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bh=PQvGVDijJ+YppuEhb9VuGiseHxG6npgOgv0LU/nsEZw=; b=YSFI+tVZiSXO8HqxstmnleIi8cIQ5kl5qVep0JT3nPKwwHfMDwwNURSf2+8E6iWVmJopnW nIBrX7QGyrZcrBWh03MoYXm4cB5NihPVf3EN6KTfqciApVMoEbTMdit4UYgB5C7kFd+7UV HANSRxJFf66seEfxS6tdyeegVzKh5Wc= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-587-Q2QmWS9KPsaGiKAgKcSZkg-1; Tue, 13 Apr 2021 07:53:40 -0400 X-MC-Unique: Q2QmWS9KPsaGiKAgKcSZkg-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A716E801814; Tue, 13 Apr 2021 11:53:39 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-114-55.ams2.redhat.com [10.36.114.55]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 2F5CF5D76F; Tue, 13 Apr 2021 11:53:37 +0000 (UTC) Subject: Re: [PATCH for-6.0] block/nbd: fix possible use after free of s->connect_thread To: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org References: <20210406155114.1057355-1-vsementsov@virtuozzo.com> From: Max Reitz Message-ID: Date: Tue, 13 Apr 2021 13:53:35 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <20210406155114.1057355-1-vsementsov@virtuozzo.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kwolf@redhat.com, rvkagan@yandex-team.ru, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 06.04.21 17:51, Vladimir Sementsov-Ogievskiy wrote: > If on nbd_close() we detach the thread (in > nbd_co_establish_connection_cancel() thr->state becomes > CONNECT_THREAD_RUNNING_DETACHED), after that point we should not use > s->connect_thread (which is set to NULL), as running thread may free it > at any time. > > Still nbd_co_establish_connection() does exactly this: it saves > s->connect_thread to local variable (just for better code style) and > use it even after yield point, when thread may be already detached. > > Fix that. Also check thr to be non-NULL on > nbd_co_establish_connection() start for safety. > > After this patch "case CONNECT_THREAD_RUNNING_DETACHED" becomes > impossible in the second switch in nbd_co_establish_connection(). > Still, don't add extra abort() just before the release. If it somehow > possible to reach this "case:" it won't hurt. Anyway, good refactoring > of all this reconnect mess will come soon. > > Signed-off-by: Vladimir Sementsov-Ogievskiy > --- > > Hi all! I faced a crash, just running 277 iotest in a loop. I can't > reproduce it on master, it reproduces only on my branch with nbd > reconnect refactorings. > > Still, it seems very possible that it may crash under some conditions. > So I propose this patch for 6.0. It's written so that it's obvious that > it will not hurt: > > pre-patch, on first hunk we'll just crash if thr is NULL, > on second hunk it's safe to return -1, and using thr when > s->connect_thread is already zeroed is obviously wrong. > > block/nbd.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) > > diff --git a/block/nbd.c b/block/nbd.c > index c26dc5a54f..1d4668d42d 100644 > --- a/block/nbd.c > +++ b/block/nbd.c > @@ -443,6 +443,11 @@ nbd_co_establish_connection(BlockDriverState *bs, Error **errp) > BDRVNBDState *s = bs->opaque; > NBDConnectThread *thr = s->connect_thread; > > + if (!thr) { > + /* detached */ > + return -1; > + } > + > qemu_mutex_lock(&thr->mutex); > > switch (thr->state) { First, it is a bit strange not to set *errp in these cases. I don’t think it’d make a difference anyway, but now that I look into it... The error would be propagated to s->connect_err, but that isn’t used anywhere, so... What’s the point? Anyway. What I really wanted to ask is: nbd_co_establish_connection() may create a thread, which accesses the thread. Is that a problem? Like, can this happen: - nbd_co_establish_connection(): thr->state := THREAD_RUNNING - nbd_co_establish_connection(): thread is created - nbd_co_establish_connection(): thr->mutex unlocked - connect_thread_func(): thr->mutex locked - connect_thread_func(): thr->state := something else - connect_thread_func(): thr->mutex unlocked - nbd_co_establish_connection(): yields - (As I understood it, this yield leads to nbd_co_establish_connection_cancel() continuing) - nbd_co_EC_cancel(): thr->mutex locked - nbd_co_EC_cancel(): do_free true - nbd_co_EC_cancel(): nbd_free_connect_thread() - connect_thread_func(): nbd_free_connect_thread() ? Max