[PATCH v5] libio: Fix CVE-2026-18374 heap buffer overflow in ccs= handling

[손동균/Process & Infra Lab(SR)/삼성전자]

Thanks for the feedback, Florian!

Changes since v4 (responding to Florian's review):
 - Unified the two if statements into a single condition since their bodies
   were identical. The first condition checks for empty charset after strip(),
   and the second checks for conversion module loading failure. Both result
   in the same error handling.
 - Removed the test case from libio/tst-fopenloc.c as suggested. Florian
   will submit a test separately.

When fopen() is called with a ,ccs= parameter whose value becomes empty
after strip(), the code must reject it with EINVAL instead of attempting
to use it.  The original upstr() fallback could read past the ',' delimiter
and cause a heap buffer overflow.

The fix checks if the charset specification is empty after strip() and
returns EINVAL immediately, preventing the overflow and following the
approach described in BZ #34574.

CVE-2026-18374 - CVSS 4.9 (AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L)
Reported-by: AISLE in partnership with Red Hat
Signed-off-by: Dongkyun Son <dongkyun.s@samsung.com>
---
 libio/fileops.c | 10 +++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/libio/fileops.c b/libio/fileops.c
index 9348d7c3a1..5a249725ee 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -355,8 +355,14 @@ _IO_new_file_fopen (FILE *fp, const char *filename, const char *mode,
 	  *((char *) __mempcpy (ccs, cs + 5, endp - (cs + 5))) = '\0';
 	  strip (ccs, ccs);
 
-	  if (__wcsmbs_named_conv (&fcts, ccs[2] == '\0'
-				   ? upstr (ccs, cs + 5) : ccs) != 0)
+	  /* After stripping, ccs[2] == '\0' means the charset name is empty.
+	     This is not a valid charset and would cause problems downstream.
+	     Reject it with EINVAL (BZ #34574, CVE-2026-18374).  */
+	  if (ccs[2] == '\0' || __wcsmbs_named_conv (&fcts, ccs) != 0)
 	    {
+	      /* Either the charset name is empty after strip(), or conversion
+		 modules cannot be loaded.  This means we cannot proceed since
+		 the user explicitly asked for character conversion.  */
 	      (void) _IO_file_close_it (fp);
 	      free (ccs);
 	      __set_errno (EINVAL);