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[83.57.168.62]) by smtp.gmail.com with ESMTPSA id k8sm1153363wrn.91.2021.11.17.15.24.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 17 Nov 2021 15:24:28 -0800 (PST) From: =?utf-8?q?Philippe_Mathieu-Daud=C3=A9?= To: qemu-devel@nongnu.org Subject: [PATCH-for-6.2 v2 1/2] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196 Date: Thu, 18 Nov 2021 00:24:21 +0100 Message-Id: <20211117232422.1026411-2-philmd@redhat.com> X-Mailer: git-send-email 2.31.1 In-Reply-To: <20211117232422.1026411-1-philmd@redhat.com> References: <20211117232422.1026411-1-philmd@redhat.com> MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=philmd@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Received-SPF: pass client-ip=170.10.133.124; envelope-from=philmd@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.701, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Laurent Vivier , Thomas Huth , Prasad J Pandit , qemu-block@nongnu.org, =?utf-8?q?Philippe_Mathieu-Daud=C3=A9?= , Markus Armbruster , Alexander Bulekov , Hanna Reitz , Paolo Bonzini , Gaoning Pan , John Snow Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" Guest might select another drive on the bus by setting the DRIVE_SEL bit of the DIGITAL OUTPUT REGISTER (DOR). The current controller model doesn't expect a BlockBackend to be NULL. A simple way to fix CVE-2021-20196 is to create an empty BlockBackend when it is missing. All further accesses will be safely handled, and the controller state machines keep behaving correctly. Fixes: CVE-2021-20196 Reported-by: Gaoning Pan (Ant Security Light-Year Lab) BugLink: https://bugs.launchpad.net/qemu/+bug/1912780 Resolves: https://gitlab.com/qemu-project/qemu/-/issues/338 Signed-off-by: Philippe Mathieu-Daudé --- hw/block/fdc.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) diff --git a/hw/block/fdc.c b/hw/block/fdc.c index fa933cd3263..eab17e946d6 100644 --- a/hw/block/fdc.c +++ b/hw/block/fdc.c @@ -1161,7 +1161,19 @@ static FDrive *get_drv(FDCtrl *fdctrl, int unit) static FDrive *get_cur_drv(FDCtrl *fdctrl) { - return get_drv(fdctrl, fdctrl->cur_drv); + FDrive *cur_drv = get_drv(fdctrl, fdctrl->cur_drv); + + if (!cur_drv->blk) { + /* + * Kludge: empty drive line selected. Create an anonymous + * BlockBackend to avoid NULL deref with various BlockBackend + * API calls within this model (CVE-2021-20196). + * Due to the controller QOM model limitations, we don't + * attach the created to the controller. + */ + cur_drv->blk = blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL); + } + return cur_drv; } /* Status A register : 0x00 (read-only) */