[PATCH] ui/vnc-clipboard: fix integer underflow in vnc_client_cut_text_ext

[Mauro Matteo Cascella <mcascell@redhat.com>]

Extended ClientCutText messages start with a 4-byte header. If len < 4,
an integer underflow occurs in vnc_client_cut_text_ext. The result is
used to decompress data in a while loop in inflate_buffer, leading to
CPU consumption and denial of service. Prevent this by checking dlen in
protocol_client_msg.

Fixes: CVE-2022-3165
Fixes: 0bf41cab93e5 ("ui/vnc: clipboard support")
Reported-by: TangPeng <tangpeng@qianxin.com>
Signed-off-by: Mauro Matteo Cascella <mcascell@redhat.com>
---
Extended Clipboard Pseudo-Encoding:
https://github.com/rfbproto/rfbproto/blob/master/rfbproto.rst#extended-clipboard-pseudo-encoding

 ui/vnc.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/ui/vnc.c b/ui/vnc.c
index 6a05d06147..acb3629cd8 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -2442,8 +2442,8 @@ static int protocol_client_msg(VncState *vs, uint8_t *data, size_t len)
         if (len == 1) {
             return 8;
         }
+        uint32_t dlen = abs(read_s32(data, 4));
         if (len == 8) {
-            uint32_t dlen = abs(read_s32(data, 4));
             if (dlen > (1 << 20)) {
                 error_report("vnc: client_cut_text msg payload has %u bytes"
                              " which exceeds our limit of 1MB.", dlen);
@@ -2456,8 +2456,13 @@ static int protocol_client_msg(VncState *vs, uint8_t *data, size_t len)
         }
 
         if (read_s32(data, 4) < 0) {
-            vnc_client_cut_text_ext(vs, abs(read_s32(data, 4)),
-                                    read_u32(data, 8), data + 12);
+            if (dlen < 4) {
+                error_report("vnc: malformed payload (header less than 4 bytes)"
+                             " in extended clipboard pseudo-encoding.");
+                vnc_client_error(vs);
+                break;
+            }
+            vnc_client_cut_text_ext(vs, dlen, read_u32(data, 8), data + 12);
             break;
         }
         vnc_client_cut_text(vs, read_u32(data, 4), data + 8);
-- 
2.37.3