Re: [PATCH v3 2/4] seccomp: Add SECCOMP_FILTER_FLAG_KILL_PROCESS

[Tyler Hicks]

On 08/09/2017 02:01 PM, Kees Cook wrote:
> Right now, SECCOMP_RET_KILL kills the current thread. There have been
> a few requests for RET_KILL to kill the entire process (the thread
> group), but since seccomp's u32 return values are ABI, and ordered by
> lowest value, with RET_KILL as 0, there isn't a trivial way to provide
> an even smaller value that would mean the more restrictive action of
> killing the thread group.
> 
> Changing the definition of SECCOMP_RET_KILL to mean "kill process" and
> then adding a new higher definition of SECCOMP_RET_KILL_THREAD isn't
> possible since there are already userspace programs depending on the
> kill-thread behavior.
> 
> Instead, create a simulated SECCOMP_RET_KILL_PROCESS action which
> has the semantics of such a return had it been possible to create
> it naturally. This is done by adding a filter flag that indicates
> that a RET_KILL from this filter must kill the process rather
> than the thread. This can be set (and not cleared) via the new
> SECCOMP_FILTER_FLAG_KILL_PROCESS flag.

I think the name of the pkill utility has made it impossible for my
brain to type FILTER_FLAG_KILL_PROCESS. My fingers want to type
FILTER_FLAG_PROCESS_KILL every single time. :)

> 
> Pros:
>  - the logic for the filter action is contained in the filter.
>  - userspace can detect support for the feature since earlier kernels
>    will reject the new flag.
>  - can be distinguished from "normal" RET_KILL in the same filter
>    chain (in case a program wants to choose to kill thread or process).
> Cons:
>  - depends on adding an assignment to the seccomp_run_filters() loop
>    (previous patch).
>  - adds logic to the seccomp_run_filters() loop (though it is a single
>    unlikely test for zero, so the cycle count change isn't measurable).
> 
> Alternatives to this approach with pros/cons:
> 
> - Change userspace API to use an s32 for BPF return value, and use a
>   negative value to indicate SECCOMP_RET_KILL_PROCESS.
>   Pros:
>    - no change needed to seccomp_run_filters() loop.
>    - the logic for the filter action is contained in the filter.
>    - can be distinguished from "normal" RET_KILL in the same filter
>      chain (in case a program wants to choose to kill thread or process).
>   Cons:
>    - there isn't a trivial way for userspace to detect if the kernel
>      supports the feature (earlier kernels will silently ignore the
>      new value, only kill the thread).
>    - need to teach libseccomp about new details, extensive updates
>      to documentation, and hope there is not confusion about signedness
>      in existing userspace.
> 
> - Use a new test during seccomp_run_filters() that treats the RET_DATA
>   mask of a RET_KILL action as special. If a new bit is set in the data,
>   then treat the return value as -1 (lower than 0).
>   Pros:
>    - the logic for the filter action is contained in the filter.
>    - can be distinguished from "normal" RET_KILL in the same filter
>      chain (in case a program wants to choose to kill thread or process).
>   Cons:
>    - adds more than a single unlikely test to time-sensitive
>      seccomp_run_filters() loop.
>    - there isn't a trivial way for userspace to detect if the kernel
>      supports the feature (earlier kernels will silently ignore the
>      RET_DATA and only kill the thread).
>    - Violates the "most recent RET_DATA" return value used for all
>      other actions (since processing must continue to find the first
>      RET_KILL with the flag set).
>    - May create problems for existing programs that were using RET_DATA
>      to distinguish between various SECCOMP_RET_KILL locations.
> 
> - Have SECCOMP_FILTER_FLAG_KILL_PROCESS attach to the seccomp struct
>   rather than the filter.
>   Pros:
>    - no change needed to seccomp_run_filters() loop.
>    - userspace can detect support for the feature since earlier kernels
>      will reject the new flag.
>   Cons:
>    - does not provide a way for a set of filters to distinguish
>      between wanting to kill a thread or a process.
>    - the logic for the filter action isn't contained in the filter,
>      which may lead to synchronization bugs (e.g. vs TSYNC, vs CRIU, etc).
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  include/linux/seccomp.h      |  3 ++-
>  include/uapi/linux/seccomp.h |  3 ++-
>  kernel/seccomp.c             | 54 ++++++++++++++++++++++++++++++++++++++++----
>  3 files changed, 54 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> index ecc296c137cd..59d001ba655c 100644
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -3,7 +3,8 @@
>  
>  #include <uapi/linux/seccomp.h>
>  
> -#define SECCOMP_FILTER_FLAG_MASK	(SECCOMP_FILTER_FLAG_TSYNC)
> +#define SECCOMP_FILTER_FLAG_MASK	(SECCOMP_FILTER_FLAG_TSYNC | \
> +					 SECCOMP_FILTER_FLAG_KILL_PROCESS)
>  
>  #ifdef CONFIG_SECCOMP
>  
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 0f238a43ff1e..4b75d8c297b6 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -15,7 +15,8 @@
>  #define SECCOMP_SET_MODE_FILTER	1
>  
>  /* Valid flags for SECCOMP_SET_MODE_FILTER */
> -#define SECCOMP_FILTER_FLAG_TSYNC	1
> +#define SECCOMP_FILTER_FLAG_TSYNC		1
> +#define SECCOMP_FILTER_FLAG_KILL_PROCESS	2
>  
>  /*
>   * All BPF programs must return a 32-bit value.
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 1f3347fc2605..6a196d1495e6 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -44,6 +44,7 @@
>   *         is only needed for handling filters shared across tasks.
>   * @prev: points to a previously installed, or inherited, filter
>   * @prog: the BPF program to evaluate
> + * @kill_process: if true, RET_KILL will kill process rather than thread.
>   *
>   * seccomp_filter objects are organized in a tree linked via the @prev
>   * pointer.  For any task, it appears to be a singly-linked list starting
> @@ -57,6 +58,7 @@
>   */
>  struct seccomp_filter {
>  	refcount_t usage;
> +	bool kill_process;
>  	struct seccomp_filter *prev;
>  	struct bpf_prog *prog;
>  };
> @@ -201,8 +203,25 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
>  	 */
>  	for (; f; f = f->prev) {
>  		u32 cur_ret = BPF_PROG_RUN(f->prog, sd);
> +		u32 action = cur_ret & SECCOMP_RET_ACTION;
>  
> -		if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION)) {
> +		/*
> +		 * In order to distinguish between SECCOMP_RET_KILL and
> +		 * "higher priority" synthetic SECCOMP_RET_KILL_PROCESS
> +		 * identified by the kill_process filter flag, treat any
> +		 * case as immediately stopping filter processing. No
> +		 * higher priority action can exist, and we can't stop
> +		 * on the first RET_KILL (which may not have set
> +		 * f->kill_process) when a RET_KILL further up the filter
> +		 * list may have f->kill_process set which would go
> +		 * unnoticed.
> +		 */
> +		if (unlikely(action == SECCOMP_RET_KILL && f->kill_process)) {
> +			*match = f;
> +			return cur_ret;
> +		}

