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-rw-r--r--security/landlock/tsync.c561
1 files changed, 561 insertions, 0 deletions
diff --git a/security/landlock/tsync.c b/security/landlock/tsync.c
new file mode 100644
index 000000000000..0d2b9c646030
--- /dev/null
+++ b/security/landlock/tsync.c
@@ -0,0 +1,561 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Landlock - Cross-thread ruleset enforcement
+ *
+ * Copyright © 2025 Google LLC
+ */
+
+#include <linux/atomic.h>
+#include <linux/cleanup.h>
+#include <linux/completion.h>
+#include <linux/cred.h>
+#include <linux/errno.h>
+#include <linux/overflow.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+#include <linux/sched/signal.h>
+#include <linux/sched/task.h>
+#include <linux/slab.h>
+#include <linux/task_work.h>
+
+#include "cred.h"
+#include "tsync.h"
+
+/*
+ * Shared state between multiple threads which are enforcing Landlock rulesets
+ * in lockstep with each other.
+ */
+struct tsync_shared_context {
+ /* The old and tentative new creds of the calling thread. */
+ const struct cred *old_cred;
+ const struct cred *new_cred;
+
+ /* True if sibling tasks need to set the no_new_privs flag. */
+ bool set_no_new_privs;
+
+ /* An error encountered in preparation step, or 0. */
+ atomic_t preparation_error;
+
+ /*
+ * Barrier after preparation step in restrict_one_thread.
+ * The calling thread waits for completion.
+ *
+ * Re-initialized on every round of looking for newly spawned threads.
+ */
+ atomic_t num_preparing;
+ struct completion all_prepared;
+
+ /* Sibling threads wait for completion. */
+ struct completion ready_to_commit;
+
+ /*
+ * Barrier after commit step (used by syscall impl to wait for
+ * completion).
+ */
+ atomic_t num_unfinished;
+ struct completion all_finished;
+};
+
+struct tsync_work {
+ struct callback_head work;
+ struct task_struct *task;
+ struct tsync_shared_context *shared_ctx;
+};
+
+/*
+ * restrict_one_thread - update a thread's Landlock domain in lockstep with the
+ * other threads in the same process
+ *
+ * When this is run, the same function gets run in all other threads in the same
+ * process (except for the calling thread which called landlock_restrict_self).
+ * The concurrently running invocations of restrict_one_thread coordinate
+ * through the shared ctx object to do their work in lockstep to implement
+ * all-or-nothing semantics for enforcing the new Landlock domain.
+ *
+ * Afterwards, depending on the presence of an error, all threads either commit
+ * or abort the prepared credentials. The commit operation can not fail any
+ * more.
+ */
+static void restrict_one_thread(struct tsync_shared_context *ctx)
+{
+ int err;
+ struct cred *cred = NULL;
+
+ if (current_cred() == ctx->old_cred) {
+ /*
+ * Switch out old_cred with new_cred, if possible.
+ *
+ * In the common case, where all threads initially point to the same
+ * struct cred, this optimization avoids creating separate redundant
+ * credentials objects for each, which would all have the same contents.
+ *
+ * Note: We are intentionally dropping the const qualifier here, because
+ * it is required by commit_creds() and abort_creds().
+ */
+ cred = (struct cred *)get_cred(ctx->new_cred);
+ } else {
+ /* Else, prepare new creds and populate them. */
+ cred = prepare_creds();
+
+ if (!cred) {
+ atomic_set(&ctx->preparation_error, -ENOMEM);
+
+ /*
+ * Even on error, we need to adhere to the protocol and coordinate
+ * with concurrently running invocations.
+ */
+ if (atomic_dec_return(&ctx->num_preparing) == 0)
+ complete_all(&ctx->all_prepared);
+
+ goto out;
+ }
+
+ landlock_cred_copy(landlock_cred(cred),
+ landlock_cred(ctx->new_cred));
+ }
+
+ /*
+ * Barrier: Wait until all threads are done preparing.
+ * After this point, we can have no more failures.
+ */
+ if (atomic_dec_return(&ctx->num_preparing) == 0)
+ complete_all(&ctx->all_prepared);
+
+ /*
+ * Wait for signal from calling thread that it's safe to read the
+ * preparation error now and we are ready to commit (or abort).
+ */
+ wait_for_completion(&ctx->ready_to_commit);
+
+ /* Abort the commit if any of the other threads had an error. */
+ err = atomic_read(&ctx->preparation_error);
+ if (err) {
+ abort_creds(cred);
+ goto out;
+ }
+
+ /*
+ * Make sure that all sibling tasks fulfill the no_new_privs prerequisite.
