rewrite wcsnrtombs to fix buffer overflow and other bugs

the original wcsnrtombs implementation, which has been largely
untouched since 0.5.0, attempted to build input-length-limiting
conversion on top of wcsrtombs, which only limits output length. as
best I recall, this choice was made out of a mix of disdain over
having yet another variant function to implement (added in POSIX 2008;
not standard C) and preference not to switch things around and
implement the wcsrtombs in terms of the more general new function,
probably over namespace issues. the strategy employed was to impose
output limits that would ensure the input limit wasn't exceeded, then
finish up the tail character-at-a-time. unfortunately, none of that
worked correctly.

first, the logic in the wcsrtombs loop was wrong in that it could
easily get stuck making no forward progress, by imposing an output
limit too small to convert even one character.

the character-at-a-time loop that followed was even worse. it made no
effort to ensure that the converted multibyte character would fit in
the remaining output space, only that there was a nonzero amount of
output space remaining. it also employed an incorrect interpretation
of wcrtomb's interface contract for converting the null character,
thereby failing to act on end of input, and remaining space accounting
was subject to unsigned wrap-around. together these errors allow
unbounded overflow of the destination buffer, controlled by input
length limit and input wchar_t string contents.

given the extent to which this function was broken, it's plausible
that most applications that would have been rendered exploitable were
sufficiently broken not to be usable in the first place. however, it's
also plausible that common (especially ASCII-only) inputs succeeded in
the wcsrtombs loop, which mostly worked, while leaving the wildly
erroneous code in the second loop exposed to particular non-ASCII
inputs.

CVE-2020-28928 has been assigned for this issue.

1 files changed, 19 insertions, 27 deletions

diff --git a/src/multibyte/wcsnrtombs.c b/src/multibyte/wcsnrtombs.c
index 676932b5..95e25e70 100644
--- a/src/multibyte/wcsnrtombs.c
+++ b/src/multibyte/wcsnrtombs.c
@@ -1,41 +1,33 @@
 #include <wchar.h>
+#include <limits.h>
+#include <string.h>
 
 size_t wcsnrtombs(char *restrict dst, const wchar_t **restrict wcs, size_t wn, size_t n, mbstate_t *restrict st)
 {
-	size_t l, cnt=0, n2;
-	char *s, buf[256];
 	const wchar_t *ws = *wcs;
-	const wchar_t *tmp_ws;
-
-	if (!dst) s = buf, n = sizeof buf;
-	else s = dst;
-
-	while ( ws && n && ( (n2=wn)>=n || n2>32 ) ) {
-		if (n2>=n) n2=n;
-		tmp_ws = ws;
-		l = wcsrtombs(s, &ws, n2, 0);
-		if (!(l+1)) {
-			cnt = l;
-			n = 0;
+	size_t cnt = 0;
+	if (!dst) n=0;
+	while (ws && wn) {
+		char tmp[MB_LEN_MAX];
+		size_t l = wcrtomb(n<MB_LEN_MAX ? tmp : dst, *ws, 0);
+		if (l==-1) {
+			cnt = -1;
 			break;
 		}
-		if (s != buf) {
-			s += l;
+		if (dst) {
+			if (n<MB_LEN_MAX) {
+				if (l>n) break;
+				memcpy(dst, tmp, l);
+			}
+			dst += l;
 			n -= l;
 		}
-		wn = ws ? wn - (ws - tmp_ws) : 0;
-		cnt += l;
-	}
-	if (ws) while (n && wn) {
-		l = wcrtomb(s, *ws, 0);
-		if ((l+1)<=1) {
-			if (!l) ws = 0;
-			else cnt = l;
+		if (!*ws) {
+			ws = 0;
 			break;
 		}
-		ws++; wn--;
-		/* safe - this loop runs fewer than sizeof(buf) times */
-		s+=l; n-=l;
+		ws++;
+		wn--;
 		cnt += l;
 	}
 	if (dst) *wcs = ws;