libxml2: Fix CVE-2026-86138

This patch applies the upstream fix as referenced in [2],
using the commit shown in [1].

[1] https://github.com/GNOME/libxml2/commit/a4cba4b5b5a8c42e155ed42d2d2a44955465a2e4
[2] https://nvd.nist.gov/vuln/detail/CVE-2026-86138

(From OE-Core rev: 1d57767b489ef6bc240020b40b834d2e1313ec44)

Signed-off-by: Hetvi Thakar <hthakar@cisco.com>
Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Paul Barker <paul@pbarker.dev>

meta/recipes-core/libxml/libxml2/CVE-2026-86138.patch

@@ -0,0 +1,52 @@
+From d2e3efc8502313a1099ccc4aac19e8e03734e8c9 Mon Sep 17 00:00:00 2001
+From: mohammadmseet-hue <mohammadmseet@gmail.com>
+Date: Thu, 16 Apr 2026 02:54:24 +0200
+Subject: [PATCH] fix: add overflow checks to xmlDictAddQString in dict.c
+
+xmlDictAddString has overflow guards for pool size calculations, but its
+sibling xmlDictAddQString lacks these entirely. The namelen + plen + 1
+addition can overflow unsigned int, and 4 * (overflowed_value) produces
+a small allocation, leading to heap buffer overflow when memcpy writes
+the prefix and name.
+
+Add the same SIZE_MAX-based overflow guards and safe size_t cast.
+
+CVE: CVE-2026-86138
+Upstream-Status: Backport [https://github.com/GNOME/libxml2/commit/a4cba4b5b5a8c42e155ed42d2d2a44955465a2e4]
+
+(cherry picked from commit a4cba4b5b5a8c42e155ed42d2d2a44955465a2e4)
+Signed-off-by: Hetvi Thakar <hthakar@cisco.com>
+---
+ dict.c | 19 +++++++++++++++----
+ 1 file changed, 15 insertions(+), 4 deletions(-)
+
+diff --git a/dict.c b/dict.c
+index d7156ed3..ae0210ea 100644
+--- a/dict.c
++++ b/dict.c
+@@ -225,10 +225,21 @@ xmlDictAddQString(xmlDictPtr dict, const xmlChar *prefix, unsigned int plen,
+             return(NULL);
+         }
+ 
+-        if (size == 0) size = 1000;
+-	else size *= 4; /* exponential growth */
+-        if (size < 4 * (namelen + plen + 1))
+-	    size = 4 * (namelen + plen + 1); /* just in case ! */
++        if (size == 0) {
++            size = 1000;
++        } else {
++            if (size < (SIZE_MAX - sizeof(xmlDictStrings)) / 4)
++                size *= 4; /* exponential growth */
++            else
++                size = SIZE_MAX - sizeof(xmlDictStrings);
++        }
++        if (size / 4 < namelen + plen + 1) {
++            if ((size_t) namelen + plen + 1 <
++                    (SIZE_MAX - sizeof(xmlDictStrings)) / 4)
++                size = 4 * ((size_t) namelen + plen + 1); /* just in case ! */
++            else
++                return(NULL);
++        }
+ 	pool = (xmlDictStringsPtr) xmlMalloc(sizeof(xmlDictStrings) + size);
+ 	if (pool == NULL)
+ 	    return(NULL);

meta/recipes-core/libxml/libxml2_2.12.10.bb

@@ -33,6 +33,7 @@ SRC_URI += "http://www.w3.org/XML/Test/xmlts20130923.tar;subdir=${BP};name=testt
            file://CVE-2026-1757.patch \
            file://CVE-2026-11979.patch \
            file://CVE-2026-86137.patch \
+           file://CVE-2026-86138.patch \
            "
 
 SRC_URI[archive.sha256sum] = "c3d8c0c34aa39098f66576fe51969db12a5100b956233dc56506f7a8679be995"