[PATCH 5.10 90/99] fs: add mode_strip_sgid() helper

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Yang Xu <xuyang2018.jy@fujitsu.com>

commit 2b3416ceff5e6bd4922f6d1c61fb68113dd82302 upstream.

[remove userns argument of helper for 5.10.y backport]

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]
Link: https://lore.kernel.org/r/1657779088-2242-1-git-send-email-xuyang2018.jy@fujitsu.com
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-and-Tested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/inode.c         |   34 ++++++++++++++++++++++++++++++----
 include/linux/fs.h |    1 +
 2 files changed, 31 insertions(+), 4 deletions(-)

--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2147,10 +2147,8 @@ void inode_init_owner(struct inode *inod
 		/* 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(inode->i_gid) &&
-			 !capable_wrt_inode_uidgid(dir, CAP_FSETID))
-			mode &= ~S_ISGID;
+		else
+			mode = mode_strip_sgid(dir, mode);
 	} else
 		inode->i_gid = current_fsgid();
 	inode->i_mode = mode;
@@ -2382,3 +2380,31 @@ int vfs_ioc_fssetxattr_check(struct inod
 	return 0;
 }
 EXPORT_SYMBOL(vfs_ioc_fssetxattr_check);
+
+/**
+ * mode_strip_sgid - handle the sgid bit for non-directories
+ * @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(const struct inode *dir, umode_t mode)
+{
+	if ((mode & (S_ISGID | S_IXGRP)) != (S_ISGID | S_IXGRP))
+		return mode;
+	if (S_ISDIR(mode) || !dir || !(dir->i_mode & S_ISGID))
+		return mode;
+	if (in_group_p(dir->i_gid))
+		return mode;
+	if (capable_wrt_inode_uidgid(dir, CAP_FSETID))
+		return mode;
+
+	return mode & ~S_ISGID;
+}
+EXPORT_SYMBOL(mode_strip_sgid);
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1768,6 +1768,7 @@ extern long compat_ptr_ioctl(struct file
 extern void inode_init_owner(struct inode *inode, const struct inode *dir,
 			umode_t mode);
 extern bool may_open_dev(const struct path *path);
+umode_t mode_strip_sgid(const struct inode *dir, umode_t mode);
 
 /*
  * This is the "filldir" function type, used by readdir() to let