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Date: Wed, 20 Jun 2018 15:05:05 +1000
From: "Tobin C . Harding" <me@...in.cc>
To: Tycho Andersen <tycho@...ho.ws>
Cc: linux-kernel@...r.kernel.org,
containers@...ts.linux-foundation.org,
Kees Cook <keescook@...omium.org>,
Andy Lutomirski <luto@...capital.net>,
Oleg Nesterov <oleg@...hat.com>,
"Eric W . Biederman" <ebiederm@...ssion.com>,
"Serge E . Hallyn" <serge@...lyn.com>,
Christian Brauner <christian.brauner@...ntu.com>,
Tyler Hicks <tyhicks@...onical.com>,
Akihiro Suda <suda.akihiro@....ntt.co.jp>
Subject: Re: [PATCH v3 1/4] seccomp: add a return code to trap to userspace
On Thu, May 31, 2018 at 08:49:46AM -0600, Tycho Andersen wrote:
> This patch introduces a means for syscalls matched in seccomp to notify
> some other task that a particular filter has been triggered.
>
> The motivation for this is primarily for use with containers. For example,
> if a container does an init_module(), we obviously don't want to load this
> untrusted code, which may be compiled for the wrong version of the kernel
> anyway. Instead, we could parse the module image, figure out which module
> the container is trying to load and load it on the host.
>
> As another example, containers cannot mknod(), since this checks
> capable(CAP_SYS_ADMIN). However, harmless devices like /dev/null or
> /dev/zero should be ok for containers to mknod, but we'd like to avoid hard
> coding some whitelist in the kernel. Another example is mount(), which has
> many security restrictions for good reason, but configuration or runtime
> knowledge could potentially be used to relax these restrictions.
>
> This patch adds functionality that is already possible via at least two
> other means that I know about, both of which involve ptrace(): first, one
> could ptrace attach, and then iterate through syscalls via PTRACE_SYSCALL.
> Unfortunately this is slow, so a faster version would be to install a
> filter that does SECCOMP_RET_TRACE, which triggers a PTRACE_EVENT_SECCOMP.
> Since ptrace allows only one tracer, if the container runtime is that
> tracer, users inside the container (or outside) trying to debug it will not
> be able to use ptrace, which is annoying. It also means that older
> distributions based on Upstart cannot boot inside containers using ptrace,
> since upstart itself uses ptrace to start services.
>
> The actual implementation of this is fairly small, although getting the
> synchronization right was/is slightly complex.
>
> Finally, it's worth noting that the classic seccomp TOCTOU of reading
> memory data from the task still applies here, but can be avoided with
> careful design of the userspace handler: if the userspace handler reads all
> of the task memory that is necessary before applying its security policy,
> the tracee's subsequent memory edits will not be read by the tracer.
>
> v2: * make id a u64; the idea here being that it will never overflow,
> because 64 is huge (one syscall every nanosecond => wrap every 584
> years) (Andy)
> * prevent nesting of user notifications: if someone is already attached
> the tree in one place, nobody else can attach to the tree (Andy)
> * notify the listener of signals the tracee receives as well (Andy)
> * implement poll
> v3: * lockdep fix (Oleg)
> * drop unnecessary WARN()s (Christian)
> * rearrange error returns to be more rpetty (Christian)
> * fix build in !CONFIG_SECCOMP_USER_NOTIFICATION case
>
> Signed-off-by: Tycho Andersen <tycho@...ho.ws>
> CC: Kees Cook <keescook@...omium.org>
> CC: Andy Lutomirski <luto@...capital.net>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: Eric W. Biederman <ebiederm@...ssion.com>
> CC: "Serge E. Hallyn" <serge@...lyn.com>
> CC: Christian Brauner <christian.brauner@...ntu.com>
> CC: Tyler Hicks <tyhicks@...onical.com>
> CC: Akihiro Suda <suda.akihiro@....ntt.co.jp>
> ---
> arch/Kconfig | 7 +
> include/linux/seccomp.h | 3 +-
> include/uapi/linux/seccomp.h | 18 +-
> kernel/seccomp.c | 398 +++++++++++++++++-
> tools/testing/selftests/seccomp/seccomp_bpf.c | 195 ++++++++-
> 5 files changed, 615 insertions(+), 6 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 75dd23acf133..1c1ae8d8c8b9 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -401,6 +401,13 @@ config SECCOMP_FILTER
>
> See Documentation/prctl/seccomp_filter.txt for details.
