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 ws5-mx01.kavi.com (ws5-mx01.kavi.com [34.193.7.191]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 32E2EE6FE46 for ; Fri, 22 Sep 2023 15:06:42 +0000 (UTC) Received: from lists.oasis-open.org (oasis.ws5.connectedcommunity.org [10.110.1.242]) by ws5-mx01.kavi.com (Postfix) with ESMTP id 25BED2AC66 for ; Fri, 22 Sep 2023 15:06:40 +0000 (UTC) Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id 058C09866B6 for ; Fri, 22 Sep 2023 15:06:40 +0000 (UTC) Received: from host09.ws5.connectedcommunity.org (host09.ws5.connectedcommunity.org [10.110.1.97]) by lists.oasis-open.org (Postfix) with QMQP id E609498669F; Fri, 22 Sep 2023 15:06:39 +0000 (UTC) Mailing-List: contact virtio-dev-help@lists.oasis-open.org; run by ezmlm List-ID: Sender: Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id D766C9866A2 for ; Fri, 22 Sep 2023 15:06:39 +0000 (UTC) X-Virus-Scanned: amavisd-new at kavi.com X-MC-Unique: mkVoBDqoM6OYNzXTagfwxg-1 X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1695395194; x=1695999994; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=ccSkKXRl4ZUJDdEPCtbe2GJP0dGnTGCO3AL6vFMXhgc=; b=VbZhz3fWGqbfNzq7CyEdXZshon4Np0nXsI4vaqWgboSuGdV800d/Ta/99yIGU2bVMl OQIPia9DpCy/Qd1SzqRKSRMLgM6dGuP2FnkMf/Gw9tH6cIGMho3d6YKoR/oBqazuijl4 KHYKktardhERzkkGu4rcyMouWRdeEVl5InCpr/eJheEi4uWbXjUegOZBQJ+/nvoLs3EW KQDKLRIUx57K4AGLdCUbsUsA5HfkhAa6CwcEtdSd5JQHKcIOKSJmciRPJEsyQh1ZiCRC 2oqsPmzJOQo2YZXR4UTDmNFzcsuuSMarmG8xG6kCQ3qjyyCGFhlx9mPMTkjvG58ZejzR 7H0w== X-Gm-Message-State: AOJu0YzImDnY6mv90cU4/MBIthj9OnhUpfxzjKSMiwNat0UwYexkwCjJ 6Aw76U/yZpoqMnVaXzxV+bg9mETkAEqqkbsbvznoKtQ+0bnMBrztn2hrtePAWxDs7C1GlWLT9CH U5lUy/qrn0yXKGSZT5Hd9zupzL3Tg X-Received: by 2002:a2e:7205:0:b0:2bc:c004:cc22 with SMTP id n5-20020a2e7205000000b002bcc004cc22mr7149622ljc.33.1695395194223; Fri, 22 Sep 2023 08:06:34 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFJbQZhKnaX7DfN3CsLUCNncD5LwsUj8UO6Fq7ZzsUSOf/AicLrnDMj9t6kQNvM/4jkUj8ywg== X-Received: by 2002:a2e:7205:0:b0:2bc:c004:cc22 with SMTP id n5-20020a2e7205000000b002bcc004cc22mr7149599ljc.33.1695395193778; Fri, 22 Sep 2023 08:06:33 -0700 (PDT) Date: Fri, 22 Sep 2023 11:06:24 -0400 From: "Michael S. Tsirkin" To: Babis Chalios Cc: virtio-comment@lists.oasis-open.org, virtio-dev@lists.oasis-open.org, "Cali, Marco" , "Graf (AWS), Alexander" , "Jason A. Donenfeld" , aams@amazon.de Message-ID: <20230922110329-mutt-send-email-mst@kernel.org> References: <20230918083722-mutt-send-email-mst@kernel.org> <20230918091506-mutt-send-email-mst@kernel.org> <20230918100355-mutt-send-email-mst@kernel.org> <4ab1b10e-4524-4b6c-a3df-863360fd225d@amazon.es> <20230919055811-mutt-send-email-mst@kernel.org> <4c6d6710-43c8-402b-9ae6-d2f4cafbe9a7@amazon.es> MIME-Version: 1.0 In-Reply-To: <4c6d6710-43c8-402b-9ae6-d2f4cafbe9a7@amazon.es> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Subject: Re: [virtio-dev] [PATCH RFC 3/3] rng: leak detection support On Tue, Sep 19, 2023 at 12:11:37PM +0200, Babis Chalios wrote: > On 19/9/23 12:01, Michael S. Tsirkin wrote: > > On Tue, Sep 19, 2023 at 09:32:08AM +0200, Babis Chalios wrote: > > > Resending to fix e-mail formatting issues (sorry for the spam) > > > > > > On 18/9/23 18:30, Babis Chalios wrote: > > > > > > > > Yes, that's what the driver does now in the RFC patch. > > > > > > > > However, this