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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 982C4C74A5B for ; Wed, 29 Mar 2023 23:59:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230489AbjC2X73 (ORCPT ); Wed, 29 Mar 2023 19:59:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58664 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229763AbjC2X71 (ORCPT ); Wed, 29 Mar 2023 19:59:27 -0400 Received: from gandalf.ozlabs.org (gandalf.ozlabs.org [150.107.74.76]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EB4523C03 for ; Wed, 29 Mar 2023 16:59:24 -0700 (PDT) Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mail.ozlabs.org (Postfix) with ESMTPSA id 4Pn3S816GNz4xDn; Thu, 30 Mar 2023 10:59:19 +1100 (AEDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ellerman.id.au; s=201909; t=1680134360; bh=QjU/3+HiBzvoD78RQQNgOACB6ehBJw/P/6iRJxUBDyo=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=in9YqqklVEuhZ4RAJvrGnZpVvzZKa0iFvTasr4dGVU11K1xIULrTEsOjqio4zrxBq 0abG6VEPkZ0qDCtyUZeOyJcz/LbuD0tZ7aNNY1PhwmPslm/ozPoel60SBWi7eFbsM0 wVAkpDhQZMzB/OoMdLXqRX6CLwyi+CvndbdWepQWLnhftOtUw9Qw0dH2YSOHtAkVzq tfIhsBnGCrq/Rfr0BpNQow712QXrXMSiV8YtjtiL2pP8LlC0R/u5P+t//P7Dbceelv GnSiNYwQYjSgpyQANKqMkT3zd/GZx6ekk+jPbZYCywnApYpfqrxqMtg+wBcxchl16x Nd9uj5uptR/WQ== From: Michael Ellerman To: Kautuk Consul Cc: Nicholas Piggin , Christophe Leroy , Fabiano Rosas , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] arch/powerpc/kvm: kvmppc_core_vcpu_create_hv: check for kzalloc failure In-Reply-To: References: <20230323074718.2810914-1-kconsul@linux.vnet.ibm.com> <87pm8tcir3.fsf@mpe.ellerman.id.au> <87fs9pcce6.fsf@mpe.ellerman.id.au> Date: Thu, 30 Mar 2023 10:59:19 +1100 Message-ID: <87tty3az3c.fsf@mpe.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Kautuk Consul writes: > On 2023-03-28 23:02:09, Michael Ellerman wrote: >> Kautuk Consul writes: >> > On 2023-03-28 15:44:02, Kautuk Consul wrote: >> >> On 2023-03-28 20:44:48, Michael Ellerman wrote: >> >> > Kautuk Consul writes: >> >> > > kvmppc_vcore_create() might not be able to allocate memory through >> >> > > kzalloc. In that case the kvm->arch.online_vcores shouldn't be >> >> > > incremented. >> >> > >> >> > I agree that looks wrong. >> >> > >> >> > Have you tried to test what goes wrong if it fails? It looks like it >> >> > will break the LPCR update, which likely will cause the guest to crash >> >> > horribly. >> > Also, are you referring to the code in kvmppc_update_lpcr()? >> > That code will not crash as it checks for the vc before trying to >> > dereference it. >> >> Yeah that's what I was looking at. I didn't mean it would crash, but >> that it would bail out early when it sees a NULL vcore, leaving other >> vcores with the wrong LPCR value. >> >> But as you say it doesn't happen because qemu quits on the first ENOMEM. >> >> And regardless if qemu does something that means the guest is broken >> that's just a qemu bug, no big deal as far as the kernel is concerned. > But there could be another user-mode application other than qemu that > actually tries to create a vcpu after it gets a -ENOMEM for another > vcpu. Shouldn't the kernel be independent of qemu? Yes, the kernel is independent of qemu. On P8 we had kvmtool, and there's several other VMMs these days, though most don't support Power. I didn't mean qemu specifically above. If any VMM continues blindly after getting ENOMEM back from the KVM API then that's a bug in that VMM. >> > But the following 2 places that utilize the arch.online_vcores will have >> > problems in logic if the usermode test-case doesn't pull down the >> > kvm context after the -ENOMEM vcpu allocation failure: >> > book3s_hv.c:3030: if (!kvm->arch.online_vcores) { >> > book3s_hv_rm_mmu.c:44: if (kvm->arch.online_vcores == 1 && local_paca->kvm_hstate.kvm_vcpu) >> >> OK. Both of those look harmless to the host. > Harmless to the host in terms of a crash, not in terms of behavior. > For example in the case of kvmhv_set_smt_mode: > If we got a kzalloc failure once (and online_vcores was wrongly incremented), > then if kvmhv_set_smt_mode() is called after that then it would be > not be setting the arch.smt_mode and arch.emul_smt_mode correctly and it > would be wrongly returning with -EBUSY instead of 0. But again that bug only affects that VM, which the VMM should have terminated when it got the ENOMEM back. It's definitely a bug that we increment online_vcores incorrectly, but it only affects that VM, and a correctly operating VMM will terminate the VM anyway because of the ENOMEM. cheers