>
> +config SECCOMP_USER_NOTIFICATION
> + bool "Enable the SECCOMP_RET_USER_NOTIF seccomp action"
> + depends on SECCOMP_FILTER
> + help
> + Enable SECCOMP_RET_USER_NOTIF, a return code which can be used by seccomp
> + programs to notify a userspace listener that a particular event happened.
> +
> config HAVE_GCC_PLUGINS
> bool
> help
> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> index c723a5c4e3ff..0fd3e0676a1c 100644
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -5,7 +5,8 @@
> #include <uapi/linux/seccomp.h>
>
> #define SECCOMP_FILTER_FLAG_MASK (SECCOMP_FILTER_FLAG_TSYNC | \
> - SECCOMP_FILTER_FLAG_LOG)
> + SECCOMP_FILTER_FLAG_LOG | \
> + SECCOMP_FILTER_FLAG_GET_LISTENER)
>
> #ifdef CONFIG_SECCOMP
>
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 2a0bd9dd104d..8160e6cad528 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -17,8 +17,9 @@
> #define SECCOMP_GET_ACTION_AVAIL 2
>
> /* Valid flags for SECCOMP_SET_MODE_FILTER */
> -#define SECCOMP_FILTER_FLAG_TSYNC 1
> -#define SECCOMP_FILTER_FLAG_LOG 2
> +#define SECCOMP_FILTER_FLAG_TSYNC 1
> +#define SECCOMP_FILTER_FLAG_LOG 2
> +#define SECCOMP_FILTER_FLAG_GET_LISTENER 4
>
> /*
> * All BPF programs must return a 32-bit value.
> @@ -34,6 +35,7 @@
> #define SECCOMP_RET_KILL SECCOMP_RET_KILL_THREAD
> #define SECCOMP_RET_TRAP 0x00030000U /* disallow and force a SIGSYS */
> #define SECCOMP_RET_ERRNO 0x00050000U /* returns an errno */
> +#define SECCOMP_RET_USER_NOTIF 0x7fc00000U /* notifies userspace */
> #define SECCOMP_RET_TRACE 0x7ff00000U /* pass to a tracer or disallow */
> #define SECCOMP_RET_LOG 0x7ffc0000U /* allow after logging */
> #define SECCOMP_RET_ALLOW 0x7fff0000U /* allow */
> @@ -59,4 +61,16 @@ struct seccomp_data {
> __u64 args[6];
> };
>
> +struct seccomp_notif {
> + __u64 id;
> + pid_t pid;
> + struct seccomp_data data;
> +};
> +
> +struct seccomp_notif_resp {
> + __u64 id;
> + __s32 error;
> + __s64 val;
> +};
> +
> #endif /* _UAPI_LINUX_SECCOMP_H */
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index dc77548167ef..f69327d5f7c7 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -31,6 +31,7 @@
> #endif
>
> #ifdef CONFIG_SECCOMP_FILTER
> +#include <linux/file.h>
> #include <linux/filter.h>
> #include <linux/pid.h>
> #include <linux/ptrace.h>
> @@ -38,6 +39,52 @@
> #include <linux/tracehook.h>
> #include <linux/uaccess.h>
>
> +#ifdef CONFIG_SECCOMP_USER_NOTIFICATION
> +#include <linux/anon_inodes.h>
> +
> +enum notify_state {
> + SECCOMP_NOTIFY_INIT,
> + SECCOMP_NOTIFY_SENT,
> + SECCOMP_NOTIFY_REPLIED,
> +};
> +
> +struct seccomp_knotif {
> + /* The pid whose filter triggered the notification */
> + pid_t pid;
> +
> + /*
> + * The "cookie" for this request; this is unique for this filter.
> + */
> + u32 id;
Perhaps
/* The "cookie" for this request; this is unique for this filter. */
Epic patch review :)
thanks,
Tobin.
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