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

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

on 2022/4/26 0:51, Darrick J. Wong wrote:
> 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.

I will use vfs_prepare_mode().

>
> --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
>>