On Sat, May 21, 2022 at 12:10:36AM +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.
>
> Moving S_ISGID stripping from filesystem callpaths into the vfs callpaths
> means thatfilesystems 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
> sys_mknod()
> -> do_mknodat(mode) // calls vfs_prepare_mode()
> -> .mknod = ovl_mknod(mode)
> -> ovl_create(mode)
> -> vfs_mknod(mode)
>
> But it is safe as overlayfs passes on the mode on from its own run
> through the vfs and then via vfs_*() to the underlying 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.
>
> This patch also changed grpid behaviour for ext4/xfs because the mode
> passed to them may have been changed by vfs_prepare_mode.
>
> 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>
> Reviewed-and-Tested-by: Jeff Layton <jlayton@xxxxxxxxxx>
> Signed-off-by: Yang Xu <xuyang2018.jy@xxxxxxxxxxx>
> ---
> v8->v9:
> 1.move vfs_prepare_mode info fs/namei.c
> 2. add grpid behaviour change in commit message
> 3. also mention the overflay in commit meessage because it will use call vfs_mknod directly
> fs/inode.c | 2 --
> fs/namei.c | 33 ++++++++++++++++++++-------------
> fs/ocfs2/namei.c | 1 +
> 3 files changed, 21 insertions(+), 15 deletions(-)
>
> diff --git a/fs/inode.c b/fs/inode.c
> index 37bd85981d38..42ecaf79aaaf 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..8b60914861f5 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2998,6 +2998,17 @@ void unlock_rename(struct dentry *p1, struct dentry *p2)
> }
> EXPORT_SYMBOL(unlock_rename);
>
> +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;
> +}
> +
> /**
> * vfs_create - create new file
> * @mnt_userns: user namespace of the mount the inode was found from
> @@ -3287,8 +3298,7 @@ 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 +3531,7 @@ 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 +3859,12 @@ 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);
This needs to move into vfs_mknod() itself to allow overlayfs to
continue to rely on setgid stripping.
See the relevant part in the _draft_ patch in [1].
> 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 +3951,7 @@ 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 +3959,13 @@ 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);
This needs to move into vfs_mkdir() itself to allow overlayfs to
continue to rely on setgid stripping.
See the relevant part in the _draft_ patch in [1].
[1]: https://lore.kernel.org/ceph-devel/20220427092201.wvsdjbnc7b4dttaw@wittgenstein
