Re: [PATCH v6 3/4] fs: strip file's S_ISGID mode on vfs instead of on underlying filesystem

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Apr 25, 2022 at 11:09:40AM +0800, Yang Xu wrote:
> 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 filesystem 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
> "
> arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
> arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
> fs/9p/vfs_inode.c:      inode_init_owner(&init_user_ns, inode, NULL, mode);
> fs/bfs/dir.c:   inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/btrfs/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
> fs/btrfs/tests/btrfs-tests.c:   inode_init_owner(&init_user_ns, inode, NULL, S_IFREG);
> fs/ext2/ialloc.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ext4/ialloc.c:               inode_init_owner(mnt_userns, inode, dir, mode);
> fs/f2fs/namei.c:        inode_init_owner(mnt_userns, inode, dir, mode);
> fs/hfsplus/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/hugetlbfs/inode.c:           inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/jfs/jfs_inode.c:     inode_init_owner(&init_user_ns, inode, parent, mode);
> fs/minix/bitmap.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/nilfs2/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ntfs3/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
> fs/ocfs2/dlmfs/dlmfs.c:         inode_init_owner(&init_user_ns, inode, NULL, mode);
> fs/ocfs2/dlmfs/dlmfs.c: inode_init_owner(&init_user_ns, inode, parent, mode);
> fs/ocfs2/namei.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/omfs/inode.c:        inode_init_owner(&init_user_ns, inode, NULL, mode);
> fs/overlayfs/dir.c:     inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
> fs/ramfs/inode.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/reiserfs/namei.c:    inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/sysv/ialloc.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ubifs/dir.c: inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/udf/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ufs/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/xfs/xfs_inode.c:             inode_init_owner(mnt_userns, inode, dir, mode);
> fs/zonefs/super.c:      inode_init_owner(&init_user_ns, inode, parent, S_IFDIR | 0555);
> kernel/bpf/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
> mm/shmem.c:             inode_init_owner(&init_user_ns, inode, dir, mode);
> "
> 
> They are used in filesystem to 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 prepare_mode() 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 new helper 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, four 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 this ioctl
>    is only useful when backport reflink features to old kernels. ocfs2 still use vfs
>    remap_range code to do reflink.
> 3) spufs which doesn't really go hrough the regular VFS callpath because it has
>    separate system call spu_create, but it t only allows the creation of
>    directories and only allows bits in 0777 and can ignore
> 4) bpf use vfs_mkobj in bpf_obj_do_pin with
>    "S_IFREG | ((S_IRUSR | S_IWUSR) & ~current_umask()) mode and
>    use bpf_mkobj_ops in bpf_iter_link_pin_kernel with S_IFREG | S_IRUSR mode,
>    so bpf is also not affected
> 
> 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>
> ---
>  fs/inode.c         |  2 --
>  fs/namei.c         | 22 +++++++++-------------
>  fs/ocfs2/namei.c   |  1 +
>  include/linux/fs.h | 11 +++++++++++
>  4 files changed, 21 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 78e7ef567e04..041c0837f248 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
> -			mode = 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 73646e28fae0..5b8e6288d503 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();
> +		mode = 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();
> +	mode = prepare_mode(mnt_userns, dir, mode);
>  	error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
>  	if (error)
>  		goto out_err;
> @@ -3850,13 +3848,12 @@ 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);
> +	mode = 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,
> @@ -3943,6 +3940,7 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
>  	struct path path;
>  	int error;
>  	unsigned int lookup_flags = LOOKUP_DIRECTORY;
> +	struct user_namespace *mnt_userns;
>  
>  retry:
>  	dentry = filename_create(dfd, name, &path, lookup_flags);
> @@ -3950,15 +3948,13 @@ 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();
> +	mnt_userns = mnt_user_ns(path.mnt);
> +	mode = prepare_mode(mnt_userns, path.dentry->d_inode, mode);
>  	error = security_path_mkdir(&path, dentry, mode);
> -	if (!error) {
> -		struct user_namespace *mnt_userns;
> -		mnt_userns = mnt_user_ns(path.mnt);
> +	if (!error)
>  		error = vfs_mkdir(mnt_userns, path.dentry->d_inode, dentry,
>  				  mode);
> -	}
> +
>  	done_path_create(&path, dentry);
>  	if (retry_estale(error, lookup_flags)) {
>  		lookup_flags |= LOOKUP_REVAL;
> diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
> index c75fd54b9185..21f3da2e66c9 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);
> +	mode = inode_sgid_strip(&init_user_ns, dir, mode);
>  	inode_init_owner(&init_user_ns, inode, dir, mode);
>  	status = dquot_initialize(inode);
>  	if (status)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 532de76c9b91..ca70cdf9c9e2 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3459,6 +3459,17 @@ static inline bool dir_relax_shared(struct inode *inode)
>  	return !IS_DEADDIR(inode);
>  }
>  
> +static inline umode_t prepare_mode(struct user_namespace *mnt_userns,

You probably ought to make this more obviously a *file* mode preparation
function by naming it "vfs_prepare_mode" or something.

--D

> +				   const struct inode *dir, umode_t mode)
> +{
> +	mode = inode_sgid_strip(mnt_userns, dir, mode);
> +
> +	if (!IS_POSIXACL(dir))
> +		mode &= ~current_umask();
> +
> +	return mode;
> +}
> +
>  extern bool path_noexec(const struct path *path);
>  extern void inode_nohighmem(struct inode *inode);
>  
> -- 
> 2.27.0
>