From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8F6BBC2D0C2 for ; Tue, 31 Dec 2019 18:12:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6D5F5206E0 for ; Tue, 31 Dec 2019 18:12:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1577815972; bh=gGMnwHoI+uzaVop3AvM64UyZ6itwwMeAjyrW0WCa2II=; h=From:To:Cc:Subject:Date:List-ID:From; b=u0W0gaZ8EN5Ufk2MFOpJ+WJOcsU83sp/aiaJMoG5poh9rFGrhlSo9Etj9BsJLl7gF juEIzBJ0/oribj3L1B3GSZazZPazN6wcPgON2PksYbxh5f2SZBLt4bmdPSrOmoRHMM dY7lN7HdZB5kCYh+gfR8c/Z7xxxFoAkmcnq47aTE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727064AbfLaSMw (ORCPT ); Tue, 31 Dec 2019 13:12:52 -0500 Received: from mail.kernel.org ([198.145.29.99]:50142 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726720AbfLaSMw (ORCPT ); Tue, 31 Dec 2019 13:12:52 -0500 Received: from zzz.tds (h75-100-12-111.burkwi.broadband.dynamic.tds.net [75.100.12.111]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 4165D206E0; Tue, 31 Dec 2019 18:12:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1577815971; bh=gGMnwHoI+uzaVop3AvM64UyZ6itwwMeAjyrW0WCa2II=; h=From:To:Cc:Subject:Date:From; b=2ipVsB9t84xDlI5XkStOUnkqNdrznfPUDq3ocS1q5YKZLCWpFHmduVWPM8P+qvXcZ pU45uGE0+Grc8kv7GYrUiEfprSljAYwaK+uFan9xC+dS22nIeYGmaOkXm5Spbc8EQt Qo0I7zp4+sHNCqEZD1M+J5Rw6Bt15mJUoxKRWd6E= From: Eric Biggers To: linux-ext4@vger.kernel.org Cc: linux-fscrypt@vger.kernel.org Subject: [PATCH] ext4: fix deadlock allocating bio_post_read_ctx from mempool Date: Tue, 31 Dec 2019 12:12:22 -0600 Message-Id: <20191231181222.47684-1-ebiggers@kernel.org> X-Mailer: git-send-email 2.24.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-fscrypt-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fscrypt@vger.kernel.org From: Eric Biggers Without any form of coordination, any case where multiple allocations from the same mempool are needed at a time to make forward progress can deadlock under memory pressure. This is the case for struct bio_post_read_ctx, as one can be allocated to decrypt a Merkle tree page during fsverity_verify_bio(), which itself is running from a post-read callback for a data bio which has its own struct bio_post_read_ctx. Fix this by freeing the first bio_post_read_ctx before calling fsverity_verify_bio(). This works because verity (if enabled) is always the last post-read step. This deadlock can be reproduced by trying to read from an encrypted verity file after reducing NUM_PREALLOC_POST_READ_CTXS to 1 and patching mempool_alloc() to pretend that pool->alloc() always fails. Note that since NUM_PREALLOC_POST_READ_CTXS is actually 128, to actually hit this bug in practice would require reading from lots of encrypted verity files at the same time. But it's theoretically possible, as N available objects isn't enough to guarantee forward progress when > N/2 threads each need 2 objects at a time. Fixes: 22cfe4b48ccb ("ext4: add fs-verity read support") Signed-off-by: Eric Biggers --- fs/ext4/readpage.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-) diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c index fef7755300c3..410c904cf59b 100644 --- a/fs/ext4/readpage.c +++ b/fs/ext4/readpage.c @@ -57,6 +57,7 @@ enum bio_post_read_step { STEP_INITIAL = 0, STEP_DECRYPT, STEP_VERITY, + STEP_MAX, }; struct bio_post_read_ctx { @@ -106,10 +107,22 @@ static void verity_work(struct work_struct *work) { struct bio_post_read_ctx *ctx = container_of(work, struct bio_post_read_ctx, work); + struct bio *bio = ctx->bio; - fsverity_verify_bio(ctx->bio); + /* + * fsverity_verify_bio() may call readpages() again, and although verity + * will be disabled for that, decryption may still be needed, causing + * another bio_post_read_ctx to be allocated. So to guarantee that + * mempool_alloc() never deadlocks we must free the current ctx first. + * This is safe because verity is the last post-read step. + */ + BUILD_BUG_ON(STEP_VERITY + 1 != STEP_MAX); + mempool_free(ctx, bio_post_read_ctx_pool); + bio->bi_private = NULL; - bio_post_read_processing(ctx); + fsverity_verify_bio(bio); + + __read_end_io(bio); } static void bio_post_read_processing(struct bio_post_read_ctx *ctx) -- 2.24.1