[tor-commits] [tor/maint-0.2.9] Fixed tor_vasprintf on systems without vasprintf.

[teor at torproject.org]

commit 0d5a0b4f0ccc804913fbca20acf5fc62f52570b8
Author: Tobias Stoeckmann <tobias at stoeckmann.org>
Date:   Wed May 29 09:33:24 2019 -0400

    Fixed tor_vasprintf on systems without vasprintf.
    
    If tor is compiled on a system with neither vasprintf nor _vscprintf,
    the fallback implementation exposes a logic flaw which prevents
    proper usage of strings longer than 127 characters:
    
    * tor_vsnprintf returns -1 if supplied buffer is not large enough,
      but tor_vasprintf uses this function to retrieve required length
    * the result of tor_vsnprintf is not properly checked for negative
      return values
    
    Both aspects together could in theory lead to exposure of uninitialized
    stack memory in the resulting string. This requires an invalid format
    string or data that exceeds integer limitations.
    
    Fortunately tor is not even able to run with this implementation because
    it runs into asserts early on during startup. Also the unit tests fail
    during a "make check" run.
    
    Signed-off-by: Tobias Stoeckmann <tobias at stoeckmann.org>
    
      [backported to 0.2.9 by nickm]
---
 src/common/compat.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/src/common/compat.c b/src/common/compat.c
index 975875112..d3bc2f5fe 100644
--- a/src/common/compat.c
+++ b/src/common/compat.c
@@ -558,10 +558,17 @@ tor_vasprintf(char **strp, const char *fmt, va_list args)
   int len, r;
   va_list tmp_args;
   va_copy(tmp_args, args);
-  /* vsnprintf() was properly checked but tor_vsnprintf() available so
-   * why not use it? */
-  len = tor_vsnprintf(buf, sizeof(buf), fmt, tmp_args);
+  /* Use vsnprintf to retrieve needed length.  tor_vsnprintf() is not an option
+   * here because it will simply return -1 if buf is not large enough to hold the
+   * complete string.
+   */
+  len = vsnprintf(buf, sizeof(buf), fmt, tmp_args);
   va_end(tmp_args);
+  buf[sizeof(buf) - 1] = '\0';
+  if (len < 0) {
+    *strp = NULL;
+    return -1;
+  }
   if (len < (int)sizeof(buf)) {
     *strp = tor_strdup(buf);
     return len;