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 5E881C4167B for ; Thu, 2 Nov 2023 11:20:40 +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 A4DAA26A45 for ; Thu, 2 Nov 2023 11:20:39 +0000 (UTC) Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id 98C88986C06 for ; Thu, 2 Nov 2023 11:20:39 +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 8CC78986BF7; Thu, 2 Nov 2023 11:20:39 +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 77E6C986BF5 for ; Thu, 2 Nov 2023 11:20:29 +0000 (UTC) X-Virus-Scanned: amavisd-new at kavi.com X-MC-Unique: bLDUstYCOAqNFovjmepDpw-1 X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1698924025; x=1699528825; 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=88J+wET4Nuz1g/cdMSTL+aVrkpDwdltVhtxM9uILjJI=; b=dVKpWX4+1hJmISPL4ER6nxXLN2Bjm4yklGbOgKq9hwcr1sp0B0OPWzJnKpjlM+PhB/ 9gILAkVrGCeKKwByLZQv0m6XXAEknsqnlYH9fcZbuQqoIcmvOeHI2If342H8ikP304D/ C9nCgYzSilYyk22+D8t77R7sr6AkAVpDqW2ySAhUaWJD5m7mE4WEIi9ky7htLj5vFj4C eoZkR91HwFXyZa4oPGGt20cx9I9ynwPpkwyhtYbXdwbIrTPstaOBEXIf4J/KG6lA0eQN Iy5LXbNUOi/ROWqx89mba1iZ6f12x1b3P/9ewou1j3B/2jMDXAOg01o9a5hxm+3Zr/E6 iRdQ== X-Gm-Message-State: AOJu0YzcayEM3d1AXaY2SWG1yrGiw4Mux9C4l2oC/WoUOTVcyCem0vxn KqGEanJTxyhywSXyxrWTChuitunoOWq5vbB7dTql7ZajI5Yyp42NoryKpQ1Qsw45ZYaOtMEZdPL CijbTGBNQalc93unJKoG435KePddg X-Received: by 2002:a05:6512:2806:b0:509:4b78:69b5 with SMTP id cf6-20020a056512280600b005094b7869b5mr1270875lfb.36.1698924025476; Thu, 02 Nov 2023 04:20:25 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHmIos2Go/QCZvtPtJ73lkKTd7kunP3773zLctZ0MPwsjfLuW+qggVrVAgmPCbCdC60A444Bg== X-Received: by 2002:a05:6512:2806:b0:509:4b78:69b5 with SMTP id cf6-20020a056512280600b005094b7869b5mr1270842lfb.36.1698924025026; Thu, 02 Nov 2023 04:20:25 -0700 (PDT) Date: Thu, 2 Nov 2023 07:20:20 -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: <20231102071934-mutt-send-email-mst@kernel.org> References: <4ab1b10e-4524-4b6c-a3df-863360fd225d@amazon.es> <20230919055811-mutt-send-email-mst@kernel.org> <4c6d6710-43c8-402b-9ae6-d2f4cafbe9a7@amazon.es> <20230922110329-mutt-send-email-mst@kernel.org> <20230922115920-mutt-send-email-mst@kernel.org> <19967fde-8893-4fe8-9f9a-7ce49bf38772@amazon.es> <20230927174611-mutt-send-email-mst@kernel.org> <4d22d699-b9f3-4d67-940a-8f11bcd29819@amazon.es> MIME-Version: 1.0 In-Reply-To: <4d22d699-b9f3-4d67-940a-8f11bcd29819@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 Thu, Sep 28, 2023 at 08:16:11PM +0200, Babis Chalios wrote: > > > On 27/9/23 23:47, Michael S. Tsirkin wrote: > > On Wed, Sep 27, 2023 at 12:43:20PM +0200, Babis Chalios wrote: > > > On 22/9/23 18:01, Michael S. Tsirkin wrote: > > > > On Fri, Sep 22, 2023 at 05:40:50PM +0200, Babis Chalios wrote: > > > > > On 22/9/23 17:06, Michael S. Tsirkin wrote: > > > > > > 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. > > > > > That is true, but does it matter? The intention of the proposal > > > > > is that if it is not safe to take snapshots (i.e. no buffers in the > > > > > queue) don't take snapshots. > > > > > > > > > > > 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? > > > > > I am not familiar with MAP/UNMAP descriptors. Is there > > > > > a link where I can read about them? > > > > > > > > > > Cheers, > > > > > Babis > > > > Heh no I just came up with the name. Will write up in a couple > > > > of days, but the idea is that driver does get_user_pages, > > > > adds buffer to queue, and device will remember the address > > > > and change that memory on a snapshot. If there are buffers > > > > in the queue it will also use these to tell driver, > > > > but if there are no buffers then it won't. > > > That sounds like a nice mechanism. However in our case the page > > > holding the counter that gets increased by the hardware is a kernel > > > page. > > > > > > The reason for that is that things other than us (virtio-rng) might > > > want to notify for leak events. For example, I think that Jason > > > intended to use this mechanism to periodically notify user-space > > > PRNGs that they need to reseed. > > > > > > Cheers, > > > Babis > > > > Now I'm lost. > > when you write, e.g.: > > 4. vcpu 0: get getrandom() entropy and cache epoch value > > how does vcpu access the epoch? > > The kernel provides a user space API to map a pointer to the epoch > value. User space then caches its value and checks it every time it > needs to make sure that no entropy leak has happened before using > cached kernel entropy. > > virtio-rng driver adds a copy on leak command to the queue for > increasing this value (that's what we are speaking about in this thread). > But other systems might want to report "leaks", such as random.c > itself. > > Cheers, > Babis This idea would be fine but I don't see how it's so different from VMGENID. Care to explain? --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org