[tor-commits] [tor/maint-0.4.1] Prevent UB on signed overflow.

[teor at torproject.org]

commit 0d4a689d3ae8f7e05b3baf8ad71d983a767ef55b
Author: Tobias Stoeckmann <tobias at stoeckmann.org>
Date:   Mon Jun 24 22:08:49 2019 +0200

    Prevent UB on signed overflow.
    
    Overflowing a signed integer in C is an undefined behaviour.
    It is possible to trigger this undefined behaviour in tor_asprintf on
    Windows or systems lacking vasprintf.
    
    On these systems, eiter _vscprintf or vsnprintf is called to retrieve
    the required amount of bytes to hold the string. These functions can
    return INT_MAX. The easiest way to recreate this is the use of a
    specially crafted configuration file, e.g. containing the line:
    
    FirewallPorts AAAAA<in total 2147483610 As>
    
    This line triggers the needed tor_asprintf call which eventually
    leads to an INT_MAX return value from _vscprintf or vsnprintf.
    
    The needed byte for \0 is added to the result, triggering the
    overflow and therefore the undefined behaviour.
    
    Casting the value to size_t before addition fixes the behaviour.
    
    Signed-off-by: Tobias Stoeckmann <tobias at stoeckmann.org>
---
 src/common/compat.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/src/common/compat.c b/src/common/compat.c
index 975875112..6f7ac7bd7 100644
--- a/src/common/compat.c
+++ b/src/common/compat.c
@@ -540,8 +540,8 @@ tor_vasprintf(char **strp, const char *fmt, va_list args)
     *strp = NULL;
     return -1;
   }
-  strp_tmp = tor_malloc(len + 1);
-  r = _vsnprintf(strp_tmp, len+1, fmt, args);
+  strp_tmp = tor_malloc((size_t)len + 1);
+  r = _vsnprintf(strp_tmp, (size_t)len+1, fmt, args);
   if (r != len) {
     tor_free(strp_tmp);
     *strp = NULL;
@@ -566,9 +566,9 @@ tor_vasprintf(char **strp, const char *fmt, va_list args)
     *strp = tor_strdup(buf);
     return len;
   }
-  strp_tmp = tor_malloc(len+1);
+  strp_tmp = tor_malloc((size_t)len+1);
   /* use of tor_vsnprintf() will ensure string is null terminated */
-  r = tor_vsnprintf(strp_tmp, len+1, fmt, args);
+  r = tor_vsnprintf(strp_tmp, (size_t)len+1, fmt, args);
   if (r != len) {
     tor_free(strp_tmp);
     *strp = NULL;
@@ -3543,4 +3543,3 @@ tor_get_avail_disk_space(const char *path)
   return -1;
 #endif
 }
-