[PATCH v3 1/2] seccomp: Add wait_killable semantic to seccomp user notifier

From: Sargun Dhillon
Date: Thu Apr 28 2022 - 22:32:29 EST

This introduces a per-filter flag (SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV)
that makes it so that when notifications are received by the supervisor
the notifying process will transition to wait killable semantics. Although
wait killable isn't a set of semantics formally exposed to userspace,
the concept is searchable. If the notifying process is signaled prior
to the notification being received by the userspace agent, it will
be handled as normal.

One quirk about how this is handled is that the notifying process
only switches to TASK_KILLABLE if it receives a wakeup from either
an addfd or a signal. This is to avoid an unnecessary wakeup of
the notifying task.

Signed-off-by: Sargun Dhillon <sargun@xxxxxxxxx>
---
.../userspace-api/seccomp_filter.rst | 8 ++++
include/linux/seccomp.h | 3 +-
include/uapi/linux/seccomp.h | 2 +
kernel/seccomp.c | 42 ++++++++++++++++++-
4 files changed, 52 insertions(+), 3 deletions(-)

diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
index 539e9d4a4860..204cf5ba511a 100644
--- a/Documentation/userspace-api/seccomp_filter.rst
+++ b/Documentation/userspace-api/seccomp_filter.rst
@@ -271,6 +271,14 @@ notifying process it will be replaced. The supervisor can also add an FD, and
respond atomically by using the ``SECCOMP_ADDFD_FLAG_SEND`` flag and the return
value will be the injected file descriptor number.

+The notifying process can be preempted, resulting in the notification being
+aborted. This can be problematic when trying to take actions on behalf of the
+notifying process that are long-running and typically retryable (mounting a
+filesytem). Alternatively, the at filter installation time, the
+``SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV`` flag can be set. This flag makes it
+such that when a user notification is received by the supervisor, the notifying
+process will ignore non-fatal signals until the response is sent.
+
It is worth noting that ``struct seccomp_data`` contains the values of register
arguments to the syscall, but does not contain pointers to memory. The task's
memory is accessible to suitably privileged traces via ``ptrace()`` or
diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index 0c564e5d40ff..d31d76be4982 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -8,7 +8,8 @@
SECCOMP_FILTER_FLAG_LOG | \
SECCOMP_FILTER_FLAG_SPEC_ALLOW | \
SECCOMP_FILTER_FLAG_NEW_LISTENER | \
- SECCOMP_FILTER_FLAG_TSYNC_ESRCH)
+ SECCOMP_FILTER_FLAG_TSYNC_ESRCH | \
+ SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV)

/* sizeof() the first published struct seccomp_notif_addfd */
#define SECCOMP_NOTIFY_ADDFD_SIZE_VER0 24
diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
index 78074254ab98..0fdc6ef02b94 100644
--- a/include/uapi/linux/seccomp.h
+++ b/include/uapi/linux/seccomp.h
@@ -23,6 +23,8 @@
#define SECCOMP_FILTER_FLAG_SPEC_ALLOW (1UL << 2)
#define SECCOMP_FILTER_FLAG_NEW_LISTENER (1UL << 3)
#define SECCOMP_FILTER_FLAG_TSYNC_ESRCH (1UL << 4)
+/* Received notifications wait in killable state (only respond to fatal signals) */
+#define SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV (1UL << 5)

/*
* All BPF programs must return a 32-bit value.
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index db10e73d06e0..9291b0843cb2 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -201,6 +201,8 @@ static inline void seccomp_cache_prepare(struct seccomp_filter *sfilter)
* the filter can be freed.
* @cache: cache of arch/syscall mappings to actions
* @log: true if all actions except for SECCOMP_RET_ALLOW should be logged
+ * @wait_killable_recv: Put notifying process in killable state once the
+ * notification is received by the userspace listener.
* @prev: points to a previously installed, or inherited, filter
* @prog: the BPF program to evaluate
* @notif: the struct that holds all notification related information
@@ -221,6 +223,7 @@ struct seccomp_filter {
refcount_t refs;
refcount_t users;
bool log;
+ bool wait_killable_recv;
struct action_cache cache;
struct seccomp_filter *prev;
struct bpf_prog *prog;
@@ -894,6 +897,10 @@ static long seccomp_attach_filter(unsigned int flags,
if (flags & SECCOMP_FILTER_FLAG_LOG)
filter->log = true;

+ /* Set wait killable flag, if present. */
+ if (flags & SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV)
+ filter->wait_killable_recv = true;
+
/*
* If there is an existing filter, make it the prev and don't drop its
* task reference.
@@ -1081,6 +1088,12 @@ static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd, struct seccomp_kn
complete(&addfd->completion);
}

+static bool should_sleep_killable(struct seccomp_filter *match,
+ struct seccomp_knotif *n)
+{
+ return match->wait_killable_recv && n->state == SECCOMP_NOTIFY_SENT;
+}
+
static int seccomp_do_user_notification(int this_syscall,
struct seccomp_filter *match,
const struct seccomp_data *sd)
@@ -1111,11 +1124,25 @@ static int seccomp_do_user_notification(int this_syscall,
* This is where we wait for a reply from userspace.
*/
do {
+ bool wait_killable = should_sleep_killable(match, &n);
+
mutex_unlock(&match->notify_lock);
- err = wait_for_completion_interruptible(&n.ready);
+ if (wait_killable)
+ err = wait_for_completion_killable(&n.ready);
+ else
+ err = wait_for_completion_interruptible(&n.ready);
mutex_lock(&match->notify_lock);
- if (err != 0)
+
+ if (err != 0) {
+ /*
+ * Check to see if the notifcation got picked up and
+ * whether we should switch to wait killable.
+ */
+ if (!wait_killable && should_sleep_killable(match, &n))
+ continue;
+
goto interrupted;
+ }

addfd = list_first_entry_or_null(&n.addfd,
struct seccomp_kaddfd, list);
@@ -1485,6 +1512,9 @@ static long seccomp_notify_recv(struct seccomp_filter *filter,
mutex_lock(&filter->notify_lock);
knotif = find_notification(filter, unotif.id);
if (knotif) {
+ /* Reset the process to make sure it's not stuck */
+ if (should_sleep_killable(filter, knotif))
+ complete(&knotif->ready);
knotif->state = SECCOMP_NOTIFY_INIT;
up(&filter->notif->request);
}
@@ -1830,6 +1860,14 @@ static long seccomp_set_mode_filter(unsigned int flags,
((flags & SECCOMP_FILTER_FLAG_TSYNC_ESRCH) == 0))
return -EINVAL;

+ /*
+ * The SECCOMP_FILTER_FLAG_WAIT_KILLABLE_SENT flag doesn't make sense
+ * without the SECCOMP_FILTER_FLAG_NEW_LISTENER flag.
+ */
+ if ((flags & SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV) &&
+ ((flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) == 0))
+ return -EINVAL;
+
/* Prepare the new filter before holding any locks. */
prepared = seccomp_prepare_user_filter(filter);
if (IS_ERR(prepared))
--
2.25.1