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Date: Fri, 5 Sep 2014 12:36:05 -0500
From: Seth Forshee <seth.forshee@...onical.com>
To: Serge Hallyn <serge.hallyn@...ntu.com>
Cc: Miklos Szeredi <miklos@...redi.hu>,
Alexander Viro <viro@...iv.linux.org.uk>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
fuse-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org, seth.forshee@...onical.com
Subject: Re: [PATCH v2 3/3] fuse: Add support for mounts from user namespaces
On Fri, Sep 05, 2014 at 04:48:11PM +0000, Serge Hallyn wrote:
> Quoting Seth Forshee (seth.forshee@...onical.com):
> > Update fuse to support mounts from within user namespaces. This
> > is mostly a matter of translating uids and gids into the
> > namespace of the process reading requests before handing the
> > requests off to userspace.
> >
> > Due to security concerns the namespace used should be fixed,
> > otherwise a user might be able to pass the fuse fd to
> > init_user_ns and inject suid files owned by a user outside the
> > namespace in order to gain elevated privileges. For fuse we
> > stash current_user_ns() when a filesystem is first mounted and
> > abort the mount if this namespace is different than the one used
> > to open the fd passed in the mount options.
> >
> > The allow_others options could also be a problem, as a userns
> > mount could bypass system policy for this option and thus open
> > the possiblity of DoS attacks. This is prevented by restricting
> > the scope of allow_other to apply only to that superblock's
> > userns and its children, giving the expected behavior within the
> > userns while preventing DoS attacks on more privileged contexts.
> >
> > Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
>
> Thanks, Seth, just two little questions below.
>
> > ---
> > fs/fuse/dev.c | 4 ++--
> > fs/fuse/dir.c | 46 +++++++++++++++++++++++++++++++++-------------
> > fs/fuse/fuse_i.h | 4 ++++
> > fs/fuse/inode.c | 28 ++++++++++++++++++++--------
> > 4 files changed, 59 insertions(+), 23 deletions(-)
> >
> > diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> > index 839caebd34f1..03c8785ed731 100644
> > --- a/fs/fuse/dev.c
> > +++ b/fs/fuse/dev.c
> > @@ -127,8 +127,8 @@ static void __fuse_put_request(struct fuse_req *req)
> >
> > static void fuse_req_init_context(struct fuse_conn *fc, struct fuse_req *req)
> > {
> > - req->in.h.uid = from_kuid_munged(&init_user_ns, current_fsuid());
> > - req->in.h.gid = from_kgid_munged(&init_user_ns, current_fsgid());
> > + req->in.h.uid = from_kuid_munged(fc->user_ns, current_fsuid());
> > + req->in.h.gid = from_kgid_munged(fc->user_ns, current_fsgid());
> > req->in.h.pid = pid_nr_ns(task_pid(current), fc->pid_ns);
> > }
> >
> > diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> > index de1d84af9f7c..c0b9968db6a1 100644
> > --- a/fs/fuse/dir.c
> > +++ b/fs/fuse/dir.c
> > @@ -905,8 +905,8 @@ static void fuse_fillattr(struct inode *inode, struct fuse_attr *attr,
> > stat->ino = attr->ino;
> > stat->mode = (inode->i_mode & S_IFMT) | (attr->mode & 07777);
> > stat->nlink = attr->nlink;
> > - stat->uid = make_kuid(&init_user_ns, attr->uid);
> > - stat->gid = make_kgid(&init_user_ns, attr->gid);
> > + stat->uid = make_kuid(fc->user_ns, attr->uid);
> > + stat->gid = make_kgid(fc->user_ns, attr->gid);
> > stat->rdev = inode->i_rdev;
> > stat->atime.tv_sec = attr->atime;
> > stat->atime.tv_nsec = attr->atimensec;
> > @@ -1085,12 +1085,20 @@ int fuse_reverse_inval_entry(struct super_block *sb, u64 parent_nodeid,
> > */
> > int fuse_allow_current_process(struct fuse_conn *fc)
> > {
> > - const struct cred *cred;
> > + const struct cred *cred = current_cred();
> >
> > - if (fc->flags & FUSE_ALLOW_OTHER)
> > - return 1;
> > + if (fc->flags & FUSE_ALLOW_OTHER) {
> > + if (kuid_has_mapping(fc->user_ns, cred->euid) &&
> > + kuid_has_mapping(fc->user_ns, cred->suid) &&
> > + kuid_has_mapping(fc->user_ns, cred->uid) &&
> > + kgid_has_mapping(fc->user_ns, cred->egid) &&
> > + kgid_has_mapping(fc->user_ns, cred->sgid) &&
> > + kgid_has_mapping(fc->user_ns, cred->gid))
>
> Should fsuid be checked here?
The point of restricting access here is to prevent a DoS type of attack
on a more privileged context by making a filesystem operation block
indefinitely. Coming from that perspective I was thinking that these
checks ought to be sufficient, but I could be wrong.
>
> > + return 1;
> > +
> > + return 0;
> > + }
> >
> > - cred = current_cred();
> > if (uid_eq(cred->euid, fc->user_id) &&
> > uid_eq(cred->suid, fc->user_id) &&
> > uid_eq(cred->uid, fc->user_id) &&
> > @@ -1556,17 +1564,25 @@ static bool update_mtime(unsigned ivalid, bool trust_local_mtime)
> > return true;
> > }
> >
> > -static void iattr_to_fattr(struct iattr *iattr, struct fuse_setattr_in *arg,
> > - bool trust_local_cmtime)
> > +static int iattr_to_fattr(struct fuse_conn *fc, struct iattr *iattr,
> > + struct fuse_setattr_in *arg, bool trust_local_cmtime)
> > {
> > unsigned ivalid = iattr->ia_valid;
> >
> > if (ivalid & ATTR_MODE)
> > arg->valid |= FATTR_MODE, arg->mode = iattr->ia_mode;
> > - if (ivalid & ATTR_UID)
> > - arg->valid |= FATTR_UID, arg->uid = from_kuid(&init_user_ns, iattr->ia_uid);
> > - if (ivalid & ATTR_GID)
> > - arg->valid |= FATTR_GID, arg->gid = from_kgid(&init_user_ns, iattr->ia_gid);
> > + if (ivalid & ATTR_UID) {
> > + arg->uid = from_kuid(fc->user_ns, iattr->ia_uid);
> > + if (arg->uid == (uid_t)-1)
>
> Any reason not to use uid_valid() here (and gid_valid() below)?
Yes. arg->uid is a uid_t and not a kuid_t, so it wouldn't be valid to
pass that to uid_valid(). And from_kuid() can return -1 for values other
than INVALID_UID.
Thanks,
Seth
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