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Subject[PATCH v2 2/4] seccomp: Add SECCOMP_FILTER_FLAG_KILL_PROCESS
Date
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.

Instead, create 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.

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.
Cons:
- depends on adding an assignment to the seccomp_run_filters() loop
(previous patch).

Alternatives to this approach with pros/cons:

- 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.
Cons:
- added complexity 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).

- Have SECCOMP_FILTER_FLAG_KILL_PROCESS attach to the seccomp struct
rather than the filter.
Pros:
- no change needed to seccomp_run_filters() loop.
Cons:
- the change in behavior technically originates external to the
filter, which allows for later filters to "enhance" a previously
applied filter's RET_KILL to kill the entire process, which may
be unexpected.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/seccomp.h | 3 ++-
include/uapi/linux/seccomp.h | 3 ++-
kernel/seccomp.c | 22 +++++++++++++++++++++-
3 files changed, 25 insertions(+), 3 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..297f8bfc3b72 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;
};
@@ -450,6 +452,10 @@ 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.
@@ -665,7 +671,21 @@ 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);
+ /*
+ * 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.
+ */
+ if (!match || match->kill_process)
+ do_group_exit(SIGSYS);
+ else
+ do_exit(SIGSYS);
}

unreachable();
--
2.7.4
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