Why not let the application enforce this via the seccomp filter? In
other words, the first filter loaded with
SECCOMP_FILTER_FLAG_KILL_PROCESS set could have a rule in the filter
that only allows seccomp(2) to be called in the future with the
SECCOMP_FILTER_FLAG_KILL_PROCESS flag set.

I understand the reasoning for wanting to enforce this automatically at
the kernel level but I think mixing return action priorities with filter
flags could be confusing and inflexible in the long run since filters
are inherited and your parent's desire to kill the entire thread group
may not mix with your desire to only kill a single thread.

Another way that this doesn't mix perfectly with the existing design is
when the action is unknown. In that situation, we treat it as RET_KILL.
However, this patch hard-codes the comparison with RET_KILL so we get
into this situation where an unknown action is treated as RET_KILL
except when the filter has the FILTER_FLAG_KILL_PROCESS flag set and
then this short-circuit doesn't kick in. It is a corner case, for sure,
but worth mentioning.

Tyler

> +
> +		if (action < (ret & SECCOMP_RET_ACTION)) {
>  			ret = cur_ret;
>  			*match = f;
>  		}
> @@ -450,6 +469,10 @@ static long seccomp_attach_filter(unsigned int flags,
>  			return ret;
>  	}
>  
> +	/* Set process-killing flag, if present. */
> +	if (flags & SECCOMP_FILTER_FLAG_KILL_PROCESS)
> +		filter->kill_process = true;
> +
>  	/*
>  	 * If there is an existing filter, make it the prev and don't drop its
>  	 * task reference.
> @@ -574,6 +597,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>  	u32 filter_ret, action;
>  	struct seccomp_filter *match = NULL;
>  	int data;
> +	bool kill_process;
>  
>  	/*
>  	 * Make sure that any changes to mode from another thread have
> @@ -655,8 +679,26 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>  	case SECCOMP_RET_KILL:
>  	default:
>  		audit_seccomp(this_syscall, SIGSYS, action);
> -		/* Dump core only if this is the last remaining thread. */
> -		if (get_nr_threads(current) == 1) {
> +
> +		/*
> +		 * The only way match can be NULL here is if something
> +		 * went very wrong in seccomp_run_filters() (e.g. a NULL
> +		 * filter list in struct seccomp) and the return action
> +		 * falls back to failing closed. In this case, take the
> +		 * strongest possible action.
> +		 *
> +		 * If we get here with match->kill_process set, we need
> +		 * to kill the entire thread group. Otherwise, kill only
> +		 * the offending thread.
> +		 */
> +		kill_process = (!match || match->kill_process);
> +
> +		/*
> +		 * Dumping core kills the entire thread group, so only
> +		 * coredump if that has been requested or this was already
> +		 * the only thread running.
> +		 */
> +		if (kill_process || get_nr_threads(current) == 1) {
>  			siginfo_t info;
>  
>  			/* Show the original registers in the dump. */
> @@ -665,7 +707,11 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>  			seccomp_init_siginfo(&info, this_syscall, data);
>  			do_coredump(&info);
>  		}
> -		do_exit(SIGSYS);
> +
> +		if (kill_process)
> +			do_group_exit(SIGSYS);
> +		else
> +			do_exit(SIGSYS);
>  	}
>  
>  	unreachable();
>

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