Fix buffer overflow when decrypting client SPICE ticket

commit 8af619009660b24e0b41ad26b30289eea288fcc2     upstream

reds_handle_ticket uses a fixed size 'password' buffer for the decrypted
password whose size is SPICE_MAX_PASSWORD_LENGTH. However,
RSA_private_decrypt which we call for the decryption expects the
destination buffer to be at least RSA_size(link->tiTicketing.rsa)
bytes long. On my spice-server build, SPICE_MAX_PASSWORD_LENGTH
is 60 while RSA_size() is 128, so we end up overflowing 'password'
when using long passwords (this was reproduced using the string:
'fullscreen=1proxy=#enter proxy here; e.g spice_proxy = http://[proxy]:[port]'
as a password).

When the overflow occurs, QEMU dies with:
*** stack smashing detected ***: qemu-system-x86_64 terminated

This commit ensures we use a corectly sized 'password' buffer,
and that it's correctly nul-terminated so that we can use strcmp
instead of strncmp. To keep using strncmp, we'd need to figure out
which one of 'password' and 'taTicket.password' is the smaller buffer,
and use that size.

This fixes rhbz#999839
diff --git a/server/reds.c b/server/reds.c
index 30d0652..6f262b0 100644
--- a/server/reds.c
+++ b/server/reds.c
@@ -1931,39 +1931,59 @@ static void reds_handle_link(RedLinkInfo *link)
 static void reds_handle_ticket(void *opaque)
 {
     RedLinkInfo *link = (RedLinkInfo *)opaque;
-    char password[SPICE_MAX_PASSWORD_LENGTH];
+    char *password;
     time_t ltime;
+    int password_size;
 
     //todo: use monotonic time
     time(&ltime);
-    RSA_private_decrypt(link->tiTicketing.rsa_size,
-                        link->tiTicketing.encrypted_ticket.encrypted_data,
-                        (unsigned char *)password, link->tiTicketing.rsa, RSA_PKCS1_OAEP_PADDING);
+    if (RSA_size(link->tiTicketing.rsa) < SPICE_MAX_PASSWORD_LENGTH) {
+        spice_warning("RSA modulus size is smaller than SPICE_MAX_PASSWORD_LENGTH (%d < %d), "
+                      "SPICE ticket sent from client may be truncated",
+                      RSA_size(link->tiTicketing.rsa), SPICE_MAX_PASSWORD_LENGTH);
+    }
+
+    password = g_malloc0(RSA_size(link->tiTicketing.rsa) + 1);
+    password_size = RSA_private_decrypt(link->tiTicketing.rsa_size,
+                                        link->tiTicketing.encrypted_ticket.encrypted_data,
+                                        (unsigned char *)password,
+                                        link->tiTicketing.rsa,
+                                        RSA_PKCS1_OAEP_PADDING);
+    if (password_size == -1) {
+        spice_warning("failed to decrypt RSA encrypted password: %s",
+                      ERR_error_string(ERR_get_error(), NULL));
+        goto error;
+    }
+    password[password_size] = '\0';
 
     if (ticketing_enabled && !link->skip_auth) {
         int expired =  taTicket.expiration_time < ltime;
 
         if (strlen(taTicket.password) == 0) {
-            reds_send_link_result(link, SPICE_LINK_ERR_PERMISSION_DENIED);
             spice_warning("Ticketing is enabled, but no password is set. "
-                        "please set a ticket first");
-            reds_link_free(link);
-            return;
+                          "please set a ticket first");
+            goto error;
         }
 
-        if (expired || strncmp(password, taTicket.password, SPICE_MAX_PASSWORD_LENGTH) != 0) {
+        if (expired || strcmp(password, taTicket.password) != 0) {
             if (expired) {
                 spice_warning("Ticket has expired");
             } else {
                 spice_warning("Invalid password");
             }
-            reds_send_link_result(link, SPICE_LINK_ERR_PERMISSION_DENIED);
-            reds_link_free(link);
-            return;
+            goto error;
         }
     }
 
     reds_handle_link(link);
+    goto end;
+
+error:
+    reds_send_link_result(link, SPICE_LINK_ERR_PERMISSION_DENIED);
+    reds_link_free(link);
+
+end:
+    g_free(password);
 }
 
 static inline void async_read_clear_handlers(AsyncRead *obj)