[Qemu-devel] [PATCH 1/2] CVE-2015-1779: incrementally decode websocket frames

["Daniel P. Berrange" <berrange@redhat.com>]

The logic for decoding websocket frames wants to fully
decode the frame header and payload, before allowing the
VNC server to see any of the payload data. There is no
size limit on websocket payloads, so this allows a
malicious network client to consume 2^64 bytes in memory
in QEMU. It can trigger this denial of service before
the VNC server even performs any authentication.

The fix is to decode the header, and then incrementally
decode the payload data as it is needed. With this fix
the websocket decoder will allow at most 4k of data to
be buffered before decoding and processing payload.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
---
 ui/vnc-ws.c | 105 ++++++++++++++++++++++++++++++++++++++++--------------------
 ui/vnc-ws.h |   9 ++++--
 ui/vnc.h    |   2 ++
 3 files changed, 80 insertions(+), 36 deletions(-)

diff --git a/ui/vnc-ws.c b/ui/vnc-ws.c
index 85dbb7e..e8146d0 100644
--- a/ui/vnc-ws.c
+++ b/ui/vnc-ws.c
@@ -107,7 +107,7 @@ long vnc_client_read_ws(VncState *vs)
 {
     int ret, err;
     uint8_t *payload;
-    size_t payload_size, frame_size;
+    size_t payload_size, header_size;
     VNC_DEBUG("Read websocket %p size %zd offset %zd\n", vs->ws_input.buffer,
             vs->ws_input.capacity, vs->ws_input.offset);
     buffer_reserve(&vs->ws_input, 4096);
@@ -117,18 +117,39 @@ long vnc_client_read_ws(VncState *vs)
     }
     vs->ws_input.offset += ret;
 
-    /* make sure that nothing is left in the ws_input buffer */
+    ret = 0;
+    /* consume as much of ws_input buffer as possible */
     do {
-        err = vncws_decode_frame(&vs->ws_input, &payload,
-                              &payload_size, &frame_size);
-        if (err <= 0) {
-            return err;
+        if (vs->ws_payload_remain == 0) {
+            err = vncws_decode_frame_header(&vs->ws_input,
+                                            &header_size,
+                                            &vs->ws_payload_remain,
+                                            &vs->ws_payload_mask);
+            if (err <= 0) {
+                return err;
+            }
+
+            buffer_advance(&vs->ws_input, header_size);
         }
+        if (vs->ws_payload_remain != 0) {
+            err = vncws_decode_frame_payload(&vs->ws_input,
+                                             &vs->ws_payload_remain,
+                                             &vs->ws_payload_mask,
+                                             &payload,
+                                             &payload_size);
+            if (err < 0) {
+                return err;
+            }
+            if (err == 0) {
+                return ret;
+            }
+            ret += err;
 
-        buffer_reserve(&vs->input, payload_size);
-        buffer_append(&vs->input, payload, payload_size);
+            buffer_reserve(&vs->input, payload_size);
+            buffer_append(&vs->input, payload, payload_size);
 
-        buffer_advance(&vs->ws_input, frame_size);
+            buffer_advance(&vs->ws_input, payload_size);
+        }
     } while (vs->ws_input.offset > 0);
 
     return ret;
@@ -265,15 +286,14 @@ void vncws_encode_frame(Buffer *output, const void *payload,
     buffer_append(output, payload, payload_size);
 }
 
-int vncws_decode_frame(Buffer *input, uint8_t **payload,
-                           size_t *payload_size, size_t *frame_size)
+int vncws_decode_frame_header(Buffer *input,
+                              size_t *header_size,
+                              size_t *payload_remain,
+                              WsMask *payload_mask)
 {
     unsigned char opcode = 0, fin = 0, has_mask = 0;
-    size_t header_size = 0;
-    uint32_t *payload32;
+    size_t payload_len;
     WsHeader *header = (WsHeader *)input->buffer;
-    WsMask mask;
-    int i;
 
     if (input->offset < WS_HEAD_MIN_LEN + 4) {
         /* header not complete */
@@ -283,7 +303,7 @@ int vncws_decode_frame(Buffer *input, uint8_t **payload,
     fin = (header->b0 & 0x80) >> 7;
     opcode = header->b0 & 0x0f;
     has_mask = (header->b1 & 0x80) >> 7;
-    *payload_size = header->b1 & 0x7f;
+    payload_len = header->b1 & 0x7f;
 
     if (opcode == WS_OPCODE_CLOSE) {
         /* disconnect */
@@ -300,40 +320,57 @@ int vncws_decode_frame(Buffer *input, uint8_t **payload,
         return -2;
     }
 