+ * (This is in line with Seccomp's SECCOMP_FILTER_FLAG_TSYNC logic in
+ * kernel/seccomp.c)
+ */
+ if (ctx->set_no_new_privs)
+ task_set_no_new_privs(current);
+
+ commit_creds(cred);
+
+out:
+ /* Notify the calling thread once all threads are done */
+ if (atomic_dec_return(&ctx->num_unfinished) == 0)
+ complete_all(&ctx->all_finished);
+}
+
+/*
+ * restrict_one_thread_callback - task_work callback for restricting a thread
+ *
+ * Calls restrict_one_thread with the struct landlock_shared_tsync_context.
+ */
+static void restrict_one_thread_callback(struct callback_head *work)
+{
+ struct tsync_work *ctx = container_of(work, struct tsync_work, work);
+
+ restrict_one_thread(ctx->shared_ctx);
+}
+
+/*
+ * struct tsync_works - a growable array of per-task contexts
+ *
+ * The zero-initialized struct represents the empty array.
+ */
+struct tsync_works {
+ struct tsync_work **works;
+ size_t size;
+ size_t capacity;
+};
+
+/*
+ * tsync_works_provide - provides a preallocated tsync_work for the given task
+ *
+ * This also stores a task pointer in the context and increments the reference
+ * count of the task.
+ *
+ * This function may fail in the case where we did not preallocate sufficient
+ * capacity. This can legitimately happen if new threads get started after we
+ * grew the capacity.
+ *
+ * Returns:
+ * A pointer to the preallocated context struct, with task filled in.
+ *
+ * NULL, if we ran out of preallocated context structs.
+ */
+static struct tsync_work *tsync_works_provide(struct tsync_works *s,
+ struct task_struct *task)
+{
+ struct tsync_work *ctx;
+
+ if (s->size >= s->capacity)
+ return NULL;
+
+ ctx = s->works[s->size];
+ s->size++;
+
+ ctx->task = get_task_struct(task);
+ return ctx;
+}
+
+/*
+ * tsync_works_grow_by - preallocates space for n more contexts in s
+ *
+ * On a successful return, the subsequent n calls to tsync_works_provide() are
+ * guaranteed to succeed. (size + n <= capacity)
+ *
+ * Returns:
+ * -ENOMEM if the (re)allocation fails
+
+ * 0 if the allocation succeeds, partially succeeds, or no reallocation
+ * was needed
+ */
+static int tsync_works_grow_by(struct tsync_works *s, size_t n, gfp_t flags)
+{
+ size_t i;
+ size_t new_capacity;
+ struct tsync_work **works;
+ struct tsync_work *work;
+
+ if (check_add_overflow(s->size, n, &new_capacity))
+ return -EOVERFLOW;
+
+ /* No need to reallocate if s already has sufficient capacity. */
+ if (new_capacity <= s->capacity)
+ return 0;
+
+ works = krealloc_array(s->works, new_capacity, sizeof(s->works[0]),
+ flags);
+ if (!works)
+ return -ENOMEM;
+
+ s->works = works;
+
+ for (i = s->capacity; i < new_capacity; i++) {
+ work = kzalloc(sizeof(*work), flags);
+ if (!work) {
+ /*
+ * Leave the object in a consistent state,
+ * but return an error.
+ */
+ s->capacity = i;
+ return -ENOMEM;
+ }
+ s->works[i] = work;
+ }
+ s->capacity = new_capacity;
+ return 0;
+}
+
+/*
+ * tsync_works_contains - checks for presence of task in s
+ */
+static bool tsync_works_contains_task(const struct tsync_works *s,
+ struct task_struct *task)
+{
+ size_t i;
+
+ for (i = 0; i < s->size; i++)
+ if (s->works[i]->task == task)
+ return true;
+ return false;
+}
+
+/*
+ * tsync_works_release - frees memory held by s and drops all task references
+ *
+ * This does not free s itself, only the data structures held by it.
