linux-kernel - [RFC PATCH 1/3] seccomp: Refactor notification handler to prepare for new semantics

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Message-Id: <20210220090502.7202-2-sargun@sargun.me>
Date:   Sat, 20 Feb 2021 01:05:00 -0800
From:   Sargun Dhillon <sargun@...gun.me>
To:     Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>
Cc:     Sargun Dhillon <sargun@...gun.me>,
        Mauricio Vásquez Bernal <mauricio@...volk.io>,
        Rodrigo Campos <rodrigo@...volk.io>,
        Tycho Andersen <tycho@...ho.pizza>,
        Giuseppe Scrivano <gscrivan@...hat.com>
Subject: [RFC PATCH 1/3] seccomp: Refactor notification handler to prepare for new semantics

This refactors the user notification code to have a do / while loop around
the completion condition. This has a small change in semantic, in that
previously we ignored addfd calls upon wakeup if the notification had been
responded to, but instead with the new change we check for an outstanding
addfd calls prior to returning to userspace.

Signed-off-by: Sargun Dhillon <sargun@...gun.me>
---
 kernel/seccomp.c | 30 ++++++++++++++++--------------
 1 file changed, 16 insertions(+), 14 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 952dc1c90229..b48fb0a29455 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1098,28 +1098,30 @@ static int seccomp_do_user_notification(int this_syscall,
 
 	up(&match->notif->request);
 	wake_up_poll(&match->wqh, EPOLLIN | EPOLLRDNORM);
-	mutex_unlock(&match->notify_lock);
 
 	/*
 	 * This is where we wait for a reply from userspace.
 	 */
-wait:
-	err = wait_for_completion_interruptible(&n.ready);
-	mutex_lock(&match->notify_lock);
-	if (err == 0) {
-		/* Check if we were woken up by a addfd message */
+	do {
+		mutex_unlock(&match->notify_lock);
+		err = wait_for_completion_interruptible(&n.ready);
+		mutex_lock(&match->notify_lock);
+		if (err != 0)
+			goto interrupted;
+
 		addfd = list_first_entry_or_null(&n.addfd,
 						 struct seccomp_kaddfd, list);
-		if (addfd && n.state != SECCOMP_NOTIFY_REPLIED) {
+		/* Check if we were woken up by a addfd message */
+		if (addfd)
 			seccomp_handle_addfd(addfd);
-			mutex_unlock(&match->notify_lock);
-			goto wait;
-		}
-		ret = n.val;
-		err = n.error;
-		flags = n.flags;
-	}
 
+	}  while (n.state != SECCOMP_NOTIFY_REPLIED);
+
+	ret = n.val;
+	err = n.error;
+	flags = n.flags;
+
+interrupted:
 	/* If there were any pending addfd calls, clear them out */
 	list_for_each_entry_safe(addfd, tmp, &n.addfd, list) {
 		/* The process went away before we got a chance to handle it */
-- 
2.25.1