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Fri, 20 Mar 2026 17:04:44 -0700 (PDT) From: Jenny Guanni Qu To: qemu-devel@nongnu.org Cc: berrange@redhat.com, mcascell@redhat.com, peter.maydell@linaro.org, qemu-security@nongnu.org, Jenny Guanni Qu Subject: [PATCH v2] hw/usb/hcd-ohci: check for MPS=0 to avoid infinite loop Date: Sat, 21 Mar 2026 00:04:44 +0000 Message-Id: <20260321000444.909451-1-qguanni@gmail.com> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=2607:f8b0:4864:20::835; envelope-from=qguanni@gmail.com; helo=mail-qt1-x835.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-Mailman-Approved-At: Fri, 20 Mar 2026 20:35:14 -0400 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @gmail.com) X-ZM-MESSAGEID: 1774053363235158500 Content-Type: text/plain; charset="utf-8" When a guest sets MaxPacketSize to 0 in an OHCI Endpoint Descriptor, ohci_service_td() transfers 0 bytes per iteration. The Transfer Descriptor never completes because CBP never advances toward BE, causing ohci_service_ed_list() to loop indefinitely and hang QEMU. Add a check for MPS=3D=3D0 after extracting the field from ED flags. If MPS is zero, call ohci_die() to reset the controller and return an error, preventing the infinite loop. Fixes: CVE-2026-3890 Reported-by: Jenny Guanni Qu Signed-off-by: Jenny Guanni Qu Reviewed-by: Peter Maydell --- hw/usb/hcd-ohci.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/hw/usb/hcd-ohci.c b/hw/usb/hcd-ohci.c index 1aeed9286f..6ed8046fc2 100644 --- a/hw/usb/hcd-ohci.c +++ b/hw/usb/hcd-ohci.c @@ -956,6 +956,17 @@ static int ohci_service_td(OHCIState *ohci, struct ohc= i_ed *ed) if (len && dir !=3D OHCI_TD_DIR_IN) { /* The endpoint may not allow us to transfer it all now */ pktlen =3D (ed->flags & OHCI_ED_MPS_MASK) >> OHCI_ED_MPS_SHIFT; + /* + * The OHCI spec does not say what to do if the guest hands us + * an endpoint descriptor which specifies a MaximumPacketSize + * of zero, which would mean we can never actually make forward + * progress transferring data to it. We choose to treat it as + * an error. + */ + if (pktlen =3D=3D 0) { + ohci_die(ohci); + return 1; + } if (pktlen > len) { pktlen =3D len; } --=20 2.34.1