[PATCH v2 2/3] vfs: strip file's S_ISGID mode on vfs instead of on underlying filesystem

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

Currently, vfs only passes mode argument to filesystem, then use inode_init_owner()
to strip S_ISGID. Some filesystem(ie ext4/btrfs) will call inode_init_owner
firstly, then posxi acl setup, but xfs uses the contrary order. It will affect
S_ISGID clear especially we filter S_IXGRP by umask or acl.

Regardless of which filesystem is in use, failure to strip the SGID correctly is
considered a security failure that needs to be fixed. The current VFS infrastructure
requires the filesystem to do everything right and not step on any landmines to
strip the SGID bit, when in fact it can easily be done at the VFS and the filesystems
then don't even need to be aware that the SGID needs to be (or has been stripped) by
the operation the user asked to be done.

Vfs has all the info it needs - it doesn't need the filesystems to do everything
correctly with the mode and ensuring that they order things like posix acl setup
functions correctly with inode_init_owner() to strip the SGID bit.

Just strip the SGID bit at the VFS, and then the filesystems can't get it wrong.

Also, the inode_sgid_strip() api should be used before IS_POSIXACL() because
this api may change mode.

Only the following places use inode_init_owner
"hugetlbfs/inode.c:846:          inode_init_owner(&init_user_ns, inode, dir, mode);
 nilfs2/inode.c:354:     inode_init_owner(&init_user_ns, inode, dir, mode);
 zonefs/super.c:1289:    inode_init_owner(&init_user_ns, inode, parent, S_IFDIR | 0555);
 reiserfs/namei.c:619:   inode_init_owner(&init_user_ns, inode, dir, mode);
 jfs/jfs_inode.c:67:     inode_init_owner(&init_user_ns, inode, parent, mode);
 f2fs/namei.c:50:        inode_init_owner(mnt_userns, inode, dir, mode);
 ext2/ialloc.c:549:              inode_init_owner(&init_user_ns, inode, dir, mode);
 overlayfs/dir.c:643:    inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
 ufs/ialloc.c:292:       inode_init_owner(&init_user_ns, inode, dir, mode);
 ntfs3/inode.c:1283:     inode_init_owner(mnt_userns, inode, dir, mode);
 ramfs/inode.c:64:               inode_init_owner(&init_user_ns, inode, dir, mode);
 9p/vfs_inode.c:263:     inode_init_owner(&init_user_ns, inode, NULL, mode);
 btrfs/tests/btrfs-tests.c:65:   inode_init_owner(&init_user_ns, inode, NULL, S_IFREG);
 btrfs/inode.c:6215:     inode_init_owner(mnt_userns, inode, dir, mode);
 sysv/ialloc.c:166:      inode_init_owner(&init_user_ns, inode, dir, mode);
 omfs/inode.c:51:        inode_init_owner(&init_user_ns, inode, NULL, mode);
 ubifs/dir.c:97: inode_init_owner(&init_user_ns, inode, dir, mode);
 udf/ialloc.c:108:       inode_init_owner(&init_user_ns, inode, dir, mode);
 ext4/ialloc.c:979:              inode_init_owner(mnt_userns, inode, dir, mode);
 hfsplus/inode.c:393:    inode_init_owner(&init_user_ns, inode, dir, mode);
 xfs/xfs_inode.c:840:            inode_init_owner(mnt_userns, inode, dir, mode);
 ocfs2/dlmfs/dlmfs.c:331:                inode_init_owner(&init_user_ns, inode, NULL, mode);
 ocfs2/dlmfs/dlmfs.c:354:        inode_init_owner(&init_user_ns, inode, parent, mode);
 ocfs2/namei.c:200:      inode_init_owner(&init_user_ns, inode, dir, mode);
 minix/bitmap.c:255:     inode_init_owner(&init_user_ns, inode, dir, mode);
 bfs/dir.c:99:   inode_init_owner(&init_user_ns, inode, dir, mode);
"

They are used in filesystem init new inode function and these init inode functions are used
by following operations:
mkdir
symlink
mknod
create
tmpfile
rename

We don't care about mkdir because we don't strip SGID bit for directory except fs.xfs.irix_sgid_inherit.
symlink and rename only use valid mode that doesn't have SGID bit.

We have added inode_sgid_strip api for the remaining operations.

In addition to the above six operations, two filesystems has a little difference
1) btrfs has btrfs_create_subvol_root to create new inode but used non SGID bit mode and can ignore
2) ocfs2 reflink function should add inode_sgid_strip api manually because we don't add it in vfs

Last but not least, this patch also changed grpid behaviour for ext4/xfs because the mode passed to
them may been changed by inode_sgid_strip.

Suggested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
---
 fs/inode.c       | 4 ----
 fs/namei.c       | 5 ++++-
 fs/ocfs2/namei.c | 1 +
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index d63264998855..b08bdd73e116 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2246,10 +2246,6 @@ 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
 		inode_fsgid_set(inode, mnt_userns);
 	inode->i_mode = mode;
diff --git a/fs/namei.c b/fs/namei.c
index 3f1829b3ab5b..e03f7defdd30 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3287,6 +3287,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 	if (open_flag & O_CREAT) {
 		if (open_flag & O_EXCL)
 			open_flag &= ~O_TRUNC;
+		inode_sgid_strip(mnt_userns, dir->d_inode, &mode);
 		if (!IS_POSIXACL(dir->d_inode))
 			mode &= ~current_umask();
 		if (likely(got_write))
@@ -3521,6 +3522,7 @@ struct dentry *vfs_tmpfile(struct user_namespace *mnt_userns,
 	child = d_alloc(dentry, &slash_name);
 	if (unlikely(!child))
 		goto out_err;
+	inode_sgid_strip(mnt_userns, dir, &mode);
 	error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
 	if (error)
 		goto out_err;
@@ -3850,13 +3852,14 @@ static int do_mknodat(int dfd, struct filename *name, umode_t mode,
 	if (IS_ERR(dentry))
 		goto out1;
 
+	mnt_userns = mnt_user_ns(path.mnt);
+	inode_sgid_strip(mnt_userns, path.dentry->d_inode, &mode);
 	if (!IS_POSIXACL(path.dentry->d_inode))
 		mode &= ~current_umask();
 	error = security_path_mknod(&path, dentry, mode, dev);
 	if (error)
 		goto out2;
 
-	mnt_userns = mnt_user_ns(path.mnt);
 	switch (mode & S_IFMT) {
 		case 0: case S_IFREG:
 			error = vfs_create(mnt_userns, path.dentry->d_inode,
diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
index c75fd54b9185..f1d626697302 100644
--- a/fs/ocfs2/namei.c
+++ b/fs/ocfs2/namei.c
@@ -197,6 +197,7 @@ static struct inode *ocfs2_get_init_inode(struct inode *dir, umode_t mode)
 	 * callers. */
 	if (S_ISDIR(mode))
 		set_nlink(inode, 2);
+	inode_sgid_strip(&init_user_ns, dir, &mode);
 	inode_init_owner(&init_user_ns, inode, dir, mode);
 	status = dquot_initialize(inode);
 	if (status)
-- 
2.27.0