From: "Michael S. Tsirkin" <mst@redhat.com> To: "Jason A. Donenfeld" <Jason@zx2c4.com> Cc: "Laszlo Ersek" <lersek@redhat.com>, LKML <linux-kernel@vger.kernel.org>, "KVM list" <kvm@vger.kernel.org>, "QEMU Developers" <qemu-devel@nongnu.org>, linux-hyperv@vger.kernel.org, "Linux Crypto Mailing List" <linux-crypto@vger.kernel.org>, "Alexander Graf" <graf@amazon.com>, "Michael Kelley (LINUX)" <mikelley@microsoft.com>, "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>, adrian@parity.io, "Daniel P. Berrangé" <berrange@redhat.com>, "Dominik Brodowski" <linux@dominikbrodowski.net>, "Jann Horn" <jannh@google.com>, "Rafael J. Wysocki" <rafael@kernel.org>, "Brown, Len" <len.brown@intel.com>, "Pavel Machek" <pavel@ucw.cz>, "Linux PM" <linux-pm@vger.kernel.org>, "Colm MacCarthaigh" <colmmacc@amazon.com>, "Theodore Ts'o" <tytso@mit.edu>, "Arnd Bergmann" <arnd@arndb.de> Subject: Re: propagating vmgenid outward and upward Date: Wed, 2 Mar 2022 11:22:46 -0500 [thread overview] Message-ID: <20220302111737-mutt-send-email-mst@kernel.org> (raw) In-Reply-To: <Yh+PET49oHNpxn+H@zx2c4.com> On Wed, Mar 02, 2022 at 04:36:49PM +0100, Jason A. Donenfeld wrote: > Hi Michael, > > On Wed, Mar 02, 2022 at 10:20:25AM -0500, Michael S. Tsirkin wrote: > > So writing some code: > > > > 1: > > put plaintext in a buffer > > put a key in a buffer > > put the nonce for that encryption in a buffer > > > > if vm gen id != stored vm gen id > > stored vm gen id = vm gen id > > goto 1 > > > > I think this is race free, but I don't see why does it matter whether we > > read gen id atomically or not. > > Because that 16 byte read of vmgenid is not atomic. Let's say you read > the first 8 bytes, and then the VM is forked. But at this point when VM was forked plaintext key and nonce are all in buffer, and you previously indicated a fork at this point is harmless. You wrote "If it changes _after_ that point of check ... it doesn't matter:" > In the forked VM, the next > 8 bytes are the same as last time, but the first 8 bytes, which you > already read, have changed. In that case, your != becomes a ==, and the > test fails. Yes I'm aware what an atomic read is. If the read is not atomic a part of value can change ;) -- MST
WARNING: multiple messages have this Message-ID (diff)
From: "Michael S. Tsirkin" <mst@redhat.com> To: "Jason A. Donenfeld" <Jason@zx2c4.com> Cc: "Brown, Len" <len.brown@intel.com>, linux-hyperv@vger.kernel.org, "Colm MacCarthaigh" <colmmacc@amazon.com>, "Daniel P. Berrangé" <berrange@redhat.com>, adrian@parity.io, "KVM list" <kvm@vger.kernel.org>, "Jann Horn" <jannh@google.com>, "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>, "Linux PM" <linux-pm@vger.kernel.org>, "Rafael J. Wysocki" <rafael@kernel.org>, LKML <linux-kernel@vger.kernel.org>, "Dominik Brodowski" <linux@dominikbrodowski.net>, "QEMU Developers" <qemu-devel@nongnu.org>, "Alexander Graf" <graf@amazon.com>, "Linux Crypto Mailing List" <linux-crypto@vger.kernel.org>, "Pavel Machek" <pavel@ucw.cz>, "Theodore Ts'o" <tytso@mit.edu>, "Michael Kelley (LINUX)" <mikelley@microsoft.com>, "Laszlo Ersek" <lersek@redhat.com>, "Arnd Bergmann" <arnd@arndb.de> Subject: Re: propagating vmgenid outward and upward Date: Wed, 2 Mar 2022 11:22:46 -0500 [thread overview] Message-ID: <20220302111737-mutt-send-email-mst@kernel.org> (raw) In-Reply-To: <Yh+PET49oHNpxn+H@zx2c4.com> On Wed, Mar 02, 2022 at 04:36:49PM +0100, Jason A. Donenfeld wrote: > Hi Michael, > > On Wed, Mar 02, 2022 at 10:20:25AM -0500, Michael S. Tsirkin wrote: > > So writing some code: > > > > 1: > > put plaintext in a buffer > > put a key in a buffer > > put the nonce for that encryption in a buffer > > > > if vm gen id != stored vm gen id > > stored vm gen id = vm gen id > > goto 1 > > > > I think this is race free, but I don't see why does it matter whether we > > read gen id atomically or not. > > Because that 16 byte read of