+ */
+static void tsync_works_release(struct tsync_works *s)
+{
+ size_t i;
+
+ for (i = 0; i < s->size; i++) {
+ if (!s->works[i]->task)
+ continue;
+
+ put_task_struct(s->works[i]->task);
+ }
+
+ for (i = 0; i < s->capacity; i++)
+ kfree(s->works[i]);
+ kfree(s->works);
+ s->works = NULL;
+ s->size = 0;
+ s->capacity = 0;
+}
+
+/*
+ * count_additional_threads - counts the sibling threads that are not in works
+ */
+static size_t count_additional_threads(const struct tsync_works *works)
+{
+ struct task_struct *thread, *caller;
+ size_t n = 0;
+
+ caller = current;
+
+ guard(rcu)();
+
+ for_each_thread(caller, thread) {
+ /* Skip current, since it is initiating the sync. */
+ if (thread == caller)
+ continue;
+
+ /* Skip exited threads. */
+ if (thread->flags & PF_EXITING)
+ continue;
+
+ /* Skip threads that we have already seen. */
+ if (tsync_works_contains_task(works, thread))
+ continue;
+
+ n++;
+ }
+ return n;
+}
+
+/*
+ * schedule_task_work - adds task_work for all eligible sibling threads
+ * which have not been scheduled yet
+ *
+ * For each added task_work, atomically increments shared_ctx->num_preparing and
+ * shared_ctx->num_unfinished.
+ *
+ * Returns:
+ * true, if at least one eligible sibling thread was found
+ */
+static bool schedule_task_work(struct tsync_works *works,
+ struct tsync_shared_context *shared_ctx)
+{
+ int err;
+ struct task_struct *thread, *caller;
+ struct tsync_work *ctx;
+ bool found_more_threads = false;
+
+ caller = current;
+
+ guard(rcu)();
+
+ for_each_thread(caller, thread) {
+ /* Skip current, since it is initiating the sync. */
+ if (thread == caller)
+ continue;
+
+ /* Skip exited threads. */
+ if (thread->flags & PF_EXITING)
+ continue;
+
+ /* Skip threads that we already looked at. */
+ if (tsync_works_contains_task(works, thread))
+ continue;
+
+ /*
+ * We found a sibling thread that is not doing its task_work yet, and
+ * which might spawn new threads before our task work runs, so we need
+ * at least one more round in the outer loop.
+ */
+ found_more_threads = true;
+
+ ctx = tsync_works_provide(works, thread);
+ if (!ctx) {
+ /*
+ * We ran out of preallocated contexts -- we need to try again with
+ * this thread at a later time!
+ * found_more_threads is already true at this point.
+ */
+ break;
+ }
+
+ ctx->shared_ctx = shared_ctx;
+
+ atomic_inc(&shared_ctx->num_preparing);
+ atomic_inc(&shared_ctx->num_unfinished);
+
+ init_task_work(&ctx->work, restrict_one_thread_callback);
+ err = task_work_add(thread, &ctx->work, TWA_SIGNAL);
+ if (err) {
+ /*
+ * task_work_add() only fails if the task is about to exit. We
+ * checked that earlier, but it can happen as a race. Resume
+ * without setting an error, as the task is probably gone in the
+ * next loop iteration. For consistency, remove the task from ctx
+ * so that it does not look like we handed it a task_work.
+ */
+ put_task_struct(ctx->task);
+ ctx->task = NULL;
+
+ atomic_dec(&shared_ctx->num_preparing);
+ atomic_dec(&shared_ctx->num_unfinished);
+ }
+ }
+
+ return found_more_threads;
+}
+
+/*
+ * cancel_tsync_works - cancel all task works where it is possible
+ *
+ * Task works can be canceled as long as they are still queued and have not
+ * started running. If they get canceled, we decrement
+ * shared_ctx->num_preparing and shared_ctx->num_unfished and mark the two
+ * completions if needed, as if the task was never scheduled.
+ */
+static void cancel_tsync_works(struct tsync_works *works,
+ struct tsync_shared_context *shared_ctx)
+{
+ int i;
+
+ for (i = 0; i < works->size; i++) {
+ if (!task_work_cancel(works->works[i]->task,
+ &works->works[i]->work))
+ continue;
+
+ /* After dequeueing, act as if the task work had executed. */
+
+ if (atomic_dec_return(&shared_ctx->num_preparing) == 0)
+ complete_all(&shared_ctx->all_prepared);
+
+ if (atomic_dec_return(&shared_ctx->num_unfinished) == 0)
+ complete_all(&shared_ctx->all_finished);
+ }
+}
+
+/*
+ * restrict_sibling_threads - enables a Landlock policy for all sibling threads
+ */
+int landlock_restrict_sibling_threads(const struct cred *old_cred,
+ const struct cred *new_cred)
+{
+ int err;
+ struct tsync_shared_context shared_ctx;
+ struct tsync_works works = {};
+ size_t newly_discovered_threads;
+ bool found_more_threads;
+
+ atomic_set(&shared_ctx.preparation_error, 0);
+ init_completion(&shared_ctx.all_prepared);
+ init_completion(&shared_ctx.ready_to_commit);
+ atomic_set(&shared_ctx.num_unfinished, 1);
+ init_completion(&shared_ctx.all_finished);
+ shared_ctx.old_cred = old_cred;
+ shared_ctx.new_cred = new_cred;
+ shared_ctx.set_no_new_privs = task_no_new_privs(current);
+
+ /*
+ * We schedule a pseudo-signal task_work for each of the calling task's
+ * sibling threads. In the task work, each thread:
+ *
+ * 1) runs prepare_creds() and writes back the error to
+ * shared_ctx.preparation_error, if needed.
