From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5AEFF1802B9; Tue, 7 May 2024 23:12:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715123532; cv=none; b=s62//80lRHwXX7i6jV4AF1zJAnQFc8P8t8j7fC2pJuepnwKIT2P8JVgDy8uTWTeHFaPpkgRMkHbb7QEEb/Vbh5V9im4DOQChPyvhaGv9YJa4R38/0j6oWr/zJ1xjDvu4foq8ijn+YvlXfpOabDWvTbq1NBxDjoOQ5VNXHyStkJM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715123532; c=relaxed/simple; bh=/nBKgpfChelk2SWz1tzhxnvoEXltEphE4iQB999gYxI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=SYTT26gK17t93X1eKqkqLNDwbHKLknjwtiZHtekCVOByxa/MT7NKTuwtkroFZO/XZPZeNpC54w+hQpgcQNNbvWu8WNZCLJdOuDk42hv4zm7EcUPGBS1IJNGrt82TpMOHBQ/BwtHpz4A7Hf2UyIhJSqTYeSyVCiw/LgoRJRoN8l8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Mw5YEZCA; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Mw5YEZCA" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0B44AC3277B; Tue, 7 May 2024 23:12:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1715123532; bh=/nBKgpfChelk2SWz1tzhxnvoEXltEphE4iQB999gYxI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Mw5YEZCAVQVgS5c6a3iPzIGoKohZ/KWKKI3jPBxVVBKjkvHEbFGvbcx0xvtp8rpfr JJfKgKJK2gDUxE9ZKAG8QvG1WXxmFciOfwwsCLFR+V9uuqJrPkYy7j0WT4KfHmAEk8 ZNUuY19mhfA543l6W+W5g7cj1Rbf4S3nEwcP8X/5UivhClOGTaYL6qJKeabTV5WrC0 PGZybv6h7aXHJlE0eyfheXTAd83uZD2yohgyPNvGGVcPhXSEXyQdoCF9UNlfdNsSxo m2e9gLGBFEvtnAanN1Osk+tDxIGB5f5Jh8/8bh1ilhpE3cxYSYHJZNJHXq8cnahtBF 9TMGmLBBq/2yw== From: Sasha Levin To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Linus Torvalds , syzbot+045b454ab35fd82a35fb@syzkaller.appspotmail.com, Jens Axboe , Sasha Levin , viro@zeniv.linux.org.uk, brauner@kernel.org, linux-fsdevel@vger.kernel.org Subject: [PATCH AUTOSEL 6.6 43/43] epoll: be better about file lifetimes Date: Tue, 7 May 2024 19:10:04 -0400 Message-ID: <20240507231033.393285-43-sashal@kernel.org> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20240507231033.393285-1-sashal@kernel.org> References: <20240507231033.393285-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore X-stable-base: Linux 6.6.30 Content-Transfer-Encoding: 8bit From: Linus Torvalds [ Upstream commit 4efaa5acf0a1d2b5947f98abb3acf8bfd966422b ] epoll can call out to vfs_poll() with a file pointer that may race with the last 'fput()'. That would make f_count go down to zero, and while the ep->mtx locking means that the resulting file pointer tear-down will be blocked until the poll returns, it means that f_count is already dead, and any use of it won't actually get a reference to the file any more: it's dead regardless. Make sure we have a valid ref on the file pointer before we call down to vfs_poll() from the epoll routines. Link: https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/ Reported-by: syzbot+045b454ab35fd82a35fb@syzkaller.appspotmail.com Reviewed-by: Jens Axboe Signed-off-by: Linus Torvalds Signed-off-by: Sasha Levin --- fs/eventpoll.c | 38 +++++++++++++++++++++++++++++++++++++- 1 file changed, 37 insertions(+), 1 deletion(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 1d9a71a0c4c16..0ed73bc7d4652 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -876,6 +876,34 @@ static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int dep return res; } +/* + * The ffd.file pointer may be in the process of being torn down due to + * being closed, but we may not have finished eventpoll_release() yet. + * + * Normally, even with the atomic_long_inc_not_zero, the file may have + * been free'd and then gotten re-allocated to something else (since + * files are not RCU-delayed, they are SLAB_TYPESAFE_BY_RCU). + * + * But for epoll, users hold the ep->mtx mutex, and as such any file in + * the process of being free'd will block in eventpoll_release_file() + * and thus the underlying file allocation will not be free'd, and the + * file re-use cannot happen. + * + * For the same reason we can avoid a rcu_read_lock() around the + * operation - 'ffd.file' cannot go away even if the refcount has + * reached zero (but we must still not call out to ->poll() functions + * etc). + */ +static struct file *epi_fget(const struct epitem *epi) +{ + struct file *file; + + file = epi->ffd.file; + if (!atomic_long_inc_not_zero(&file->f_count)) + file = NULL; + return file; +} + /* * Differs from ep_eventpoll_poll() in that internal callers already have * the ep->mtx so we need to start from depth=1, such that mutex_lock_nested() @@ -884,14 +912,22 @@ static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int dep static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt, int depth) { - struct file *file = epi->ffd.file; + struct file *file = epi_fget(epi); __poll_t res; + /* + * We could return EPOLLERR | EPOLLHUP or something, but let's + * treat this more as "file doesn't exist, poll didn't happen". + */ + if (!file) + return 0; + pt->_key = epi->event.events; if (!is_file_epoll(file)) res = vfs_poll(file, pt); else res = __ep_eventpoll_poll(file, pt, depth); + fput(file); return res & epi->event.events; } -- 2.43.0