Re: [PATCH v2 2/2] selinux: do not override context on context mounts

[Ondrej Mosnacek <omosnace@redhat.com>]

On Fri, Dec 21, 2018 at 9:45 PM Stephen Smalley <sds@tycho.nsa.gov> wrote:
> On 12/21/18 3:18 PM, Ondrej Mosnacek wrote:
> > Ignore all selinux_inode_notifysecctx() calls on mounts with SBLABEL_MNT
> > flag unset. This is achived by returning -EOPNOTSUPP for this case in
> > selinux_inode_setsecurtity() (because that function should not be called
> > in such case anyway) and translating this error to 0 in
> > selinux_inode_notifysecctx().
> >
> > This fixes behavior of kernfs-based filesystems when mounted with the
> > 'context=' option. Before this patch, if a node's context had been
> > explicitly set to a non-default value and later the filesystem has been
> > remounted with the 'context=' option, then this node would show up as
> > having the manually-set context and not the mount-specified one.
> >
> > Steps to reproduce:
> >      # mount -t cgroup2 cgroup2 /sys/fs/cgroup/unified
> >      # chcon unconfined_u:object_r:user_home_t:s0 /sys/fs/cgroup/unified/cgroup.stat
> >      # ls -lZ /sys/fs/cgroup/unified
> >      total 0
> >      -r--r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.controllers
> >      -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.max.depth
> >      -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.max.descendants
> >      -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.procs
> >      -r--r--r--. 1 root root unconfined_u:object_r:user_home_t:s0 0 Dec 13 10:41 cgroup.stat
> >      -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.subtree_control
> >      -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.threads
> >      # umount /sys/fs/cgroup/unified
> >      # mount -o context=system_u:object_r:tmpfs_t:s0 -t cgroup2 cgroup2 /sys/fs/cgroup/unified
> >
> > Result before:
> >      # ls -lZ /sys/fs/cgroup/unified
> >      total 0
> >      -r--r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.controllers
> >      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.max.depth
> >      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.max.descendants
> >      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.procs
> >      -r--r--r--. 1 root root unconfined_u:object_r:user_home_t:s0 0 Dec 13 10:41 cgroup.stat
> >      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.subtree_control
> >      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.threads
> >
> > Result after:
> >      # ls -lZ /sys/fs/cgroup/unified
> >      total 0
> >      -r--r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.controllers
> >      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.max.depth
> >      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.max.descendants
> >      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.procs
> >      -r--r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.stat
> >      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.subtree_control
> >      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.threads
> >
> > Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
>
> The patch looks good to me but I am a little puzzled by the cgroup2
> behavior here.  I would have expected that unmounting would have killed
> the superblock and thus discarded the previously set label.  I guess
> something else is keeping it alive?

It is because the context set via setxattr is stored inside the kernfs
node and the kernfs tree is preserved across mounts/unmounts. It
actually behaves sort of like a normal filesystem backed by a block
device, just the "device" is represented by an in-memory kernfs tree
which is discarded at poweroff.

>
> Reviewed-by: Stephen Smalley <sds@tycho.nsa.gov>

Thanks!

>
> > ---
> >   security/selinux/hooks.c | 9 ++++++++-
> >   1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index b4759bebeddc..fcf4af1e5157 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -3477,12 +3477,16 @@ static int selinux_inode_setsecurity(struct inode *inode, const char *name,
> >                                    const void *value, size_t size, int flags)
> >   {
> >       struct inode_security_struct *isec = inode_security_novalidate(inode);
> > +     struct superblock_security_struct *sbsec = inode->i_sb->s_security;
> >       u32 newsid;
> >       int rc;
> >
> >       if (strcmp(name, XATTR_SELINUX_SUFFIX))
> >               return -EOPNOTSUPP;
> >
> > +     if (!(sbsec->flags & SBLABEL_MNT))
> > +             return -EOPNOTSUPP;
> > +
> >       if (!value || !size)
> >               return -EACCES;
> >
> > @@ -6625,7 +6629,10 @@ static void selinux_inode_invalidate_secctx(struct inode *inode)
> >    */
> >   static int selinux_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen)
> >   {
> > -     return selinux_inode_setsecurity(inode, XATTR_SELINUX_SUFFIX, ctx, ctxlen, 0);
> > +     int rc = selinux_inode_setsecurity(inode, XATTR_SELINUX_SUFFIX,
> > +                                        ctx, ctxlen, 0);
> > +     /* Do not return error when suppressing label (SBLABEL_MNT not set). */
> > +     return rc == -EOPNOTSUPP ? 0 : rc;
> >   }
> >
> >   /*
> >

-- 
Ondrej Mosnacek <omosnace at redhat dot com>
Associate Software Engineer, Security Technologies
Red Hat, Inc.