On Fri, Oct 7, 2022 at 5:06 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> Currently setgid stripping in file_remove_privs()'s should_remove_suid()
> helper is inconsistent with other parts of the vfs. Specifically, it only
> raises ATTR_KILL_SGID if the inode is S_ISGID and S_IXGRP but not if the
> inode isn't in the caller's groups and the caller isn't privileged over the
> inode although we require this already in setattr_prepare() and
> setattr_copy() and so all filesystem implement this requirement implicitly
> because they have to use setattr_{prepare,copy}() anyway.
>
> But the inconsistency shows up in setgid stripping bugs for overlayfs in
> xfstests (e.g., generic/673, generic/683, generic/685, generic/686,
> generic/687). For example, we test whether suid and setgid stripping works
> correctly when performing various write-like operations as an unprivileged
> user (fallocate, reflink, write, etc.):
>
> echo "Test 1 - qa_user, non-exec file $verb"
> setup_testfile
> chmod a+rws $junk_file
> commit_and_check "$qa_user" "$verb" 64k 64k
>
> The test basically creates a file with 6666 permissions. While the file has
> the S_ISUID and S_ISGID bits set it does not have the S_IXGRP set. On a
> regular filesystem like xfs what will happen is:
>
> sys_fallocate()
> -> vfs_fallocate()
> -> xfs_file_fallocate()
> -> file_modified()
> -> __file_remove_privs()
> -> dentry_needs_remove_privs()
> -> should_remove_suid()
> -> __remove_privs()
> newattrs.ia_valid = ATTR_FORCE | kill;
> -> notify_change()
> -> setattr_copy()
>
> In should_remove_suid() we can see that ATTR_KILL_SUID is raised
> unconditionally because the file in the test has S_ISUID set.
>
> But we also see that ATTR_KILL_SGID won't be set because while the file
> is S_ISGID it is not S_IXGRP (see above) which is a condition for
> ATTR_KILL_SGID being raised.
>
> So by the time we call notify_change() we have attr->ia_valid set to
> ATTR_KILL_SUID | ATTR_FORCE. Now notify_change() sees that
> ATTR_KILL_SUID is set and does:
>
> ia_valid = attr->ia_valid |= ATTR_MODE
> attr->ia_mode = (inode->i_mode & ~S_ISUID);
>
> which means that when we call setattr_copy() later we will definitely
> update inode->i_mode. Note that attr->ia_mode still contains S_ISGID.
>
> Now we call into the filesystem's ->setattr() inode operation which will
> end up calling setattr_copy(). Since ATTR_MODE is set we will hit:
>
> if (ia_valid & ATTR_MODE) {
> umode_t mode = attr->ia_mode;
> vfsgid_t vfsgid = i_gid_into_vfsgid(mnt_userns, inode);
> if (!vfsgid_in_group_p(vfsgid) &&
> !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
> mode &= ~S_ISGID;
> inode->i_mode = mode;
> }
>
> and since the caller in the test is neither capable nor in the group of the
> inode the S_ISGID bit is stripped.
>
> But assume the file isn't suid then ATTR_KILL_SUID won't be raised which
> has the consequence that neither the setgid nor the suid bits are stripped
> even though it should be stripped because the inode isn't in the caller's
> groups and the caller isn't privileged over the inode.
>
> If overlayfs is in the mix things become a bit more complicated and the bug
> shows up more clearly. When e.g., ovl_setattr() is hit from
> ovl_fallocate()'s call to file_remove_privs() then ATTR_KILL_SUID and
> ATTR_KILL_SGID might be raised but because the check in notify_change() is
> questioning the ATTR_KILL_SGID flag again by requiring S_IXGRP for it to be
> stripped the S_ISGID bit isn't removed even though it should be stripped:
>
> sys_fallocate()
> -> vfs_fallocate()
> -> ovl_fallocate()
> -> file_remove_privs()
> -> dentry_needs_remove_privs()
> -> should_remove_suid()
> -> __remove_privs()
> newattrs.ia_valid = ATTR_FORCE | kill;
> -> notify_change()
> -> ovl_setattr()
> // TAKE ON MOUNTER'S CREDS
> -> ovl_do_notify_change()
> -> notify_change()
> // GIVE UP MOUNTER'S CREDS
> // TAKE ON MOUNTER'S CREDS
> -> vfs_fallocate()
> -> xfs_file_fallocate()
> -> file_modified()
> -> __file_remove_privs()
> -> dentry_needs_remove_privs()
> -> should_remove_suid()
> -> __remove_privs()
> newattrs.ia_valid = attr_force | kill;
> -> notify_change()
>
> The fix for all of this is to make file_remove_privs()'s
> should_remove_suid() helper to perform the same checks as we already
> require in setattr_prepare() and setattr_copy() and have notify_change()
> not pointlessly requiring S_IXGRP again. It doesn't make any sense in the
> first place because the caller must calculate the flags via
> should_remove_suid() anyway which would raise ATTR_KILL_SGID.