vmgenid is not atomic. Let's say you read > the first 8 bytes, and then the VM is forked. But at this point when VM was forked plaintext key and nonce are all in buffer, and you previously indicated a fork at this point is harmless. You wrote "If it changes _after_ that point of check ... it doesn't matter:" > In the forked VM, the next > 8 bytes are the same as last time, but the first 8 bytes, which you > already read, have changed. In that case, your != becomes a ==, and the > test fails. Yes I'm aware what an atomic read is. If the read is not atomic a part of value can change ;) -- MST
next prev parent reply other threads:[~2022-03-02 16:22 UTC|newest] Thread overview: 61+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-03-01 15:42 propagating vmgenid outward and upward Jason A. Donenfeld 2022-03-01 16:15 ` Laszlo Ersek 2022-03-01 16:28 ` Jason A. Donenfeld 2022-03-01 16:28 ` Jason A. Donenfeld 2022-03-01 17:17 ` Michael S. Tsirkin 2022-03-01 17:17 ` Michael S. Tsirkin 2022-03-01 18:37 ` Jason A. Donenfeld 2022-03-01 18:37 ` Jason A. Donenfeld 2022-03-02 7:42 ` Michael S. Tsirkin 2022-03-02 7:42 ` Michael S. Tsirkin 2022-03-02 7:48 ` Michael S. Tsirkin 2022-03-02 7:48 ` Michael S. Tsirkin 2022-03-02 8:30 ` Michael S. Tsirkin 2022-03-02 8:30 ` Michael S. Tsirkin 2022-03-02 11:26 ` Jason A. Donenfeld 2022-03-02 11:26 ` Jason A. Donenfeld 2022-03-02 12:58 ` Michael S. Tsirkin 2022-03-02 12:58 ` Michael S. Tsirkin 2022-03-02 13:55 ` Jason A. Donenfeld 2022-03-02 13:55 ` Jason A. Donenfeld 2022-03-02 14:46 ` Michael S. Tsirkin 2022-03-02 14:46 ` Michael S. Tsirkin 2022-03-02 15:14 ` Jason A. Donenfeld 2022-03-02 15:14 ` Jason A. Donenfeld 2022-03-02 15:20 ` Michael S. Tsirkin 2022-03-02 15:20 ` Michael S. Tsirkin 2022-03-02 15:36 ` Jason A. Donenfeld 2022-03-02 15:36 ` Jason A. Donenfeld 2022-03-02 16:22 ` Michael S. Tsirkin [this message] 2022-03-02 16:22 ` Michael S. Tsirkin 2022-03-02 16:32 ` Jason A. Donenfeld 2022-03-02 16:32 ` Jason A. Donenfeld 2022-03-02 17:27 ` Michael S. Tsirkin 2022-03-02 17:27 ` Michael S. Tsirkin 2022-03-03 13:07 ` Michael S. Tsirkin 2022-03-03 13:07 ` Michael S. Tsirkin 2022-03-02 16:29 ` Michael S. Tsirkin 2022-03-02 16:29 ` Michael S. Tsirkin 2022-03-01 16:21 ` Michael S. Tsirkin 2022-03-01 16:21 ` Michael S. Tsirkin 2022-03-01 16:35 ` Jason A. Donenfeld 2022-03-01 16:35 ` Jason A. Donenfeld 2022-03-01 18:01 ` Greg KH 2022-03-01 18:01 ` Greg KH 2022-03-01 18:24 ` Jason A. Donenfeld 2022-03-01 18:24 ` Jason A. Donenfeld 2022-03-01 19:41 ` Greg KH 2022-03-01 19:41 ` Greg KH 2022-03-01 23:12 ` Jason A. Donenfeld 2022-03-01 23:12 ` Jason A. Donenfeld 2022-03-02 14:35 ` Jason A. Donenfeld 2022-03-09 10:10 ` Alexander Graf 2022-03-09 22:02 ` Jason A. Donenfeld 2022-03-09 22:02 ` Jason A. Donenfeld 2022-03-10 11:18 ` Alexander Graf 2022-03-20 22:53 ` Michael S. Tsirkin 2022-03-20 22:53 ` Michael S. Tsirkin 2022-04-19 15:12 ` Jason A. Donenfeld 2022-04-19 15:12 ` Jason A. Donenfeld 2022-04-19 16:43 ` Michael S. Tsirkin 2022-04-19 16:43 ` Michael S. Tsirkin
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20220302111737-mutt-send-email-mst@kernel.org \ --to=mst@redhat.com \ --cc=Jason@zx2c4.com \ --cc=adrian@parity.io \ --cc=arnd@arndb.de \ --cc=berrange@redhat.com \ --cc=colmmacc@amazon.com \ --cc=graf@amazon.com \ --cc=gregkh@linuxfoundation.org \ --cc=jannh@google.com \ --cc=kvm@vger.kernel.org \ --cc=len.brown@intel.com \ --cc=lersek@redhat.com \ --cc=linux-crypto@vger.kernel.org \ --cc=linux-hyperv@vger.kernel.org \ --cc=linux-kernel@vger.kernel.org \ --cc=linux-pm@vger.kernel.org \ --cc=linux@dominikbrodowski.net \ --cc=mikelley@microsoft.com \ --cc=pavel@ucw.cz \ --cc=qemu-devel@nongnu.org \ --cc=rafael@kernel.org \ --cc=tytso@mit.edu \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes, see mirroring instructions on how to clone and mirror all data and code used by this external index.