just > > > > > > > > decreases > > > > > > > > the race window, it doesn't eliminate it. If a third > > > > > > > > leak event happens it > > > > > > > > might not > > > > > > > > find any buffers to use: > > > > > > > > > > > > > > > > 1. available buffers to queue 1-X > > > > > > > > 2. available buffers to queue X > > > > > > > > > > > > > > > > > > > > > > > > 3. poll queue X > > > > > > > > 4. used buffers in queue X <- leak event 1 will > > > > > > > > use buffers in X > > > > > > > > 5. avail buffers in queue X > > > > > > > > 6. poll queue 1-X <- leak event 2 will > > > > > > > > use buffers in 1-X > > > > > > > > 7. used buffers in queue 1-X > > > > > > > > 8. avail buffers in queue 1-X > > > > > > > > <- leak event 3 (it > > > > > > > > needs buffers in X, race with step 5) > > > > > > > > 9. goto 3 > > > > > > > I don't get it. we added buffers in step 5. > > > > > > What if the leak event 3 arrives before step 5 had time to > > > > > > actually add the > > > > > > buffers in X and make > > > > > > them visible to the device? > > > > > Then it will see a single event in 1-X instead of two events. A leak is > > > > > a leak though, I don't see does it matter how many triggered. > > > > > > > > > > > So the scenario I have in mind is the following: > > > > > > (Epoch here is terminology that I used in the Linux RFC. It is a value > > > maintained by random.c > > > that changes every time a leak event happens). > > > > > > 1. add buffers to 1-X > > > 2. add buffers to X > > > 3. poll queue X > > > 4. vcpu 0: get getrandom() entropy and cache epoch value > > > 5. Device: First snapshot, uses buffers in X > > > 6. vcpu 1: sees used buffers > > > 7. Device: Second snapshot, uses buffers in 1-X > > > 8. vcpu 0: getrandom() observes new epoch value & caches it > > > 9. Device: Third snapshot, no buffers in either queue, (vcpu 1 from step 6 > > > has not yet finished adding new buffers). > > > 10. vcpu 1 adds new buffer in X > > > 11. vcpu 0: getrandom() will not see new epoch and gets stale entropy. > > > > > > > > > In this succession of events, when the third snapshot will happen, the > > > device won't find > > > any buffers in either queue, so it won't increase the RNG epoch value. So, > > > any entropy > > > gathered after step 8 will be the same across all snapshots. Am I missing > > > something? > > > > > > Cheers, > > > Babis > > > > > Yes but notice how this is followed by: > > > > 12. vcpu 1: sees used buffers in 1-X > > > > Driver can notify getrandom I guess? > > It could, but then we have the exact race condition that VMGENID had, > userspace has already consumed stale entropy and there's nothing we > can do about that. > > Although this is indeed a corner case, it feels like it beats the purpose > of having the hardware update directly userspace (via copy on leak). > > How do you feel about the proposal a couple of emails back? It looks to > me that it avoids completely the race condition. > > Cheers, > Babis It does. The problem of course is that this means that e.g. taking a snapshot of a guest that is stuck won't work well. I have been thinking of adding MAP/UNMAP descriptors for a while now. Thus it will be possible to modify userspace memory without consuming buffers. Would something like this solve the problem? > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org