linux-kernel - [PATCH 5.16 34/43] fanotify: Fix stale file descriptor in copy_event_to

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Date:   Fri,  4 Feb 2022 10:22:41 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Dan Carpenter <dan.carpenter@...cle.com>,
        Mathias Krause <minipli@...ecurity.net>,
        Jan Kara <jack@...e.cz>
Subject: [PATCH 5.16 34/43] fanotify: Fix stale file descriptor in copy_event_to_user()

From: Dan Carpenter <dan.carpenter@...cle.com>

commit ee12595147ac1fbfb5bcb23837e26dd58d94b15d upstream.

This code calls fd_install() which gives the userspace access to the fd.
Then if copy_info_records_to_user() fails it calls put_unused_fd(fd) but
that will not release it and leads to a stale entry in the file
descriptor table.

Generally you can't trust the fd after a call to fd_install().  The fix
is to delay the fd_install() until everything else has succeeded.

Fortunately it requires CAP_SYS_ADMIN to reach this code so the security
impact is less.

Fixes: f644bc449b37 ("fanotify: fix copy_event_to_user() fid error clean up")
Link: https://lore.kernel.org/r/20220128195656.GA26981@kili
Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
Reviewed-by: Mathias Krause <minipli@...ecurity.net>
Signed-off-by: Jan Kara <jack@...e.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 fs/notify/fanotify/fanotify_user.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -656,9 +656,6 @@ static ssize_t copy_event_to_user(struct
 	if (fanotify_is_perm_event(event->mask))
 		FANOTIFY_PERM(event)->fd = fd;
 
-	if (f)
-		fd_install(fd, f);
-
 	if (info_mode) {
 		ret = copy_info_records_to_user(event, info, info_mode, pidfd,
 						buf, count);
@@ -666,6 +663,9 @@ static ssize_t copy_event_to_user(struct
 			goto out_close_fd;
 	}
 
+	if (f)
+		fd_install(fd, f);
+
 	return metadata.event_len;
 
 out_close_fd: