On Tue, Apr 26, 2022 at 1:38 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> 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.
Looks fine to me, but CC Miklos ...
Thanks,
Amir.
>
> > 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
> >
