Re: [PATCH v8 3/4] fs: move S_ISGID stripping into the vfs

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On Tue, Apr 26, 2022 at 07:11:29PM +0800, Yang Xu wrote:
> Creating files that have both the S_IXGRP and S_ISGID bit raised in
> directories that themselves have the S_ISGID bit set requires additional
> privileges to avoid security issues.
> 
> When a filesystem creates a new inode it needs to take care that the
> caller is either in the group of the newly created inode or they have
> CAP_FSETID in their current user namespace and are privileged over the
> parent directory of the new inode. If any of these two conditions is
> true then the S_ISGID bit can be raised for an S_IXGRP file and if not
> it needs to be stripped.
> 
> However, there are several key issues with the current state of things:
> 
> * The S_ISGID stripping logic is entangled with umask stripping.
> 
>   If a filesystem doesn't support or enable POSIX ACLs then umask
>   stripping is done directly in the vfs before calling into the
>   filesystem.
>   If the filesystem does support POSIX ACLs then unmask stripping may be
>   done in the filesystem itself when calling posix_acl_create().
> 
> * Filesystems that don't rely on inode_init_owner() don't get S_ISGID
>   stripping logic.
> 
>   While that may be intentional (e.g. network filesystems might just
>   defer setgid stripping to a server) it is often just a security issue.
> 
> * The first two points taken together mean that there's a
>   non-standardized ordering between setgid stripping in
>   inode_init_owner() and posix_acl_create() both on the vfs level and
>   the filesystem level. The latter part is especially problematic since
>   each filesystem is technically free to order inode_init_owner() and
>   posix_acl_create() however it sees fit meaning that S_ISGID
>   inheritance might or might not be applied.
> 
> * We do still have bugs in this areas years after the initial round of
>   setgid bugfixes.
> 
> So the current state is quite messy and while we won't be able to make
> it completely clean as posix_acl_create() is still a filesystem specific
> call we can improve the S_SIGD stripping situation quite a bit by
> hoisting it out of inode_init_owner() and into the vfs creation
> operations. This means we alleviate the burden for filesystems to handle
> S_ISGID stripping correctly and can standardize the ordering between
> S_ISGID and umask stripping in the vfs.
> 
> The S_ISGID bit is stripped before any umask is applied. This has the
> advantage that the ordering is unaffected by whether umask stripping is
> done by the vfs itself (if no POSIX ACLs are supported or enabled) or in
> the filesystem in posix_acl_create() (if POSIX ACLs are supported).
> 
> To this end a new helper vfs_prepare_mode() is added which calls the
> previously added mode_strip_setgid() helper and strips the umask
> afterwards.
> 
> All inode operations that create new filesystem objects have been
> updated to call vfs_prepare_mode() before passing the mode into the
> relevant inode operation of the filesystems. Care has been taken to
> ensure that the mode passed to the security hooks is the mode that is
> seen by the filesystem.
> 
> Following is an overview of the filesystem specific and inode operations
> specific implications:
> 
> 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);
> 
> All of the above filesystems end up calling inode_init_owner() when new
> filesystem objects are created through the following ->mkdir(),
> ->symlink(), ->mknod(), ->create(), ->tmpfile(), ->rename() inode
> operations.
> 
> Since directories always inherit the S_ISGID bit with the exception of
> xfs when irix_sgid_inherit mode is turned on S_ISGID stripping doesn't
> apply. The ->symlink() inode operation trivially inherit the mode from
> the target and the ->rename() inode operation inherits the mode from the
> source inode.
> 
> All other inode operations will have the S_ISGID bit stripped once in
> vfs_prepare_mode() before.
> 
> In addition to this there are filesystems which allow the creation of
> filesystem objects through ioctl()s or - in the case of spufs -
> circumventing the vfs in other ways. If filesystem objects are created
> through ioctl()s the vfs doesn't know about it and can't apply regular
> permission checking including S_ISGID logic. Therfore, a filesystem
> relying on S_ISGID stripping in inode_init_owner() in their ioctl()
> callpath will be affected by moving this logic into the vfs.
> 
> So we did our best to audit all filesystems in this regard:
> 
> * btrfs allows the creation of filesystem objects through various
>   ioctls(). Snapshot creation literally takes a snapshot and so the mode
>   is fully preserved and S_ISGID stripping doesn't apply.
> 
>   Creating a new subvolum relies on inode_init_owner() in
>   btrfs_new_inode() but only creates directories and doesn't raise
>   S_ISGID.
> 
> * ocfs2 has a peculiar implementation of reflinks. In contrast to e.g.
>   xfs and btrfs FICLONE/FICLONERANGE ioctl() that is only concerned with
>   the actual extents ocfs2 uses a separate ioctl() that also creates the
>   target file.
> 
>   Iow, ocfs2 circumvents the vfs entirely here and did indeed rely on
>   inode_init_owner() to strip the S_ISGID bit. This is the only place
>   where a filesystem needs to call mode_strip_sgid() directly but this
>   is self-inflicted pain tbh.
> 
> * spufs doesn't go through the vfs at all and doesn't use ioctl()s
>   either. Instead it has a dedicated system call spufs_create() which
>   allows the creation of filesystem objects. But spufs only creates
>   directories and doesn't allo S_SIGID bits, i.e. it specifically only
>   allows 0777 bits.
> 
> * bpf uses vfs_mkobj() but also doesn't allow S_ISGID bits to be created.
> 
> While we did our best to audit everything there's a risk of regressions
> in here. However, for the sake of maintenance and given that we've seen
> a range of bugs years after S_ISGID inheritance issues were fixed (see
> [1]-[3]) the risk seems worth taking. In the worst case we will have to
> revert.
> 
> Associated with this change is a new set of fstests to enforce the
> semantics for all new filesystems.
> 
> Link: e014f37db1a2 ("xfs: use setattr_copy to set vfs inode attributes") [1]
> Link: 01ea173e103e ("xfs: fix up non-directory creation in SGID directories") [2]
> Link: fd84bfdddd16 ("ceph: fix up non-directory creation in SGID directories") [3]
> Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>
> Suggested-by: Dave Chinner <david@xxxxxxxxxxxxx>
> Signed-off-by: Yang Xu <xuyang2018.jy@xxxxxxxxxxx>
> ---