>
> Running xfstests with this doesn't report any regressions. We should really
> try and use consistent checks.
>
> Co-Developed-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Signed-off-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>
> ---
>
> Notes:
> /* v2 */
> Amir Goldstein <amir73il@xxxxxxxxx>:
> - mention xfstests that failed prior to that
>
> Christian Brauner <brauner@xxxxxxxxxx>:
> - Use should_remove_sgid() in chown_common() just like we do in do_truncate().
>
> fs/attr.c | 2 +-
> fs/fuse/file.c | 2 +-
> fs/inode.c | 24 ++++++++----------------
> fs/internal.h | 3 ++-
> fs/ocfs2/file.c | 4 ++--
> fs/open.c | 8 ++++----
> include/linux/fs.h | 2 +-
> 7 files changed, 19 insertions(+), 26 deletions(-)
>
> diff --git a/fs/attr.c b/fs/attr.c
> index d0bb1dae425e..888b34e8c268 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -421,7 +421,7 @@ int notify_change(struct user_namespace *mnt_userns, struct dentry *dentry,
> }
> }
> if (ia_valid & ATTR_KILL_SGID) {
> - if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
> + if (mode & S_ISGID) {
> if (!(ia_valid & ATTR_MODE)) {
> ia_valid = attr->ia_valid |= ATTR_MODE;
> attr->ia_mode = inode->i_mode;
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 1a3afd469e3a..fccc2c7e88fd 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1313,7 +1313,7 @@ static ssize_t fuse_cache_write_iter(struct kiocb *iocb, struct iov_iter *from)
> return err;
>
> if (fc->handle_killpriv_v2 &&
> - should_remove_suid(file_dentry(file))) {
> + should_remove_suid(&init_user_ns, file_dentry(file))) {
> goto writethrough;
> }
>
> diff --git a/fs/inode.c b/fs/inode.c
> index ba1de23c13c1..092a66324c65 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -1949,26 +1949,17 @@ void touch_atime(const struct path *path)
> }
> EXPORT_SYMBOL(touch_atime);
>
> -/*
> - * The logic we want is
> - *
> - * if suid or (sgid and xgrp)
> - * remove privs
> - */
> -int should_remove_suid(struct dentry *dentry)
> +int should_remove_suid(struct user_namespace *mnt_userns, struct dentry *dentry)
> {
> - umode_t mode = d_inode(dentry)->i_mode;
> + struct inode *inode = d_inode(dentry);
> + umode_t mode = inode->i_mode;
> int kill = 0;
>
> /* suid always must be killed */
> if (unlikely(mode & S_ISUID))
> kill = ATTR_KILL_SUID;
>
> - /*
> - * sgid without any exec bits is just a mandatory locking mark; leave
> - * it alone. If some exec bits are set, it's a real sgid; kill it.
> - */
> - if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
> + if (unlikely(should_remove_sgid(mnt_userns, inode)))
> kill |= ATTR_KILL_SGID;
kill |= should_remove_sgid(mnt_userns, inode);
>
> if (unlikely(kill && !capable(CAP_FSETID) && S_ISREG(mode)))
> @@ -1983,7 +1974,8 @@ EXPORT_SYMBOL(should_remove_suid);
> * response to write or truncate. Return 0 if nothing has to be changed.
> * Negative value on error (change should be denied).
