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Tsirkin" , qemu-stable@nongnu.org Subject: [PATCH] backends/rng: cap request size to avoid oversized allocation Date: Wed, 15 Jul 2026 16:13:00 +0200 Message-ID: <20260715141300.2295392-1-lvivier@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 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=lists1p.gnu.org; Received-SPF: permerror client-ip=170.10.129.124; envelope-from=lvivier@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 12 X-Spam_score: 1.2 X-Spam_bar: + X-Spam_report: (1.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.01, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, T_SPF_PERMERROR=0.01 autolearn=no autolearn_force=no X-Spam_action: no action 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 @redhat.com) X-ZM-MESSAGEID: 1784124842898158500 Content-Type: text/plain; charset="utf-8" rng_backend_request_entropy() uses the requested size to allocate a buffer with g_malloc(). With virtio-rng, this size comes from guest-supplied descriptor lengths. A malicious guest can set a very large descriptor length, causing QEMU to attempt a multi-gigabyte allocation and abort. Cap the allocation to 64 KiB. The virtio-rng queue size is hardcoded to 8 entries, the EGD backend protocol limits requests to 255 bytes, the Linux kernel hwrng framework requests at most SMP_CACHE_BYTES per call (64 bytes on x86_64), and the Windows viorng driver uses a 4 KiB buffer. The worst legitimate case is 8 x 4 KiB =3D 32 KiB, so 64 KiB is well above any legitimate use. Fixes: 14417039653d ("virtio-rng: use virtqueue_get_avail_bytes, fix migrat= ion") Cc: qemu-stable@nongnu.org Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3983 Signed-off-by: Laurent Vivier Reviewed-by: Thomas Huth --- backends/rng.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/backends/rng.c b/backends/rng.c index ab94dfea850a..d8904f89dc43 100644 --- a/backends/rng.c +++ b/backends/rng.c @@ -11,11 +11,14 @@ */ =20 #include "qemu/osdep.h" +#include "qemu/units.h" #include "system/rng.h" #include "qapi/error.h" #include "qemu/module.h" #include "qom/object_interfaces.h" =20 +#define RNG_MAX_REQUEST_SIZE (64 * KiB) + void rng_backend_request_entropy(RngBackend *s, size_t size, EntropyReceiveFunc *receive_entropy, void *opaque) @@ -27,7 +30,7 @@ void rng_backend_request_entropy(RngBackend *s, size_t si= ze, req =3D g_malloc(sizeof(*req)); =20 req->offset =3D 0; - req->size =3D size; + req->size =3D MIN(size, RNG_MAX_REQUEST_SIZE); req->receive_entropy =3D receive_entropy; req->opaque =3D opaque; req->data =3D g_malloc(req->size); --=20 2.54.0