vtls: fix potential ssl_buffer stack overflow

In Curl_multissl_version() it was possible to overflow the passed in
buffer if the generated version string exceeded the size of the buffer.
Fix by inverting the logic, and also make sure to not exceed the local
buffer during the string generation.

Closes #3863
Reported-by: nevv on HackerOne/curl
Reviewed-by: Jay Satiro
Reviewed-by: Daniel Stenberg

1 files changed, 5 insertions, 4 deletions

diff --git a/lib/vtls/vtls.c b/lib/vtls/vtls.c
index 8a405c05c..25391443f 100644
--- a/lib/vtls/vtls.c
+++ b/lib/vtls/vtls.c
@@ -1239,16 +1239,17 @@ static size_t Curl_multissl_version(char *buffer, size_t size)
 
   if(current != selected) {
     char *p = backends;
+    char *end = backends + sizeof(backends);
     int i;
 
     selected = current;
 
-    for(i = 0; available_backends[i]; i++) {
+    for(i = 0; available_backends[i] && p < (end - 4); i++) {
       if(i)
         *(p++) = ' ';
       if(selected != available_backends[i])
         *(p++) = '(';
-      p += available_backends[i]->version(p, backends + sizeof(backends) - p);
+      p += available_backends[i]->version(p, end - p - 2);
       if(selected != available_backends[i])
         *(p++) = ')';
     }
@@ -1256,14 +1257,14 @@ static size_t Curl_multissl_version(char *buffer, size_t size)
     total = p - backends;
   }
 
-  if(size < total)
+  if(size > total)
     memcpy(buffer, backends, total + 1);
   else {
     memcpy(buffer, backends, size - 1);
     buffer[size - 1] = '\0';
   }
 
-  return total;
+  return CURLMIN(size - 1, total);
 }
 
 static int multissl_init(const struct Curl_ssl *backend)