[PATCH 3/3] seccomp: introduce read protection for struct seccomp

[Tycho Andersen <tycho@tycho.ws>]

As Jann pointed out, there is a race between SECCOMP_FILTER_FLAG_TSYNC and
the ptrace code that can inspect a filter of another process. Let's
introduce read locking into the two ptrace accesses so that we don't race.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Reported-by: Jann Horn <jannh@google.com>
CC: Kees Cook <keescook@chromium.org>
CC: Andy Lutomirski <luto@amacapital.net>
---
 include/linux/seccomp.h |  4 ++--
 kernel/seccomp.c        | 10 ++++++++++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index 8429bdda947a..30b27e898162 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -22,8 +22,8 @@ struct seccomp_filter;
  * @filter: must always point to a valid seccomp-filter or NULL as it is
  *          accessed without locking during system call entry.
  *
- *          @filter must only be accessed from the context of current as there
- *          is no read locking.
+ *          @filter is read-protected by task->signal->cred_guard_mutex when
+ *          outside of current context.
  */
 struct seccomp {
 	int mode;
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index ef80dd19f268..f65d47650ac1 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1042,7 +1042,12 @@ int seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
 		return -EACCES;
 	}
 
+	ret = mutex_lock_killable(&task->signal->cred_guard_mutex);
+	if (ret < 0)
+		return ret;
+
 	filter = get_nth_filter(task, filter_off);
+	mutex_unlock(&task->signal->cred_guard_mutex);
 	if (IS_ERR(filter))
 		return PTR_ERR(filter);
 
@@ -1088,7 +1093,12 @@ int seccomp_get_metadata(struct task_struct *task,
 	if (copy_from_user(&kmd.filter_off, data, sizeof(kmd.filter_off)))
 		return -EFAULT;
 
+	ret = mutex_lock_killable(&task->signal->cred_guard_mutex);
+	if (ret < 0)
+		return ret;
+
 	filter = get_nth_filter(task, kmd.filter_off);
+	mutex_unlock(&task->signal->cred_guard_mutex);
 	if (IS_ERR(filter))
 		return PTR_ERR(filter);
 
-- 
2.17.1