diff --git a/fs/smb/server/smb2pdu.c b/fs/smb/server/smb2pdu.c
index b73167785e87..320b46db9377 100644
--- a/fs/smb/server/smb2pdu.c
+++ b/fs/smb/server/smb2pdu.c
@@ -6781,6 +6781,7 @@ static int set_file_allocation_info(struct ksmbd_work *work,
 	 */
 
 	loff_t alloc_blks;
+	u64 alloc_size;
 	struct inode *inode;
 	struct kstat stat;
 	int rc;
@@ -6796,7 +6797,19 @@ static int set_file_allocation_info(struct ksmbd_work *work,
 	if (rc)
 		return rc;
 
-	alloc_blks = (le64_to_cpu(file_alloc_info->AllocationSize) + 511) >> 9;
+	/*
+	 * AllocationSize is fully client-controlled (the caller only
+	 * validates the fixed 8-byte buffer length). Reject values that
+	 * would overflow the "round up to 512-byte blocks" conversion
+	 * below instead of silently wrapping it to a tiny block count,
+	 * which would truncate the file to a size the client never
+	 * asked for.
+	 */
+	alloc_size = le64_to_cpu(file_alloc_info->AllocationSize);
+	if (alloc_size > MAX_LFS_FILESIZE - 511)
+		return -EINVAL;
+
+	alloc_blks = (alloc_size + 511) >> 9;
 	inode = file_inode(fp->filp);
 
 	if (alloc_blks > stat.blocks) {