[PATCH] fs/namespace: reject bufsize < sizeof(struct statmount) in prepare_kstatmount()

[Hui Peng]

In prepare_kstatmount(), kstatmount.buf_size is initialized from the
caller's bufsize argument without rejecting buffers smaller than
sizeof(struct statmount), allowing string offset calculations relative
to sizeof(struct statmount) to underflow or write past the user buffer.
Reject bufsize < sizeof(struct statmount) in prepare_kstatmount().

Fixes: 68385d77c05b ("statmount: simplify string option retrieval")
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@xxxxxxxxx>
---
diff --git a/fs/namespace.c b/fs/namespace.c
index ae5dc64f8b45..7f7905286fa6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5882,7 +5882,7 @@ static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
 	ks->bufsize = bufsize;
 
 	if (ks->mask & STATMOUNT_STRING_REQ) {
-		if (bufsize == sizeof(ks->sm))
+		if (bufsize <= sizeof(ks->sm))
 			return -EOVERFLOW;
 
 		ks->seq.buf = kvmalloc(seq_size, GFP_KERNEL_ACCOUNT);