[PATCH v8 1/4] fs: add mode_strip_sgid() helper

[Yang Xu <xuyang2018.jy@fujitsu.com>]

Add a dedicated helper to handle the setgid bit when creating a new file
in a setgid directory. This is a preparatory patch for moving setgid
stripping into the vfs. The patch contains no functional changes.

Currently the setgid stripping logic is open-coded directly in
inode_init_owner() and the individual filesystems are responsible for
handling setgid inheritance. Since this has proven to be brittle as
evidenced by old issues we uncovered over the last months (see [1] to
[3] below) we will try to move this logic into the vfs.

Link: e014f37db1a2 ("xfs: use setattr_copy to set vfs inode attributes") [1]
Link: 01ea173e103e ("xfs: fix up non-directory creation in SGID directories") [2]
Link: fd84bfdddd16 ("ceph: fix up non-directory creation in SGID directories") [3]
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
---
 fs/inode.c         | 37 +++++++++++++++++++++++++++++++++----
 include/linux/fs.h |  2 ++
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 9d9b422504d1..e9a5f2ec2f89 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2246,10 +2246,8 @@ void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
 		/* Directories are special, and always inherit S_ISGID */
 		if (S_ISDIR(mode))
 			mode |= S_ISGID;
-		else if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP) &&
-			 !in_group_p(i_gid_into_mnt(mnt_userns, dir)) &&
-			 !capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
-			mode &= ~S_ISGID;
+		else
+			mode = mode_strip_sgid(mnt_userns, dir, mode);
 	} else
 		inode_fsgid_set(inode, mnt_userns);
 	inode->i_mode = mode;
@@ -2405,3 +2403,34 @@ struct timespec64 current_time(struct inode *inode)
 	return timestamp_truncate(now, inode);
 }
 EXPORT_SYMBOL(current_time);
+
+/**
+ * mode_strip_sgid - handle the sgid bit for non-directories
+ * @mnt_userns: User namespace of the mount the inode was created from
+ * @dir: parent directory inode
+ * @mode: mode of the file to be created in @dir
+ *
+ * If the @mode of the new file has both the S_ISGID and S_IXGRP bit
+ * raised and @dir has the S_ISGID bit raised ensure that the caller is
+ * either in the group of the parent directory or they have CAP_FSETID
+ * in their user namespace and are privileged over the parent directory.
+ * In all other cases, strip the S_ISGID bit from @mode.
+ *
+ * Return: the new mode to use for the file
+ */
+umode_t mode_strip_sgid(struct user_namespace *mnt_userns,
+			 const struct inode *dir, umode_t mode)
+{
+	if (S_ISDIR(mode) || !dir || !(dir->i_mode & S_ISGID))
+		return mode;
+	if ((mode & (S_ISGID | S_IXGRP)) != (S_ISGID | S_IXGRP))
+		return mode;
+	if (in_group_p(i_gid_into_mnt(mnt_userns, dir)))
+		return mode;
+	if (capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
+		return mode;
+
+	mode &= ~S_ISGID;
+	return mode;
+}
+EXPORT_SYMBOL(mode_strip_sgid);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index bbde95387a23..98b44a2732f5 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1897,6 +1897,8 @@ extern long compat_ptr_ioctl(struct file *file, unsigned int cmd,
 void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
 		      const struct inode *dir, umode_t mode);
 extern bool may_open_dev(const struct path *path);
+umode_t mode_strip_sgid(struct user_namespace *mnt_userns,
+			 const struct inode *dir, umode_t mode);
 
 /*
  * This is the "filldir" function type, used by readdir() to let
-- 
2.27.0