[Buildroot] [PATCH] package/musl: add upstream security fix for CVE-2020-28928

[Peter Korsgaard]

The wcsnrtombs function has been found to have multiple bugs in handling of
destination buffer size when limiting the input character count, which can
lead to infinite loop with no forward progress (no overflow) or writing past
the end of the destination buffer.

For more details, see the advisory:
https://www.openwall.com/lists/oss-security/2020/11/20/4

Signed-off-by: Peter Korsgaard <peter at korsgaard.com>
---
 ...bs-to-fix-buffer-overflow-and-other-.patch | 114 ++++++++++++++++++
 1 file changed, 114 insertions(+)
 create mode 100644 package/musl/0003-rewrite-wcsnrtombs-to-fix-buffer-overflow-and-other-.patch

diff --git a/package/musl/0003-rewrite-wcsnrtombs-to-fix-buffer-overflow-and-other-.patch b/package/musl/0003-rewrite-wcsnrtombs-to-fix-buffer-overflow-and-other-.patch
new file mode 100644
index 0000000000..2fb29940a9
--- /dev/null
+++ b/package/musl/0003-rewrite-wcsnrtombs-to-fix-buffer-overflow-and-other-.patch
@@ -0,0 +1,114 @@
+From 3ab2a4e02682df1382955071919d8aa3c3ec40d4 Mon Sep 17 00:00:00 2001
+From: Rich Felker <dalias at aerifal.cx>
+Date: Thu, 19 Nov 2020 17:12:43 -0500
+Subject: [PATCH] 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.
+
+Signed-off-by: Peter Korsgaard <peter at korsgaard.com>
+---
+ src/multibyte/wcsnrtombs.c | 46 ++++++++++++++++----------------------
+ 1 file 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;
+-- 
+2.20.1
+
-- 
2.20.1