commit dfe719fef03d752f1682fa8aeddf30ba501c8555
author Jann Horn <jannh@google.com> Mon Oct 05 03:44:01 2020 +0200
committer Kees Cook <keescook@chromium.org> Thu Oct 08 13:17:47 2020 -0700
tree 7695a6683c814dbfd5174b10d55f51e071825bad
parent 282a181b1a0d66de1f0894d82f395fcd478f51d1

seccomp: Make duplicate listener detection non-racy

Currently, init_listener() tries to prevent adding a filter with
SECCOMP_FILTER_FLAG_NEW_LISTENER if one of the existing filters already
has a listener. However, this check happens without holding any lock that
would prevent another thread from concurrently installing a new filter
(potentially with a listener) on top of the ones we already have.

Theoretically, this is also a data race: The plain load from
current->seccomp.filter can race with concurrent writes to the same
location.

Fix it by moving the check into the region that holds the siglock to guard
against concurrent TSYNC.

(The "Fixes" tag points to the commit that introduced the theoretical
data race; concurrent installation of another filter with TSYNC only
became possible later, in commit 51891498f2da ("seccomp: allow TSYNC and
USER_NOTIF together").)

Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
Reviewed-by: Tycho Andersen <tycho@tycho.pizza>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20201005014401.490175-1-jannh@google.com

kernel/seccomp.c

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index ae6b40c..8ad7a29 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1476,13 +1476,7 @@ static const struct file_operations seccomp_notify_ops = {
 
 static struct file *init_listener(struct seccomp_filter *filter)
 {
-	struct file *ret = ERR_PTR(-EBUSY);
-	struct seccomp_filter *cur;
-
-	for (cur = current->seccomp.filter; cur; cur = cur->prev) {
-		if (cur->notif)
-			goto out;
-	}
+	struct file *ret;
 
 	ret = ERR_PTR(-ENOMEM);
 	filter->notif = kzalloc(sizeof(*(filter->notif)), GFP_KERNEL);
@@ -1508,6 +1502,31 @@ static struct file *init_listener(struct seccomp_filter *filter)
 	return ret;
 }
 
+/*
+ * Does @new_child have a listener while an ancestor also has a listener?
+ * If so, we'll want to reject this filter.
+ * This only has to be tested for the current process, even in the TSYNC case,
+ * because TSYNC installs @child with the same parent on all threads.
+ * Note that @new_child is not hooked up to its parent at this point yet, so
+ * we use current->seccomp.filter.
+ */
+static bool has_duplicate_listener(struct seccomp_filter *new_child)
+{
+	struct seccomp_filter *cur;
+
+	/* must be protected against concurrent TSYNC */
+	lockdep_assert_held(&current->sighand->siglock);
+
+	if (!new_child->notif)
+		return false;
+	for (cur = current->seccomp.filter; cur; cur = cur->prev) {
+		if (cur->notif)
+			return true;
+	}
+
+	return false;
+}
+
 /**
  * seccomp_set_mode_filter: internal function for setting seccomp filter
  * @flags:  flags to change filter behavior
@@ -1579,6 +1598,11 @@ static long seccomp_set_mode_filter(unsigned int flags,
 	if (!seccomp_may_assign_mode(seccomp_mode))
 		goto out;
 
+	if (has_duplicate_listener(prepared)) {
+		ret = -EBUSY;
+		goto out;
+	}
+
 	ret = seccomp_attach_filter(flags, prepared);
 	if (ret)
 		goto out;