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CVE-2015-1779: limit size of HTTP headers from websockets clients

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The VNC server websockets decoder will read and buffer data from
websockets clients until it sees the end of the HTTP headers,
as indicated by \r\n\r\n. In theory this allows a malicious to
trick QEMU into consuming an arbitrary amount of RAM. In practice,
because QEMU runs g_strstr_len() across the buffered header data,
it will spend increasingly long burning CPU time searching for
the substring match and less & less time reading data. So while
this does cause arbitrary memory growth, the bigger problem is
that QEMU will be burning 100% of available CPU time.

A novnc websockets client typically sends headers of around
512 bytes in length. As such it is reasonable to place a 4096
byte limit on the amount of data buffered while searching for
the end of HTTP headers.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
This commit is contained in:
Daniel P. Berrange 2015-03-23 22:58:22 +00:00 committed by Gerd Hoffmann
parent a2bebfd6e0
commit 2cdb5e142f
1 changed files with 8 additions and 2 deletions
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10
ui/vnc-ws.c
View file

@ -81,8 +81,11 @@ void vncws_handshake_read(void *opaque)
VncState *vs = opaque;
uint8_t *handshake_end;
long ret;
buffer_reserve(&vs->ws_input, 4096);
ret = vnc_client_read_buf(vs, buffer_end(&vs->ws_input), 4096);
/* Typical HTTP headers from novnc are 512 bytes, so limiting
* total header size to 4096 is easily enough. */
size_t want = 4096 - vs->ws_input.offset;
buffer_reserve(&vs->ws_input, want);
ret = vnc_client_read_buf(vs, buffer_end(&vs->ws_input), want);
if (!ret) {
if (vs->csock == -1) {
@ -99,6 +102,9 @@ void vncws_handshake_read(void *opaque)
vncws_process_handshake(vs, vs->ws_input.buffer, vs->ws_input.offset);
buffer_advance(&vs->ws_input, handshake_end - vs->ws_input.buffer +
strlen(WS_HANDSHAKE_END));
} else if (vs->ws_input.offset >= 4096) {
VNC_DEBUG("End of headers not found in first 4096 bytes\n");
vnc_client_error(vs);
}
}