[2.26 COMMITTED] Fix signed integer overflow in random_r (bug 17343).

[Florian Weimer]

From: Joseph Myers <joseph@codesourcery.com>

Bug 17343 reports that stdlib/random_r.c has code with undefined
behavior because of signed integer overflow on int32_t.  This patch
changes the code so that the possibly overflowing computations use
unsigned arithmetic instead.

Note that the bug report refers to "Most code" in that file.  The
places changed in this patch are the only ones I found where I think
such overflow can occur.

Tested for x86_64 and x86.

	[BZ #17343]
	* stdlib/random_r.c (__random_r): Use unsigned arithmetic for
	possibly overflowing computations.

(cherry picked from commit 8a07b0c43c46a480da070efd53a2720195e2256f)

2018-03-20  Joseph Myers  <joseph@codesourcery.com>

	[BZ #17343]
	* stdlib/random_r.c (__random_r): Use unsigned arithmetic for
	possibly overflowing computations.

diff --git a/NEWS b/NEWS
index 106c7e0a73..7c1526004f 100644
--- a/NEWS
+++ b/NEWS
@@ -74,6 +74,7 @@ Security related changes:
 The following bugs are resolved with this release:
 
   [16750] ldd: Never run file directly.
+  [17343] Fix signed integer overflow in random_r
   [17956] crypt: Use NSPR header files in addition to NSS header files
   [20532] getaddrinfo: More robust handling of dlopen failures
   [21242] assert: Suppress pedantic warning caused by statement expression
diff --git a/stdlib/random_r.c b/stdlib/random_r.c
index c3f6f9aede..150fa9bb34 100644
--- a/stdlib/random_r.c
+++ b/stdlib/random_r.c
@@ -361,8 +361,7 @@ __random_r (struct random_data *buf, int32_t *result)
 
   if (buf->rand_type == TYPE_0)
     {
-      int32_t val = state[0];
-      val = ((state[0] * 1103515245) + 12345) & 0x7fffffff;
+      int32_t val = ((state[0] * 1103515245U) + 12345U) & 0x7fffffff;
       state[0] = val;
       *result = val;
     }
@@ -371,11 +370,11 @@ __random_r (struct random_data *buf, int32_t *result)
       int32_t *fptr = buf->fptr;
       int32_t *rptr = buf->rptr;
       int32_t *end_ptr = buf->end_ptr;
-      int32_t val;
+      uint32_t val;
 
-      val = *fptr += *rptr;
+      val = *fptr += (uint32_t) *rptr;
       /* Chucking least random bit.  */
-      *result = (val >> 1) & 0x7fffffff;
+      *result = val >> 1;
       ++fptr;
       if (fptr >= end_ptr)
 	{