Ensure we handle len == 0 in ERR_err_string_n

If len == 0 in a call to ERR_error_string_n() then we can read beyond the
end of the buffer. Really applications should not be calling this function
with len == 0, but we shouldn't be letting it through either!

Thanks to Agostino Sarubbo for reporting this issue. Agostino's blog on
this issue is available here:
https://blogs.gentoo.org/ago/2016/10/14/openssl-libcrypto-stack-based-buffer-overflow-in-err_error_string_n-err-c/

Reviewed-by: Richard Levitte <levitte@openssl.org>
(cherry picked from commit e5c1361580d8de79682958b04a5f0d262e680f8b)

diff --git a/crypto/err/err.c b/crypto/err/err.c
index c3f7212a78c5e9d64a2f8e7dae1e6c20b3e12f01..29e5a031972456afbbfa5c528c747bc2b20f3003 100644 (file)
--- a/crypto/err/err.c
+++ b/crypto/err/err.c
@@ -500,6 +500,9 @@ void ERR_error_string_n(unsigned long e, char *buf, size_t len)
     const char *ls, *fs, *rs;
     unsigned long l, f, r;
 
+    if (len == 0)
+        return;
+
     l = ERR_GET_LIB(e);
     f = ERR_GET_FUNC(e);
     r = ERR_GET_REASON(e);