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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 6F025C4338F for ; Fri, 30 Jul 2021 15:24:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5655860F4B for ; Fri, 30 Jul 2021 15:24:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239565AbhG3PYu (ORCPT ); Fri, 30 Jul 2021 11:24:50 -0400 Received: from mail.kernel.org ([198.145.29.99]:60600 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239398AbhG3PYs (ORCPT ); Fri, 30 Jul 2021 11:24:48 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id F2A2260F46; Fri, 30 Jul 2021 15:24:41 +0000 (UTC) Date: Fri, 30 Jul 2021 16:24:39 +0100 From: Catalin Marinas To: Andrey Konovalov Cc: Kuan-Ying Lee , Marco Elver , Nicholas Tang , Andrew Yang , Andrey Ryabinin , Alexander Potapenko , Chinwen Chang , Andrew Morton , kasan-dev , Linux Memory Management List , LKML , Linux ARM , "moderated list:ARM/Mediatek SoC support" Subject: Re: [PATCH 1/2] kasan, mm: reset tag when access metadata Message-ID: <20210730152438.GA2690@arm.com> References: <20210727040021.21371-1-Kuan-Ying.Lee@mediatek.com> <20210727040021.21371-2-Kuan-Ying.Lee@mediatek.com> <20210727192217.GV13920@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 30, 2021 at 04:57:20PM +0200, Andrey Konovalov wrote: > On Tue, Jul 27, 2021 at 9:22 PM Catalin Marinas wrote: > > > > On Tue, Jul 27, 2021 at 04:32:02PM +0800, Kuan-Ying Lee wrote: > > > On Tue, 2021-07-27 at 09:10 +0200, Marco Elver wrote: > > > > +Cc Catalin > > > > > > > > On Tue, 27 Jul 2021 at 06:00, Kuan-Ying Lee < > > > > Kuan-Ying.Lee@mediatek.com> wrote: > > > > > > > > > > Hardware tag-based KASAN doesn't use compiler instrumentation, we > > > > > can not use kasan_disable_current() to ignore tag check. > > > > > > > > > > Thus, we need to reset tags when accessing metadata. > > > > > > > > > > Signed-off-by: Kuan-Ying Lee > > > > > > > > This looks reasonable, but the patch title is not saying this is > > > > kmemleak, nor does the description say what the problem is. What > > > > problem did you encounter? Was it a false positive? > > > > > > kmemleak would scan kernel memory to check memory leak. > > > When it scans on the invalid slab and dereference, the issue > > > will occur like below. > > > > > > So I think we should reset the tag before scanning. > > > > > > # echo scan > /sys/kernel/debug/kmemleak > > > [ 151.905804] > > > ================================================================== > > > [ 151.907120] BUG: KASAN: out-of-bounds in scan_block+0x58/0x170 > > > [ 151.908773] Read at addr f7ff0000c0074eb0 by task kmemleak/138 > > > [ 151.909656] Pointer tag: [f7], memory tag: [fe] > > > > It would be interesting to find out why the tag doesn't match. Kmemleak > > should in principle only scan valid objects that have been allocated and > > the pointer can be safely dereferenced. 0xfe is KASAN_TAG_INVALID, so it > > either goes past the size of the object (into the red zone) or it still > > accesses the object after it was marked as freed but before being > > released from kmemleak. > > > > With slab, looking at __cache_free(), it calls kasan_slab_free() before > > ___cache_free() -> kmemleak_free_recursive(), so the second scenario is > > possible. With slub, however, slab_free_hook() first releases the object > > from kmemleak before poisoning it. Based on the stack dump, you are > > using slub, so it may be that kmemleak goes into the object red zones. > > > > I'd like this clarified before blindly resetting the tag. > > AFAIK, kmemleak scans the whole object including the leftover redzone > for kmalloc-allocated objects. > > Looking at the report, there are 11 0xf7 granules, which amounts to > 176 bytes, and the object is allocated from the kmalloc-256 cache. So > when kmemleak accesses the last 256-176 bytes, it causes faults, as > those are marked with KASAN_KMALLOC_REDZONE == KASAN_TAG_INVALID == > 0xfe. > > Generally, resetting tags in kasan_disable/enable_current() section > should be fine to suppress MTE faults, provided those sections had > been added correctly in the first place. Thanks for the explanation, the patch makes sense. FWIW: Acked-by: Catalin Marinas