[glibc/maskray/lld] posix: consume less entropy on tempname

[glibc/maskray/lld] posix: consume less entropy on tempname

[Fangrui Song]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=f430293d842031f2afc3013f156e1018065e480e

commit f430293d842031f2afc3013f156e1018065e480e
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Tue Jan 12 09:17:09 2021 -0300

    posix: consume less entropy on tempname
    
    The first getrandom is used only for __GT_NOCREATE, which is inherently
    insecure and can use the entropy as a small improvement.  On the
    second and later attempts it might help against DoS attacks.
    
    It sync with gnulib commit 854fbb81d91f7a0f2b463e7ace2499dee2f380f2.
    
    Checked on x86_64-linux-gnu.

Diff:
---
 sysdeps/posix/tempname.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/sysdeps/posix/tempname.c b/sysdeps/posix/tempname.c
index f199b25a7a..5f804b38d7 100644
--- a/sysdeps/posix/tempname.c
+++ b/sysdeps/posix/tempname.c
@@ -22,6 +22,7 @@
 
 #include <sys/types.h>
 #include <assert.h>
+#include <stdbool.h>
 
 #include <errno.h>
 
@@ -77,11 +78,11 @@ typedef uint_fast64_t random_value;
 #define BASE_62_POWER (62LL * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62)
 
 static random_value
-random_bits (random_value var)
+random_bits (random_value var, bool use_getrandom)
 {
   random_value r;
   /* Without GRND_NONBLOCK it can be blocked for minutes on some systems.  */
-  if (__getrandom (&r, sizeof r, GRND_NONBLOCK) == sizeof r)
+  if (use_getrandom && __getrandom (&r, sizeof r, GRND_NONBLOCK) == sizeof r)
     return r;
 #if _LIBC || (defined CLOCK_MONOTONIC && HAVE_CLOCK_GETTIME)
   /* Add entropy if getrandom did not work.  */
@@ -269,6 +270,13 @@ try_tempname_len (char *tmpl, int suffixlen, void *args,
   /* How many random base-62 digits can currently be extracted from V.  */
   int vdigits = 0;
 
+  /* Whether to consume entropy when acquiring random bits.  On the
+     first try it's worth the entropy cost with __GT_NOCREATE, which
+     is inherently insecure and can use the entropy to make it a bit
+     less secure.  On the (rare) second and later attempts it might
+     help against DoS attacks.  */
+  bool use_getrandom = tryfunc == try_nocreate;
+
   /* Least unfair value for V.  If V is less than this, V can generate
      BASE_62_DIGITS digits fairly.  Otherwise it might be biased.  */
   random_value const unfair_min
@@ -292,7 +300,10 @@ try_tempname_len (char *tmpl, int suffixlen, void *args,
           if (vdigits == 0)
             {
               do
-                v = random_bits (v);
+                {
+                  v = random_bits (v, use_getrandom);
+                  use_getrandom = true;
+                }
               while (unfair_min <= v);
 
               vdigits = BASE_62_DIGITS;