Merge tag 'bootconfig-fixes-v7.2-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace

Pull bootconfig fix from Masami Hiramatsu:

 - bootconfig: Fix NULL-pointer arithmetic

   Fix undefined pointer arithmetic in xbc_snprint_cmdline() when
   probing the buffer length with NULL and size 0. Track the written
   length as a size_t instead to prevent build-time UBSan/FORTIFY_SOURCE
   failures.

* tag 'bootconfig-fixes-v7.2-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  bootconfig: fix NULL-pointer arithmetic in xbc_snprint_cmdline()

1 files changed, 16 insertions, 7 deletions

diff --git a/lib/bootconfig.c b/lib/bootconfig.c
index f445b7703fdd..2ed9ee3dc81c 100644
--- a/lib/bootconfig.c
+++ b/lib/bootconfig.c
@@ -427,10 +427,18 @@ static char xbc_namebuf[XBC_KEYLEN_MAX] __initdata;
 int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
 {
 	struct xbc_node *knode, *vnode;
-	char *end = buf + size;
 	const char *val, *q;
+	size_t len = 0;
 	int ret;
 
+	/*
+	 * Track the running written length rather than advancing @buf, so we
+	 * never form "buf + size" or "buf += ret" while @buf is NULL (the
+	 * size-probe call passes buf=NULL, size=0). NULL pointer arithmetic
+	 * is undefined behavior and trips host UBSan / FORTIFY_SOURCE when
+	 * this renderer runs at kernel build time. snprintf(NULL, 0, ...)
+	 * itself is well defined and returns the would-be length.
+	 */
 	xbc_node_for_each_key_value(root, knode, val) {
 		ret = xbc_node_compose_key_after(root, knode,
 					xbc_namebuf, XBC_KEYLEN_MAX);
@@ -439,10 +447,11 @@ int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
 
 		vnode = xbc_node_get_child(knode);
 		if (!vnode) {
-			ret = snprintf(buf, rest(buf, end), "%s ", xbc_namebuf);
+			ret = snprintf(buf ? buf + len : NULL, rest(len, size),
+				       "%s ", xbc_namebuf);
 			if (ret < 0)
 				return ret;
-			buf += ret;
+			len += ret;
 			continue;
 		}
 		xbc_array_for_each_value(vnode, val) {
@@ -452,15 +461,15 @@ int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
 			 * whitespace.
 			 */
 			q = strpbrk(val, " \t\r\n") ? "\"" : "";
-			ret = snprintf(buf, rest(buf, end), "%s=%s%s%s ",
-				       xbc_namebuf, q, val, q);
+			ret = snprintf(buf ? buf + len : NULL, rest(len, size),
+				       "%s=%s%s%s ", xbc_namebuf, q, val, q);
 			if (ret < 0)
 				return ret;
-			buf += ret;
+			len += ret;
 		}
 	}
 
-	return buf - (end - size);
+	return len;
 }
 #undef rest