For line-buffered streams the __sflush() function did not correctly u…

…pdate

the FILE object's write space member when the write(2) system call returns
an error.

Depending on the nature of an application that calls libc's stdio functions
and the presence of errors returned from the write(2) system call (or an
overridden stdio write routine) a heap buffer overfly may occur.  Such
overflows may lead to data corruption or the execution of arbitrary code at
the privilege level of the calling program.

Obtained from: FreeBSD

lib/libc/stdio/fflush.c

@@ -105,10 +105,10 @@ int
 __sflush(FILE *fp)
 {
 	unsigned char *p;
-	int n, t;
+	int n, f, t;
 
-	t = fp->_flags;
-	if ((t & __SWR) == 0)
+	f = fp->_flags;
+	if ((f & __SWR) == 0)
 		return (0);
 
 	if ((p = fp->_bf._base) == NULL)
@@ -121,19 +121,18 @@ __sflush(FILE *fp)
 	 * exchange buffering (via setvbuf) in user write function.
 	 */
 	fp->_p = p;
-	fp->_w = t & (__SLBF|__SNBF) ? 0 : fp->_bf._size;
+	fp->_w = f & (__SLBF|__SNBF) ? 0 : fp->_bf._size;
 
 	for (; n > 0; n -= t, p += t) {
 		t = _swrite(fp, (char *)p, n);
 		if (t <= 0) {
-			/* Reset _p and _w. */
-			if (p > fp->_p) {
+			if (p > fp->_p)
 				/* Some was written. */
 				memmove(fp->_p, p, n);
-				fp->_p += n;
-				if ((fp->_flags & (__SLBF | __SNBF)) == 0)
-					fp->_w -= n;
-			}
+			/* Reset _p and _w. */
+			fp->_p += n;
+			if ((fp->_flags & __SNBF) == 0)
+				fp->_w -= n;
 			fp->_flags |= __SERR;
 			return (EOF);
 		}