[PATCH v3 2/4] seccomp: Add SECCOMP_FILTER_FLAG_KILL_PROCESS
From: Kees Cook
Date: Wed Aug 09 2017 - 15:02:13 EST
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.
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;
+ }
+
+ 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();
--
2.7.4