+ *
+ * 2) signals that it's done with prepare_creds() to the calling task.
+ * (completion "all_prepared").
+ *
+ * 3) waits for the completion "ready_to_commit". This is sent by the
+ * calling task after ensuring that all sibling threads have done
+ * with the "preparation" stage.
+ *
+ * After this barrier is reached, it's safe to read
+ * shared_ctx.preparation_error.
+ *
+ * 4) reads shared_ctx.preparation_error and then either does commit_creds()
+ * or abort_creds().
+ *
+ * 5) signals that it's done altogether (barrier synchronization
+ * "all_finished")
+ *
+ * Unlike seccomp, which modifies sibling tasks directly, we do not need to
+ * acquire the cred_guard_mutex and sighand->siglock:
+ *
+ * - As in our case, all threads are themselves exchanging their own struct
+ * cred through the credentials API, no locks are needed for that.
+ * - Our for_each_thread() loops are protected by RCU.
+ * - We do not acquire a lock to keep the list of sibling threads stable
+ * between our for_each_thread loops. If the list of available sibling
+ * threads changes between these for_each_thread loops, we make up for
+ * that by continuing to look for threads until they are all discovered
+ * and have entered their task_work, where they are unable to spawn new
+ * threads.
+ */
+ do {
+ /* In RCU read-lock, count the threads we need. */
+ newly_discovered_threads = count_additional_threads(&works);
+
+ if (newly_discovered_threads == 0)
+ break; /* done */
+
+ err = tsync_works_grow_by(&works, newly_discovered_threads,
+ GFP_KERNEL_ACCOUNT);
+ if (err) {
+ atomic_set(&shared_ctx.preparation_error, err);
+ break;
+ }
+
+ /*
+ * The "all_prepared" barrier is used locally to the loop body, this use
+ * of for_each_thread(). We can reset it on each loop iteration because
+ * all previous loop iterations are done with it already.
+ *
+ * num_preparing is initialized to 1 so that the counter can not go to 0
+ * and mark the completion as done before all task works are registered.
+ * We decrement it at the end of the loop body.
+ */
+ atomic_set(&shared_ctx.num_preparing, 1);
+ reinit_completion(&shared_ctx.all_prepared);
+
+ /*
+ * In RCU read-lock, schedule task work on newly discovered sibling
+ * tasks.
+ */
+ found_more_threads = schedule_task_work(&works, &shared_ctx);
+
+ /*
+ * Decrement num_preparing for current, to undo that we initialized it
+ * to 1 a few lines above.
+ */
+ if (atomic_dec_return(&shared_ctx.num_preparing) > 0) {
+ if (wait_for_completion_interruptible(
+ &shared_ctx.all_prepared)) {
+ /* In case of interruption, we need to retry the system call. */
+ atomic_set(&shared_ctx.preparation_error,
+ -ERESTARTNOINTR);
+
+ /*
+ * Cancel task works for tasks that did not start running yet,
+ * and decrement all_prepared and num_unfinished accordingly.
+ */
+ cancel_tsync_works(&works, &shared_ctx);
+
+ /*
+ * The remaining task works have started running, so waiting for
+ * their completion will finish.
+ */
+ wait_for_completion(&shared_ctx.all_prepared);
+ }
+ }
+ } while (found_more_threads &&
+ !atomic_read(&shared_ctx.preparation_error));
+
+ /*
+ * We now have all sibling threads blocking and in "prepared" state in the
+ * task work. Ask all threads to commit.
+ */
+ complete_all(&shared_ctx.ready_to_commit);
+
+ /*
+ * Decrement num_unfinished for current, to undo that we initialized it to 1
+ * at the beginning.
+ */
+ if (atomic_dec_return(&shared_ctx.num_unfinished) > 0)
+ wait_for_completion(&shared_ctx.all_finished);
+
+ tsync_works_release(&works);
+
+ return atomic_read(&shared_ctx.preparation_error);
+}
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