[glibc] Fix OOB read in stdlib thousand grouping parsing [BZ #29727]

[Szabolcs Nagy <nsz@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=17bfe5954baee1f18672aea94caa1126ec36fb81

commit 17bfe5954baee1f18672aea94caa1126ec36fb81
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date:   Tue Oct 11 15:24:41 2022 +0100

    Fix OOB read in stdlib thousand grouping parsing [BZ #29727]
    
    __correctly_grouped_prefixmb only worked with thousands_len == 1,
    otherwise it read past the end of cp or thousands.
    
    This affects scanf formats like %'d, %'f and the internal but
    exposed __strto{l,ul,f,d,..}_internal with grouping flag set
    and an LC_NUMERIC locale where thousands_len > 1.
    
    Avoid OOB access by considering thousands_len when initializing cp.
    This fixes bug 29727.
    
    Found by the morello port with strict bounds checking where
    
    FAIL: stdlib/tst-strtod4
    FAIL: stdlib/tst-strtod5i
    
    crashed using a locale with thousands_len==3.

Diff:
---
 stdlib/grouping.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/stdlib/grouping.c b/stdlib/grouping.c
index be7922f5fd..06cbe7b9c7 100644
--- a/stdlib/grouping.c
+++ b/stdlib/grouping.c
@@ -52,21 +52,19 @@ __correctly_grouped_prefixmb (const STRING_TYPE *begin, const STRING_TYPE *end,
 #endif
 			      const char *grouping)
 {
-#ifndef USE_WIDE_CHAR
-  size_t thousands_len;
-  int cnt;
-#endif
-
   if (grouping == NULL)
     return end;
 
-#ifndef USE_WIDE_CHAR
-  thousands_len = strlen (thousands);
+#ifdef USE_WIDE_CHAR
+  size_t thousands_len = 1;
+#else
+  size_t thousands_len = strlen (thousands);
+  int cnt;
 #endif
 
-  while (end > begin)
+  while (end - begin >= thousands_len)
     {
-      const STRING_TYPE *cp = end - 1;
+      const STRING_TYPE *cp = end - thousands_len;
       const char *gp = grouping;
 
       /* Check first group.  */