Thanks for using my commit message!

One thing that I just remembered and which I think I haven't mentioned
so far is that moving S_ISGID stripping from filesystem callpaths into
the vfs callpaths means that we're hoisting this logic out of vfs_*()
helpers implicitly.

So filesystems that call vfs_*() helpers directly can't rely on S_ISGID
stripping being done in vfs_*() helpers anymore unless they pass the
mode on from a prior run through the vfs.

This mostly affects overlayfs which calls vfs_*() functions directly. So
a typical overlayfs callstack would be (roughly - I'm omw to lunch):

sys_mknod()
-> do_mknodat(mode) // calls vfs_prepare_mode()
   -> .mknod = ovl_mknod(mode)
      -> ovl_create(mode)
         -> vfs_mknod(mode)

I think we are safe as overlayfs passes on the mode on from its own run
through the vfs and then via vfs_*() to the underlying filesystem but it
is worth point that out.

Ccing Amir just for confirmation.

>  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 e9a5f2ec2f89..dd357f4b556d 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 = mode_strip_sgid(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..5dbf00704ae8 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 = vfs_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 = vfs_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 = vfs_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 = vfs_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..961d1cf54388 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 = mode_strip_sgid(&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 98b44a2732f5..914c8f28bb02 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 vfs_prepare_mode(struct user_namespace *mnt_userns,
> +				   const struct inode *dir, umode_t mode)
> +{
> +	mode = mode_strip_sgid(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
> 



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