screen: backport fix for CVE-2021-26937

Security reports:
- https://lists.gnu.org/archive/html/screen-devel/2021-02/msg00000.html
This issue can be reproduced even on OpenWrt
- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=982435

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>

utils/screen/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=screen
 PKG_VERSION:=4.8.0
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=@GNU/screen

utils/screen/patches/001-CVE-2021-26937.patch

@@ -0,0 +1,64 @@
+Description: [CVE-2021-26937] Fix out of bounds array access
+Author: Michael Schröder <mls@suse.de>
+Bug-Debian: https://bugs.debian.org/982435
+Bug: https://savannah.gnu.org/bugs/?60030
+Bug: https://lists.gnu.org/archive/html/screen-devel/2021-02/msg00000.html
+Bug-OSS-Security: https://www.openwall.com/lists/oss-security/2021/02/09/3
+Origin: https://lists.gnu.org/archive/html/screen-devel/2021-02/msg00010.html
+
+--- a/encoding.c
++++ b/encoding.c
+@@ -43,7 +43,7 @@ static int  encmatch __P((char *, char *
+ # ifdef UTF8
+ static int   recode_char __P((int, int, int));
+ static int   recode_char_to_encoding __P((int, int));
+-static void  comb_tofront __P((int, int));
++static void  comb_tofront __P((int));
+ #  ifdef DW_CHARS
+ static int   recode_char_dw __P((int, int *, int, int));
+ static int   recode_char_dw_to_encoding __P((int, int *, int));
+@@ -1263,6 +1263,8 @@ int c;
+     {0x30000, 0x3FFFD},
+   };
+ 
++  if (c >= 0xdf00 && c <= 0xdfff)
++    return 1;          /* dw combining sequence */
+   return ((bisearch(c, wide, sizeof(wide) / sizeof(struct interval) - 1)) ||
+           (cjkwidth &&
+            bisearch(c, ambiguous,
+@@ -1330,11 +1332,12 @@ int c;
+ }
+ 
+ static void
+-comb_tofront(root, i)
+-int root, i;
++comb_tofront(i)
++int i;
+ {
+   for (;;)
+     {
++      int root = i >= 0x700 ? 0x801 : 0x800;
+       debug1("bring to front: %x\n", i);
+       combchars[combchars[i]->prev]->next = combchars[i]->next;
+       combchars[combchars[i]->next]->prev = combchars[i]->prev;
+@@ -1396,9 +1399,9 @@ struct mchar *mc;
+     {
+       /* full, recycle old entry */
+       if (c1 >= 0xd800 && c1 < 0xe000)
+-        comb_tofront(root, c1 - 0xd800);
++        comb_tofront(c1 - 0xd800);
+       i = combchars[root]->prev;
+-      if (c1 == i + 0xd800)
++      if (i == 0x800 || i == 0x801 || c1 == i + 0xd800)
+ 	{
+ 	  /* completely full, can't recycle */
+ 	  debug("utf8_handle_comp: completely full!\n");
+@@ -1422,7 +1425,7 @@ struct mchar *mc;
+   mc->font  = (i >> 8) + 0xd8;
+   mc->fontx = 0;
+   debug3("combinig char %x %x -> %x\n", c1, c, i + 0xd800);
+-  comb_tofront(root, i);
++  comb_tofront(i);
+ }
+ 
+ #else /* !UTF8 */