-    if (*payload_size < 126) {
-        header_size = 6;
-        mask = header->u.m;
-    } else if (*payload_size == 126 && input->offset >= 8) {
-        *payload_size = be16_to_cpu(header->u.s16.l16);
-        header_size = 8;
-        mask = header->u.s16.m16;
-    } else if (*payload_size == 127 && input->offset >= 14) {
-        *payload_size = be64_to_cpu(header->u.s64.l64);
-        header_size = 14;
-        mask = header->u.s64.m64;
+    if (payload_len < 126) {
+        *payload_remain = payload_len;
+        *header_size = 6;
+        *payload_mask = header->u.m;
+    } else if (payload_len == 126 && input->offset >= 8) {
+        *payload_remain = be16_to_cpu(header->u.s16.l16);
+        *header_size = 8;
+        *payload_mask = header->u.s16.m16;
+    } else if (payload_len == 127 && input->offset >= 14) {
+        *payload_remain = be64_to_cpu(header->u.s64.l64);
+        *header_size = 14;
+        *payload_mask = header->u.s64.m64;
     } else {
         /* header not complete */
         return 0;
     }
 
-    *frame_size = header_size + *payload_size;
+    return 1;
+}
+
+int vncws_decode_frame_payload(Buffer *input,
+                               size_t *payload_remain, WsMask *payload_mask,
+                               uint8_t **payload, size_t *payload_size)
+{
+    size_t i;
+    uint32_t *payload32;
 
-    if (input->offset < *frame_size) {
-        /* frame not complete */
+    *payload = input->buffer;
+    /* If we aren't at the end of the payload, then drop
+     * off the last bytes, so we're always multiple of 4
+     * for purpose of unmasking, except at end of payload
+     */
+    if (input->offset < *payload_remain) {
+        *payload_size = input->offset - (input->offset % 4);
+    } else {
+        *payload_size = input->offset;
+    }
+    if (*payload_size == 0) {
         return 0;
     }
-
-    *payload = input->buffer + header_size;
+    *payload_remain -= *payload_size;
 
     /* unmask frame */
     /* process 1 frame (32 bit op) */
     payload32 = (uint32_t *)(*payload);
     for (i = 0; i < *payload_size / 4; i++) {
-        payload32[i] ^= mask.u;
+        payload32[i] ^= payload_mask->u;
     }
     /* process the remaining bytes (if any) */
     for (i *= 4; i < *payload_size; i++) {
-        (*payload)[i] ^= mask.c[i % 4];
+        (*payload)[i] ^= payload_mask->c[i % 4];
     }
 
     return 1;
diff --git a/ui/vnc-ws.h b/ui/vnc-ws.h
index ef229b7..14d4230 100644
--- a/ui/vnc-ws.h
+++ b/ui/vnc-ws.h
@@ -83,7 +83,12 @@ long vnc_client_read_ws(VncState *vs);
 void vncws_process_handshake(VncState *vs, uint8_t *line, size_t size);
 void vncws_encode_frame(Buffer *output, const void *payload,
             const size_t payload_size);
-int vncws_decode_frame(Buffer *input, uint8_t **payload,
-                               size_t *payload_size, size_t *frame_size);
+int vncws_decode_frame_header(Buffer *input,
+                              size_t *header_size,
+                              size_t *payload_remain,
+                              WsMask *payload_mask);
+int vncws_decode_frame_payload(Buffer *input,
+                               size_t *payload_remain, WsMask *payload_mask,
+                               uint8_t **payload, size_t *payload_size);
 
 #endif /* __QEMU_UI_VNC_WS_H */
diff --git a/ui/vnc.h b/ui/vnc.h
index e19ac39..66af181 100644
--- a/ui/vnc.h
+++ b/ui/vnc.h
@@ -306,6 +306,8 @@ struct VncState
 #ifdef CONFIG_VNC_WS
     Buffer ws_input;
     Buffer ws_output;
+    uint64_t ws_payload_remain;
+    WsMask ws_payload_mask;
 #endif
     /* current output mode information */
     VncWritePixels *write_pixels;
-- 
2.1.0