Re: [PATCH v4] lockdown,selinux: fix wrong subject in some SELinux lockdown checks

[Paul Moore]

On Mon, Sep 13, 2021 at 10:02 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote:
>
> Commit 59438b46471a ("security,lockdown,selinux: implement SELinux
> lockdown") added an implementation of the locked_down LSM hook to
> SELinux, with the aim to restrict which domains are allowed to perform
> operations that would breach lockdown.
>
> However, in several places the security_locked_down() hook is called in
> situations where the current task isn't doing any action that would
> directly breach lockdown, leading to SELinux checks that are basically
> bogus.
>
> To fix this, add an explicit struct cred pointer argument to
> security_lockdown() and define NULL as a special value to pass instead
> of current_cred() in such situations. LSMs that take the subject
> credentials into account can then fall back to some default or ignore
> such calls altogether. In the SELinux lockdown hook implementation, use
> SECINITSID_KERNEL in case the cred argument is NULL.
>
> Most of the callers are updated to pass current_cred() as the cred
> pointer, thus maintaining the same behavior. The following callers are
> modified to pass NULL as the cred pointer instead:
> 1. arch/powerpc/xmon/xmon.c
>      Seems to be some interactive debugging facility. It appears that
>      the lockdown hook is called from interrupt context here, so it
>      should be more appropriate to request a global lockdown decision.
> 2. fs/tracefs/inode.c:tracefs_create_file()
>      Here the call is used to prevent creating new tracefs entries when
>      the kernel is locked down. Assumes that locking down is one-way -
>      i.e. if the hook returns non-zero once, it will never return zero
>      again, thus no point in creating these files. Also, the hook is
>      often called by a module's init function when it is loaded by
>      userspace, where it doesn't make much sense to do a check against
>      the current task's creds, since the task itself doesn't actually
>      use the tracing functionality (i.e. doesn't breach lockdown), just
>      indirectly makes some new tracepoints available to whoever is
>      authorized to use them.
> 3. net/xfrm/xfrm_user.c:copy_to_user_*()
>      Here a cryptographic secret is redacted based on the value returned
>      from the hook. There are two possible actions that may lead here:
>      a) A netlink message XFRM_MSG_GETSA with NLM_F_DUMP set - here the
>         task context is relevant, since the dumped data is sent back to
>         the current task.
>      b) When adding/deleting/updating an SA via XFRM_MSG_xxxSA, the
>         dumped SA is broadcasted to tasks subscribed to XFRM events -
>         here the current task context is not relevant as it doesn't
>         represent the tasks that could potentially see the secret.
>      It doesn't seem worth it to try to keep using the current task's
>      context in the a) case, since the eventual data leak can be
>      circumvented anyway via b), plus there is no way for the task to
>      indicate that it doesn't care about the actual key value, so the
>      check could generate a lot of "false alert" denials with SELinux.
>      Thus, let's pass NULL instead of current_cred() here faute de
>      mieux.
>
> Improvements-suggested-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> Improvements-suggested-by: Paul Moore <paul@xxxxxxxxxxxxxx>
> Fixes: 59438b46471a ("security,lockdown,selinux: implement SELinux lockdown")
> Acked-by: Dan Williams <dan.j.williams@xxxxxxxxx>         [cxl]
> Acked-by: Steffen Klassert <steffen.klassert@xxxxxxxxxxx> [xfrm]
> Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
> ---
>
> v4:
> - rebase on top of TODO
> - fix rebase conflicts:
>   * drivers/cxl/pci.c
>     - trivial: the lockdown reason was corrected in mainline
>   * kernel/bpf/helpers.c, kernel/trace/bpf_trace.c
>     - trivial: LOCKDOWN_BPF_READ was renamed to LOCKDOWN_BPF_READ_KERNEL
>       in mainline
>   * kernel/power/hibernate.c
>     - trivial: !secretmem_active() was added to the condition in
>       hibernation_available()
> - cover new security_locked_down() call in kernel/bpf/helpers.c
>   (LOCKDOWN_BPF_WRITE_USER in BPF_FUNC_probe_write_user case)
>
> v3: https://lore.kernel.org/lkml/20210616085118.1141101-1-omosnace@xxxxxxxxxx/
> - add the cred argument to security_locked_down() and adapt all callers
> - keep using current_cred() in BPF, as the hook calls have been shifted
>   to program load time (commit ff40e51043af ("bpf, lockdown, audit: Fix
>   buggy SELinux lockdown permission checks"))
> - in SELinux, don't ignore hook calls where cred == NULL, but use
>   SECINITSID_KERNEL as the subject instead
> - update explanations in the commit message
>
> v2: https://lore.kernel.org/lkml/20210517092006.803332-1-omosnace@xxxxxxxxxx/
> - change to a single hook based on suggestions by Casey Schaufler
>
> v1: https://lore.kernel.org/lkml/20210507114048.138933-1-omosnace@xxxxxxxxxx/

The changes between v3 and v4 all seem sane to me, but I'm going to
let this sit for a few days in hopes that we can collect a few more
Reviewed-bys and ACKs.  If I don't see any objections I'll merge it
mid-week(ish) into selinux/stable-5.15 and plan on sending it to Linus
after it goes through a build/test cycle.

-- 
paul moore
www.paul-moore.com