping(s_user_ns, inode->i_uid) ||
+	    !kgid_has_mapping(s_user_ns, inode->i_gid))
+		return -EPERM;
+
 	error = setattr_prepare(dentry, attr);
 	if (error)
 		return error;
-- 
2.13.6