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 42A45168C4; Mon, 27 May 2024 19:21:34 +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=1716837694; cv=none; b=FUye9xo7Jp6LnmFQ2+ZSh8sih3xh5DPswuDjcry/IJs1z+RRVnvF6g6MSV2FpE0rQcSXtJn9YbVqAYumefcAOvKWQUdv6LJVjX/seUXOfoh7v5CqCDpHnytRA6eUgsxtMvGqMPAB0i4W2Sk/e9OukgMrZPFR+wapsE8ud02uszg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1716837694; c=relaxed/simple; bh=UHavKZO4PEqLn8Pjpn+6N3EMTfjwkc6LcNYuKlFHxCo=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=Cq9/COAVWWmvtGha2Zq0ksxEw2LFpc8nV3Ei0Xu/AAdVWsO8zRzOPOHO0gHWlB5AeFvZ6Zd6wQQaRjrwlQ5D31SpGgvu6jHOygXkmZ4b610dClr4DMEqZMEg+AlAK6uKndjImV2FCEc2XZxhLirmExM3LVZVyJxN02n94qCBMho= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=p5p2CmIL; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="p5p2CmIL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CEF66C2BBFC; Mon, 27 May 2024 19:21:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1716837694; bh=UHavKZO4PEqLn8Pjpn+6N3EMTfjwkc6LcNYuKlFHxCo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=p5p2CmILwj8s6fBRIehq9iLRy1WVPjCoS204kRHTc2VyASlG1Dt3ub21t/NfA12LH zlrnKMbAirHaSUVZySV0MD1Qbry/YY+hg0qozJp2gj6Dy6gAqdy6ltd0AMkcf5kLX5 BM7u5lRDFIkpuEzK0Tw9yQ404qQTy0EeCCGI/cRo= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, syzbot+045b454ab35fd82a35fb@syzkaller.appspotmail.com, Jens Axboe , Linus Torvalds , Sasha Levin Subject: [PATCH 6.8 108/493] epoll: be better about file lifetimes Date: Mon, 27 May 2024 20:51:50 +0200 Message-ID: <20240527185634.056918751@linuxfoundation.org> X-Mailer: git-send-email 2.45.1 In-Reply-To: <20240527185626.546110716@linuxfoundation.org> References: <20240527185626.546110716@linuxfoundation.org> User-Agent: quilt/0.67 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.8-stable review patch. If anyone has any objections, please let me know. ------------------ 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 3534d36a14740..c5a9a483fb538 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -875,6 +875,34 @@ 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() @@ -883,14 +911,22 @@ 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