> */
> -int dentry_needs_remove_privs(struct dentry *dentry)
> +int dentry_needs_remove_privs(struct user_namespace *mnt_userns,
> + struct dentry *dentry)
> {
> struct inode *inode = d_inode(dentry);
> int mask = 0;
> @@ -1992,7 +1984,7 @@ int dentry_needs_remove_privs(struct dentry *dentry)
> if (IS_NOSEC(inode))
> return 0;
>
> - mask = should_remove_suid(dentry);
> + mask = should_remove_suid(mnt_userns, dentry);
> ret = security_inode_need_killpriv(dentry);
> if (ret < 0)
> return ret;
> @@ -2024,7 +2016,7 @@ static int __file_remove_privs(struct file *file, unsigned int flags)
> if (IS_NOSEC(inode) || !S_ISREG(inode->i_mode))
> return 0;
>
> - kill = dentry_needs_remove_privs(dentry);
> + kill = dentry_needs_remove_privs(file_mnt_user_ns(file), dentry);
> if (kill < 0)
> return kill;
>
> diff --git a/fs/internal.h b/fs/internal.h
> index 9d165ab65a2a..b46881b7f8a0 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -139,7 +139,8 @@ extern int vfs_open(const struct path *, struct file *);
> * inode.c
> */
> extern long prune_icache_sb(struct super_block *sb, struct shrink_control *sc);
> -extern int dentry_needs_remove_privs(struct dentry *dentry);
> +extern int dentry_needs_remove_privs(struct user_namespace *,
> + struct dentry *dentry);
>
> /*
> * fs-writeback.c
> diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
> index 9c67edd215d5..e421491783c3 100644
> --- a/fs/ocfs2/file.c
> +++ b/fs/ocfs2/file.c
> @@ -1991,7 +1991,7 @@ static int __ocfs2_change_file_space(struct file *file, struct inode *inode,
> }
> }
>
> - if (file && should_remove_suid(file->f_path.dentry)) {
> + if (file && should_remove_suid(&init_user_ns, file->f_path.dentry)) {
> ret = __ocfs2_write_remove_suid(inode, di_bh);
> if (ret) {
> mlog_errno(ret);
> @@ -2279,7 +2279,7 @@ static int ocfs2_prepare_inode_for_write(struct file *file,
> * inode. There's also the dinode i_size state which
> * can be lost via setattr during extending writes (we
> * set inode->i_size at the end of a write. */
> - if (should_remove_suid(dentry)) {
> + if (should_remove_suid(&init_user_ns, dentry)) {
> if (meta_level == 0) {
> ocfs2_inode_unlock_for_extent_tree(inode,
> &di_bh,
> diff --git a/fs/open.c b/fs/open.c
> index 8a813fa5ca56..d955ecef758f 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -54,7 +54,7 @@ int do_truncate(struct user_namespace *mnt_userns, struct dentry *dentry,
> }
>
> /* Remove suid, sgid, and file capabilities on truncate too */
> - ret = dentry_needs_remove_privs(dentry);
> + ret = dentry_needs_remove_privs(mnt_userns, dentry);
> if (ret < 0)
> return ret;
> if (ret)
> @@ -721,10 +721,10 @@ int chown_common(const struct path *path, uid_t user, gid_t group)
> return -EINVAL;
> if ((group != (gid_t)-1) && !setattr_vfsgid(&newattrs, gid))
> return -EINVAL;
> - if (!S_ISDIR(inode->i_mode))
> - newattrs.ia_valid |=
> - ATTR_KILL_SUID | ATTR_KILL_SGID | ATTR_KILL_PRIV;
> inode_lock(inode);
> + if (!S_ISDIR(inode->i_mode))
> + newattrs.ia_valid |= ATTR_KILL_SUID | ATTR_KILL_PRIV |
> + should_remove_sgid(mnt_userns, inode);
This is making me stop and wonder:
1. This has !S_ISDIR, should_remove_suid() has S_ISREG and
setattr_drop_sgid() has neither - is this consistent?
2. SUID and PRIV are removed unconditionally and SGID is
removed conditionally - this is not a change of behavior
(at least for non-overlayfs), but is it desired???
Those questions could be dealt with in future patches if at all.
The change itself looks legit and solves a real problem, so you may add:
Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
Instead of Co-Developed-by ;-)
Thanks,
Amir.
