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Wed, 26 Jan 2022 07:41:45 -0800 (PST) Received: from [10.0.0.97] ([24.225.241.171]) by smtp.gmail.com with ESMTPSA id x16sm11440747qko.17.2022.01.26.07.41.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Jan 2022 07:41:44 -0800 (PST) Message-ID: Date: Wed, 26 Jan 2022 10:44:55 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.3.0 Subject: Re: [PATCH] fdc: check for illegal dma length calculation To: qemu-devel@nongnu.org, Jason Wang References: <20220114013319.348012-1-jmaloy@redhat.com> From: Jon Maloy In-Reply-To: <20220114013319.348012-1-jmaloy@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=jmaloy@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=jmaloy@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.155, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham 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: jasowang@redhat.com, pjp@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 1/13/22 20:33, Jon Maloy wrote: > The function fdctrl_start_transfer() calculates the dma data length > wrongly when certain boundary conditions are fulfilled. We have > noticed that the if ((fdctrl->fifo[5] - fdctrl->fifo[6]) > 1) we get > a dma length that will be interpreted as negative by the next function > in the chain, fdctrl_transfer_handler(). This leads to a crash. > > Rather than trying to fix this obscure calculation, we just check if > the harmful condition is fulfilled, and return without action if that > is the case. Since this is a condition that can only be created by a > malicious user we deem this solution safe. > > This fix is intended to address CVE-2021-3507. > > Signed-off-by: Jon Maloy > --- > hw/block/fdc.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/hw/block/fdc.c b/hw/block/fdc.c > index 21d18ac2e3..80a1f1750a 100644 > --- a/hw/block/fdc.c > +++ b/hw/block/fdc.c > @@ -1532,6 +1532,11 @@ static void fdctrl_start_transfer(FDCtrl *fdctrl, int direction) > if (fdctrl->fifo[0] & 0x80) > tmp += fdctrl->fifo[6]; > fdctrl->data_len *= tmp; > + if (tmp < 0) { > + FLOPPY_DPRINTF("calculated illegal data_len=%u, tmp=%i\n", > + fdctrl->data_len, tmp); > + return; > + } > } > fdctrl->eot = fdctrl->fifo[6]; > if (fdctrl->dor & FD_DOR_DMAEN) { I never received any feedback on this one. Is there any? ///jon