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

["xuyang2018.jy@fujitsu.com" <xuyang2018.jy@fujitsu.com>]

on 2022/4/15 22:24, Christian Brauner wrote:
> On Fri, Apr 15, 2022 at 07:02:22PM +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 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);
>
> Your changes causes the security and the filesystem layer to potentially
> see different values for mode.
>
> You need to change the patch so prepare_mode() is called before the mode
> is passed to the security layer. This will ensure that both the security
> layer and the vfs see the same mode.

Will move it before security_path_mkdir.

Best Regards
Yang Xu
>
>>   	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
>>