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Tsirkin" To: "Jason A. Donenfeld" Cc: Laszlo Ersek , LKML , KVM list , QEMU Developers , linux-hyperv@vger.kernel.org, Linux Crypto Mailing List , Alexander Graf , "Michael Kelley (LINUX)" , Greg Kroah-Hartman , adrian@parity.io, Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Dominik Brodowski , Jann Horn , "Rafael J. Wysocki" , "Brown, Len" , Pavel Machek , Linux PM , Colm MacCarthaigh , Theodore Ts'o , Arnd Bergmann Subject: Re: propagating vmgenid outward and upward Message-ID: <20220302111737-mutt-send-email-mst@kernel.org> References: <20220301121419-mutt-send-email-mst@kernel.org> <20220302031738-mutt-send-email-mst@kernel.org> <20220302074503-mutt-send-email-mst@kernel.org> <20220302092149-mutt-send-email-mst@kernel.org> <20220302101602-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-pm@vger.kernel.org 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