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    SubjectRe: [PATCH v12 13/26] userns: Add pointer to ima_namespace to user_namespace
    On Mon, May 23, 2022 at 02:41:59PM +0200, Christian Brauner wrote:
    > On Mon, May 23, 2022 at 07:31:29AM -0400, Stefan Berger wrote:
    > >
    > >
    > > On 5/23/22 05:59, Christian Brauner wrote:
    > > > On Sun, May 22, 2022 at 01:24:26PM -0500, Serge Hallyn wrote:
    > > > > On Wed, Apr 20, 2022 at 10:06:20AM -0400, Stefan Berger wrote:
    > > > > > Add a pointer to ima_namespace to the user_namespace and initialize
    > > > > > the init_user_ns with a pointer to init_ima_ns. We need a pointer from
    > > > > > the user namespace to its associated IMA namespace since IMA namespaces
    > > > > > are piggybacking on user namespaces.
    > > > > >
    > > > > > Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
    > > > > > Acked-by: Christian Brauner <brauner@kernel.org>
    > > > > > Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
    > > > > >
    > > > > > ---
    > > > > > v11:
    > > > > > - Added lost A-b from Christian back
    > > > > > - Added sentence to patch description explaining why we need the pointer
    > > > > >
    > > > > > v9:
    > > > > > - Deferred implementation of ima_ns_from_user_ns() to later patch
    > > > > > ---
    > > > > > include/linux/ima.h | 2 ++
    > > > > > include/linux/user_namespace.h | 4 ++++
    > > > > > kernel/user.c | 4 ++++
    > > > > > 3 files changed, 10 insertions(+)
    > > > > >
    > > > > > diff --git a/include/linux/ima.h b/include/linux/ima.h
    > > > > > index 426b1744215e..fcb60a44e05f 100644
    > > > > > --- a/include/linux/ima.h
    > > > > > +++ b/include/linux/ima.h
    > > > > > @@ -14,6 +14,8 @@
    > > > > > #include <crypto/hash_info.h>
    > > > > > struct linux_binprm;
    > > > > > +extern struct ima_namespace init_ima_ns;
    > > > > > +
    > > > > > #ifdef CONFIG_IMA
    > > > > > extern enum hash_algo ima_get_current_hash_algo(void);
    > > > > > extern int ima_bprm_check(struct linux_binprm *bprm);
    > > > > > diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
    > > > > > index 33a4240e6a6f..019e8cf7b633 100644
    > > > > > --- a/include/linux/user_namespace.h
    > > > > > +++ b/include/linux/user_namespace.h
    > > > > > @@ -36,6 +36,7 @@ struct uid_gid_map { /* 64 bytes -- 1 cache line */
    > > > > > #define USERNS_INIT_FLAGS USERNS_SETGROUPS_ALLOWED
    > > > > > struct ucounts;
    > > > > > +struct ima_namespace;
    > > > > > enum ucount_type {
    > > > > > UCOUNT_USER_NAMESPACES,
    > > > > > @@ -99,6 +100,9 @@ struct user_namespace {
    > > > > > #endif
    > > > > > struct ucounts *ucounts;
    > > > > > long ucount_max[UCOUNT_COUNTS];
    > > > > > +#ifdef CONFIG_IMA_NS
    > > > >
    > > > > It's probably worth putting a comment here saying that user_ns does not
    > > > > pin ima_ns.
    > > > >
    > > > > That the only time the ima_ns will be freed is when user_ns is freed,
    > > > > and only time it will be changed is when user_ns is freed, or during
    > > > > ima_fs_ns_init() (under smp_load_acquire) during a new mount.
    > > > >
    > > > > > + struct ima_namespace *ima_ns;
    > > > >
    > > > > So, if I create a new user_ns with a new ima_ns, and in there I
    > > > > create a new user_ns again, it looks like ima_ns will be NULL in
    > > > > the new user_ns? Should it not be set to the parent->ima_ns?
    > > > > (which would cause trouble for the way it's currently being
    > > > > freed...)
    > > >
    > > > Would also work and wouldn't be difficult to do imho.
    > >
    > > We previously decide to create an ima_namespace when securityfs is mounted.
    > > This now also applies to nested containers where an IMA namespace is first
    > > configured with the hash and template to use in a particular container and
    > > then activated. If no configuration is done it will inherit the hash and
    > > template from the first ancestor that has been configure when it is
    > > activated. So the same steps and behavior applies to *all* containers, no
    > > difference at any depth of nesting. Besides that, we don't want nested
    > > containers to share policy and logs but keep them isolated from each other,
    > > or do we not?
    > >
    > > Further, how should it work if we were to apply this even to the first
    > > container? Should it just inherit the &init_ima_namespace and we'd have no
    > > isolation at all? Why would we start treating containers at deeper nesting
    > > levels differently?
    >
    > Valid points. I understood Serge as suggesting an implementation detail
    > change not a design change but might misunderstand him here.
    >
    > # Currently
    >
    > 1. create new userns -> imans set to NULL
    > 2. mount securityfs and configure imans -> set imans to &new_ima_ns
    >
    > When 2. hasn't been done then we find the relevant imans by walking
    > the userns hierarchy upwards finding the first parent userns that has a
    > non-NULL imans.

    Ah, right, thanks Christian.

    But so the code - I think where the ima_ns is defined in the user_ns
    struct, needs to make this clear. Just something like

    // Pointer to ima_ns which this user_ns created. Can be null.
    // Access checks will walk the userns->parent chain and check
    // against all active ima_ns's. Note that when the user_ns is
    // freed, the ima_ns is guaranteed to be free-able.
    struct ima_namespace *ima_ns;

    > # Serge's suggestion
    >
    > 1. create new userns -> imans is set to parent imans
    > 2. mount securityfs and configure imans -> replace parent with &new_ima_ns
    >
    > So when 2. hasn't been done we don't need to walk the userns hierarchy
    > upwards. We always find the relevant imans directly. Some massaging
    > would be needed in process_measurement() probably but it wouldn't need
    > to change semantics per se.
    >
    > But I think I misunderstood something in any case. So looking at an
    > example like ima_post_path_mknod(). You seem to not call into
    > ima_must_appraise() if the caller's userns doesn't have an imans
    > enabled. I somehow had thought that the same logic applied as in
    > process_measurement. But if it isn't then it might make sense to keep
    > the current implementation.

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