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 2AA302B9BE; Sun, 5 May 2024 18:00:03 +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=1714932004; cv=none; b=jPuvEXm/apwZQPPrqmjsIfHl/8jVpz6n0+cJ4PEgxx4lvHv7boUHKFE9vjOq840QVhBAbF7WNDEJWO2czM4lvMbcralLypXiGA8yH7eW3TD3fJbT1BnQFST6sxhAtuTaYAYVNeshHwO7I69D6kvM/53Ca7fSAQu7T+Ma7eJh7ks= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714932004; c=relaxed/simple; bh=TsViQ/wYOfXwpC4HIbCA3YZAn4taAzdXrsvEgMW9B4A=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=QCp7q94XPEgHGEIXe0ONMZhPJPx+7I/2c70O4s9MLjdYuzJ+rqZtMfFsNbYK7axDf9FPwOeITYq/F9Ab+h8+c9qJkPOqUzvG7waW0YvNxeJGJTK5l+qbaZlbhPRJnYcOX2MXs9N/nCV9L8j0NPjoXPRI6BWXFH8V2s1H+IcOzWE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=CZtZ4Kss; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="CZtZ4Kss" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 563D9C113CC; Sun, 5 May 2024 18:00:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1714932003; bh=TsViQ/wYOfXwpC4HIbCA3YZAn4taAzdXrsvEgMW9B4A=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=CZtZ4Kss4M1ugKJ7mNWZqHBV202HqoMwtZlcRjCpwHl674Vss/9EoYPFiv6Kb4FeD tHC4/QlBc/TNWEyBH7IT/ScaeO/01P3qAeR+FGsapRFZT4Yn6EJc4pRu0BLRUe+fhe 9HecR26VCfGEdlB9PzGLrIhnvWf+OU1PPMzCB+EE= From: Linus Torvalds To: torvalds@linux-foundation.org Cc: axboe@kernel.dk, brauner@kernel.org, christian.koenig@amd.com, dri-devel@lists.freedesktop.org, io-uring@vger.kernel.org, jack@suse.cz, keescook@chromium.org, laura@labbott.name, linaro-mm-sig@lists.linaro.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, minhquangbui99@gmail.com, sumit.semwal@linaro.org, syzbot+045b454ab35fd82a35fb@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk Subject: [PATCH v2] epoll: be better about file lifetimes Date: Sun, 5 May 2024 10:55:57 -0700 Message-ID: <20240505175556.1213266-2-torvalds@linux-foundation.org> X-Mailer: git-send-email 2.44.0.330.g4d18c88175 In-Reply-To: References: Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 --- Changes since v1: - add Link, Reported-by, and Jens' reviewed-by. And sign off on it because it looks fine to me and we have some testing now. - move epi_fget() closer to the user - more comments about the background - remove the rcu_read_lock(), with the comment explaining why it's not needed - note about returning zero rather than something like EPOLLERR|POLLHUP for a file that is going away fs/eventpoll.c | 42 +++++++++++++++++++++++++++++++++++++++++- 1 file changed, 41 insertions(+), 1 deletion(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 882b89edc52a..a3f0f868adc4 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -979,6 +979,37 @@ 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() @@ -987,14 +1018,23 @@ 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; } -- 2.44.0.330.g4d18c88175