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 2F354256C65; Thu, 17 Apr 2025 18:39:45 +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=1744915185; cv=none; b=m4Fl+kejgL8aFSMZ2yD7H25VhAYRr2XhXDvEVQ2ZG+6IaRoFcP9jsXmpQcQ4pfRbCvW9oml37+RsLWmatK8Iu2thq5mdL+zo3BLx3ih3d6sdwC1Ro8btjofsq/UFgrzzvSUpeiRU4djxOUGJqHcdtT+m0qshhtv2TyDclmDDErI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744915185; c=relaxed/simple; bh=gZV2mDI8AKsh44+66B8yBGSZMgjJ6WhpnP/Q4bvZzpE=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=YkD1GQrUttfpfk30AMoeEEsM1EweKK73Mk49Eg9eiWNmzgTanT/tEpksVvFZ/u700LUeJ2f4Xo6YJKzvOYpKTMkFtwMULA52J9tEuFN25jTO7wy/rRvozoQj8gHscpe0AP6d9SEf/StQoHtTQX/vMEwh6VefSAVPl1o3R+/UXL0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=k2nXkiPN; 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="k2nXkiPN" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 94FBBC4CEE4; Thu, 17 Apr 2025 18:39:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1744915185; bh=gZV2mDI8AKsh44+66B8yBGSZMgjJ6WhpnP/Q4bvZzpE=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=k2nXkiPN9zWRVz897doKnvv0wBppzb7O12tcDYmZq9XrVWjpdBy4gt6A1hUV1PCKT BBmcEIjgKYYhRKcBalubnVLxYWTVB4P4kxl4fhYiVOrlzrffImvTH+T00LdOLenbZa iU+1wAJ6eUdOlZQYxxFJgYZE7oSU4un32BP3J+JY= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Mateusz Guzik , Christian Brauner , Sasha Levin Subject: [PATCH 6.12 045/393] fs: consistently deref the files table with rcu_dereference_raw() Date: Thu, 17 Apr 2025 19:47:34 +0200 Message-ID: <20250417175109.406180834@linuxfoundation.org> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20250417175107.546547190@linuxfoundation.org> References: <20250417175107.546547190@linuxfoundation.org> User-Agent: quilt/0.68 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 6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Mateusz Guzik [ Upstream commit f381640e1bd4f2de7ccafbfe8703d33c3718aad9 ] ... except when the table is known to be only used by one thread. A file pointer can get installed at any moment despite the ->file_lock being held since the following: 8a81252b774b53e6 ("fs/file.c: don't acquire files->file_lock in fd_install()") Accesses subject to such a race can in principle suffer load tearing. While here redo the comment in dup_fd -- it only covered a race against files showing up, still assuming fd_install() takes the lock. Signed-off-by: Mateusz Guzik Link: https://lore.kernel.org/r/20250313135725.1320914-1-mjguzik@gmail.com Signed-off-by: Christian Brauner Signed-off-by: Sasha Levin --- fs/file.c | 26 +++++++++++++++++--------- 1 file changed, 17 insertions(+), 9 deletions(-) diff --git a/fs/file.c b/fs/file.c index 4cb952541dd03..b6fb6d18ac3b9 100644 --- a/fs/file.c +++ b/fs/file.c @@ -367,17 +367,25 @@ struct files_struct *dup_fd(struct files_struct *oldf, struct fd_range *punch_ho old_fds = old_fdt->fd; new_fds = new_fdt->fd; + /* + * We may be racing against fd allocation from other threads using this + * files_struct, despite holding ->file_lock. + * + * alloc_fd() might have already claimed a slot, while fd_install() + * did not populate it yet. Note the latter operates locklessly, so + * the file can show up as we are walking the array below. + * + * At the same time we know no files will disappear as all other + * operations take the lock. + * + * Instead of trying to placate userspace racing with itself, we + * ref the file if we see it and mark the fd slot as unused otherwise. + */ for (i = open_files; i != 0; i--) { - struct file *f = *old_fds++; + struct file *f = rcu_dereference_raw(*old_fds++); if (f) { get_file(f); } else { - /* - * The fd may be claimed in the fd bitmap but not yet - * instantiated in the files array if a sibling thread - * is partway through open(). So make sure that this - * fd is available to the new process. - */ __clear_open_fd(open_files - i, new_fdt); } rcu_assign_pointer(*new_fds++, f); @@ -637,7 +645,7 @@ struct file *file_close_fd_locked(struct files_struct *files, unsigned fd) return NULL; fd = array_index_nospec(fd, fdt->max_fds); - file = fdt->fd[fd]; + file = rcu_dereference_raw(fdt->fd[fd]); if (file) { rcu_assign_pointer(fdt->fd[fd], NULL); __put_unused_fd(files, fd); @@ -1219,7 +1227,7 @@ __releases(&files->file_lock) */ fdt = files_fdtable(files); fd = array_index_nospec(fd, fdt->max_fds); - tofree = fdt->fd[fd]; + tofree = rcu_dereference_raw(fdt->fd[fd]); if (!tofree && fd_is_open(fd, fdt)) goto Ebusy; get_file(file); -- 2.39.5