libvhost-user: fix buffer overflow in virtqueue_read_indirect_desc()

virtqueue_read_indirect_desc() copies an indirect descriptor table
into a buffer in chunks when the table crosses a memory region
boundary. The destination is a struct vring_desc pointer but is
advanced by a byte count, so each increment moves the pointer by
read_len elements instead of read_len bytes, writing beyond the
buffer.

Use a char pointer for the destination so that the arithmetic
advances correctly.

While at it, change the source from a struct vring_desc pointer
to a void pointer: when the table is split across regions,
vu_gpa_to_va() can return a pointer into the middle of a descriptor,
so casting it to a struct vring_desc pointer is wrong. The pointer is
only used as a memcpy() source, so a void pointer is fine.

Fixes: CVE-2026-6425
Fixes: 293084a719 ("libvhost-user: Support across-memory-boundary access")
Cc: qemu-stable@nongnu.org
Reported-by: DARKNAVY <vr@darknavy.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260417132645.121192-2-sgarzare@redhat.com>
(cherry picked from commit 85805ee887be4adddb1f1a34f526968adc95e582)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

1 files changed, 4 insertions, 3 deletions

diff --git a/subprojects/libvhost-user/libvhost-user.c b/subprojects/libvhost-user/libvhost-user.c
index 9c630c2170..014d210748 100644
--- a/subprojects/libvhost-user/libvhost-user.c
+++ b/subprojects/libvhost-user/libvhost-user.c
@@ -2391,8 +2391,9 @@ static int
 virtqueue_read_indirect_desc(VuDev *dev, struct vring_desc *desc,
                              uint64_t addr, size_t len)
 {
-    struct vring_desc *ori_desc;
+    char *dst_desc = (char *)desc;
     uint64_t read_len;
+    void *ori_desc;
 
     if (len > (VIRTQUEUE_MAX_SIZE * sizeof(struct vring_desc))) {
         return -1;
@@ -2409,10 +2410,10 @@ virtqueue_read_indirect_desc(VuDev *dev, struct vring_desc *desc,
             return -1;
         }
 
-        memcpy(desc, ori_desc, read_len);
+        memcpy(dst_desc, ori_desc, read_len);
         len -= read_len;
         addr += read_len;
-        desc += read_len;
+        dst_desc += read_len;
     }
 
     return 0;