[PATCH v3 6/7] fs: strip file's S_ISGID mode on vfs instead of on underlying filesystem

Yang Xu
2022-04-15T10:03:58Z

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.
But we even call it in do_mkdirat() since inode_sgid_strip() will skip directories anyway. This will
enforce the same ordering for all relevant operations and it will make the code more uniform and
easier to understand by using prepare_mode().

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

This patch also changed grpid behaviour for ext4/xfs because the mode passed to them may been
changed by inode_sgid_strip.

Also as Christian Brauner said"
The patch itself is useful as it would move a security sensitive operation that is currently burried in
individual filesystems into the vfs layer. But it has a decent regression potential since it might strip
filesystems that have so far relied on getting the S_ISGID bit with a mode argument. So this needs a lot
of testing and long exposure in -next for at least one full kernel cycle."

Suggested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
---
v2->v3:
1.use new helper prepare_mode to do inode sgid strip and umask strip
2.also use prepare_mode() for mkdirat
fs/inode.c | 2 --
fs/namei.c | 14 +++++---------
fs/ocfs2/namei.c | 1 +
3 files changed, 6 insertions(+), 11 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 1b569ad882ce..a250aa01d3c3 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2246,8 +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
- inode_sgid_strip(mnt_userns, dir, &mode);
} else
inode_fsgid_set(inode, mnt_userns);
inode->i_mode = mode;
diff --git a/fs/namei.c b/fs/namei.c
index bbc7c950bbdc..0fadc884af7f 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3287,8 +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;
- if (!IS_POSIXACL(dir->d_inode))
- mode &= ~current_umask();
+ prepare_mode(mnt_userns, dir->d_inode, &mode);
if (likely(got_write))
create_error = may_o_create(mnt_userns, &nd->path,
dentry, mode);
@@ -3521,8 +3520,7 @@ struct dentry *vfs_tmpfile(struct user_namespace *mnt_userns,
child = d_alloc(dentry, &slash_name);
if (unlikely(!child))
goto out_err;
- if (!IS_POSIXACL(dir))
- mode &= ~current_umask();
+ prepare_mode(mnt_userns, dir, &mode);
error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
if (error)
goto out_err;
@@ -3852,13 +3850,12 @@ static int do_mknodat(int dfd, struct filename *name, umode_t mode,
if (IS_ERR(dentry))
goto out1;

- if (!IS_POSIXACL(path.dentry->d_inode))
- mode &= ~current_umask();
+ mnt_userns = mnt_user_ns(path.mnt);
+ prepare_mode(mnt_userns, path.dentry->d_inode, &mode);
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,
@@ -3952,12 +3949,11 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
if (IS_ERR(dentry))
goto out_putname;

- if (!IS_POSIXACL(path.dentry->d_inode))
- mode &= ~current_umask();
error = security_path_mkdir(&path, dentry, mode);
if (!error) {
struct user_namespace *mnt_userns;
mnt_userns = mnt_user_ns(path.mnt);
+ prepare_mode(mnt_userns, path.dentry->d_inode, &mode);
error = vfs_mkdir(mnt_userns, path.dentry->d_inode, dentry,
mode);
}
diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
index c75fd54b9185..c81b8e0847aa 100644
--- a/fs/ocfs2/namei.c
+++ b/fs/ocfs2/namei.c
@@ -198,6 +198,7 @@ static struct inode *ocfs2_get_init_inode(struct inode *dir, umode_t mode)
if (S_ISDIR(mode))
set_nlink(inode, 2);
inode_init_owner(&init_user_ns, inode, dir, mode);
+ inode_sgid_strip(&init_user_ns, dir, &mode);
status = dquot_initialize(inode);
if (status)
return ERR_PTR(status);
--
2.27.0