Re: [kernel-hardening] [PATCH net-next v6 06/11] seccomp,landlock: Handle Landlock events per process hierarchy

From: Djalal Harouni
Date: Wed Mar 29 2017 - 06:35:43 EST

On Wed, Mar 29, 2017 at 1:46 AM, MickaÃl SalaÃn <mic@xxxxxxxxxxx> wrote:
> The seccomp(2) syscall can be used by a task to apply a Landlock rule to
> itself. As a seccomp filter, a Landlock rule is enforced for the current
> task and all its future children. A rule is immutable and a task can
> only add new restricting rules to itself, forming a chain of rules.
>
> A Landlock rule is tied to a Landlock event. If the use of a kernel
> object is allowed by the other Linux security mechanisms (e.g. DAC,
> capabilities, other LSM), then a Landlock event related to this kind of
> object is triggered. The chain of rules for this event is then
> evaluated. Each rule return a 32-bit value which can deny the use of a
> kernel object with a non-zero value. If every rules of the chain return
> zero, then the use of the object is allowed.
>
> Changes since v5:
> * remove struct landlock_node and use a similar inheritance mechanisme
> as seccomp-bpf (requested by Andy Lutomirski)
> * rename SECCOMP_ADD_LANDLOCK_RULE to SECCOMP_APPEND_LANDLOCK_RULE
> * rename file manager.c to providers.c
> * add comments
> * typo and cosmetic fixes
>
> Changes since v4:
> * merge manager and seccomp patches
> * return -EFAULT in seccomp(2) when user_bpf_fd is null to easely check
> if Landlock is supported
> * only allow a process with the global CAP_SYS_ADMIN to use Landlock
> (will be lifted in the future)
> * add an early check to exit as soon as possible if the current process
> does not have Landlock rules
>
> Changes since v3:
> * remove the hard link with seccomp (suggested by Andy Lutomirski and
> Kees Cook):
> * remove the cookie which could imply multiple evaluation of Landlock
> rules
> * remove the origin field in struct landlock_data
> * remove documentation fix (merged upstream)
> * rename the new seccomp command to SECCOMP_ADD_LANDLOCK_RULE
> * internal renaming
> * split commit
> * new design to be able to inherit on the fly the parent rules
>
> Changes since v2:
> * Landlock programs can now be run without seccomp filter but for any
> syscall (from the process) or interruption
> * move Landlock related functions and structs into security/landlock/*
> (to manage cgroups as well)
> * fix seccomp filter handling: run Landlock programs for each of their
> legitimate seccomp filter
> * properly clean up all seccomp results
> * cosmetic changes to ease the understanding
> * fix some ifdef
>
> Signed-off-by: MickaÃl SalaÃn <mic@xxxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: James Morris <james.l.morris@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
> Cc: Will Drewry <wad@xxxxxxxxxxxx>
> Link: https://lkml.kernel.org/r/c10a503d-5e35-7785-2f3d-25ed8dd63fab@xxxxxxxxxxx
> ---
> include/linux/landlock.h | 36 +++++++
> include/linux/seccomp.h | 8 ++
> include/uapi/linux/seccomp.h | 1 +
> kernel/fork.c | 14 ++-
> kernel/seccomp.c | 8 ++
> security/landlock/Makefile | 2 +-
> security/landlock/hooks.c | 37 +++++++
> security/landlock/hooks.h | 5 +
> security/landlock/init.c | 3 +-
> security/landlock/providers.c | 232 ++++++++++++++++++++++++++++++++++++++++++
> 10 files changed, 342 insertions(+), 4 deletions(-)
> create mode 100644 security/landlock/providers.c
>
> diff --git a/include/linux/landlock.h b/include/linux/landlock.h
> index 53013dc374fe..c40ee78e86e0 100644
> --- a/include/linux/landlock.h
> +++ b/include/linux/landlock.h
> @@ -12,6 +12,9 @@
> #define _LINUX_LANDLOCK_H
> #ifdef CONFIG_SECURITY_LANDLOCK
>
> +#include <linux/bpf.h> /* _LANDLOCK_SUBTYPE_EVENT_LAST */
> +#include <linux/types.h> /* atomic_t */
> +
> /*
> * This is not intended for the UAPI headers. Each userland software should use
> * a static minimal version for the required features as explained in the
> @@ -19,5 +22,38 @@
> */
> #define LANDLOCK_VERSION 1
>
> +struct landlock_rule {
> + atomic_t usage;
> + struct landlock_rule *prev;
> + struct bpf_prog *prog;
> +};
> +
> +/**
> + * struct landlock_events - Landlock event rules enforced on a thread
> + *
> + * This is used for low performance impact when forking a process. Instead of
> + * copying the full array and incrementing the usage of each entries, only
> + * create a pointer to &struct landlock_events and increments its usage. When
> + * appending a new rule, if &struct landlock_events is shared with other tasks,
> + * then duplicate it and append the rule to this new &struct landlock_events.
> + *
> + * @usage: reference count to manage the object lifetime. When a thread need to
> + * add Landlock rules and if @usage is greater than 1, then the thread
> + * must duplicate &struct landlock_events to not change the children's
> + * rules as well.
> + * @rules: array of non-NULL &struct landlock_rule pointers
> + */
> +struct landlock_events {
> + atomic_t usage;
> + struct landlock_rule *rules[_LANDLOCK_SUBTYPE_EVENT_LAST];
> +};
> +
> +void put_landlock_events(struct landlock_events *events);
> +
> +#ifdef CONFIG_SECCOMP_FILTER
> +int landlock_seccomp_append_prog(unsigned int flags,
> + const char __user *user_bpf_fd);
> +#endif /* CONFIG_SECCOMP_FILTER */
> +
> #endif /* CONFIG_SECURITY_LANDLOCK */
> #endif /* _LINUX_LANDLOCK_H */
> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> index e25aee2cdfc0..9a38de3c0e72 100644
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -10,6 +10,10 @@
> #include <linux/thread_info.h>
> #include <asm/seccomp.h>
>
> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_SECURITY_LANDLOCK)
> +struct landlock_events;
> +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_SECURITY_LANDLOCK */
> +
> struct seccomp_filter;
> /**
> * struct seccomp - the state of a seccomp'ed process
> @@ -18,6 +22,7 @@ struct seccomp_filter;
> * system calls available to a process.
> * @filter: must always point to a valid seccomp-filter or NULL as it is
> * accessed without locking during system call entry.
> + * @landlock_events: contains an array of Landlock rules.
> *
> * @filter must only be accessed from the context of current as there
> * is no read locking.
> @@ -25,6 +30,9 @@ struct seccomp_filter;
> struct seccomp {
> int mode;
> struct seccomp_filter *filter;
> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_SECURITY_LANDLOCK)
> + struct landlock_events *landlock_events;
> +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_SECURITY_LANDLOCK */
> };

Sorry if this was discussed before, but since this is mean to be a
stackable LSM, I'm wondering if later you could move the events from
seccomp, and go with a security_task_alloc() model [1] ?

Thanks!

[1] http://kernsec.org/pipermail/linux-security-module-archive/2017-March/000184.html