[PATCH 01/11] params: bound array element output to the caller's page buffer

[Kees Cook]

From: Pengpeng Hou <pengpeng@xxxxxxxxxxx>

param_array_get() appends each element's string representation into the
shared sysfs page buffer by passing buffer + off to the element getter.

That works for getters that only write a small bounded string, but
param_get_charp() and similar helpers format against PAGE_SIZE from the
pointer they receive. Once off is non-zero, an element getter can
therefore write past the end of the original sysfs page buffer.

Collect each element into a temporary PAGE_SIZE buffer first and then
copy only the remaining space into the caller's page buffer.

Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
Signed-off-by: Pengpeng Hou <pengpeng@xxxxxxxxxxx>
Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
---
 kernel/params.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/kernel/params.c b/kernel/params.c
index 74d620bc2521..752721922a15 100644
--- a/kernel/params.c
+++ b/kernel/params.c
@@ -475,22 +475,36 @@ static int param_array_set(const char *val, const struct kernel_param *kp)
 static int param_array_get(char *buffer, const struct kernel_param *kp)
 {
 	int i, off, ret;
+	char *elem_buf;
 	const struct kparam_array *arr = kp->arr;
 	struct kernel_param p = *kp;
 
+	elem_buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
+	if (!elem_buf)
+		return -ENOMEM;
+
 	for (i = off = 0; i < (arr->num ? *arr->num : arr->max); i++) {
-		/* Replace \n with comma */
-		if (i)
-			buffer[off - 1] = ',';
 		p.arg = arr->elem + arr->elemsize * i;
 		check_kparam_locked(p.mod);
-		ret = arr->ops->get(buffer + off, &p);
+		ret = arr->ops->get(elem_buf, &p);
 		if (ret < 0)
-			return ret;
+			goto out;
+		ret = min(ret, (int)(PAGE_SIZE - 1 - off));
+		if (!ret)
+			break;
+		/* Replace the previous element's trailing newline with a comma. */
+		if (i)
+			buffer[off - 1] = ',';
+		memcpy(buffer + off, elem_buf, ret);
 		off += ret;
+		if (off == PAGE_SIZE - 1)
+			break;
 	}
 	buffer[off] = '\0';
-	return off;
+	ret = off;
+out:
+	kfree(elem_buf);
+	return ret;
 }
 
 static void param_array_free(void *arg)
-- 
2.34.1