efi: capsule-loader: Fix use-after-free in efi_capsule_write

A race condition may occur if the user calls close() on another thread
during a write() operation on the device node of the efi capsule.

This is a race condition that occurs between the efi_capsule_write() and
efi_capsule_flush() functions of efi_capsule_fops, which ultimately
results in UAF.

So, the page freeing process is modified to be done in
efi_capsule_release() instead of efi_capsule_flush().

Cc: <stable@vger.kernel.org> # v4.9+
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Link: https://lore.kernel.org/all/20220907102920.GA88602@ubuntu/Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/capsule-loader.c

@@ -242,29 +242,6 @@ static ssize_t efi_capsule_write(struct file *file, const char __user *buff,
 	return ret;
 }
 
-/**
- * efi_capsule_flush - called by file close or file flush
- * @file: file pointer
- * @id: not used
- *
- *	If a capsule is being partially uploaded then calling this function
- *	will be treated as upload termination and will free those completed
- *	buffer pages and -ECANCELED will be returned.
- **/
-static int efi_capsule_flush(struct file *file, fl_owner_t id)
-{
-	int ret = 0;
-	struct capsule_info *cap_info = file->private_data;
-
-	if (cap_info->index > 0) {
-		pr_err("capsule upload not complete\n");
-		efi_free_all_buff_pages(cap_info);
-		ret = -ECANCELED;
-	}
-
-	return ret;
-}
-
 /**
  * efi_capsule_release - called by file close
  * @inode: not used
@@ -277,6 +254,13 @@ static int efi_capsule_release(struct inode *inode, struct file *file)
 {
 	struct capsule_info *cap_info = file->private_data;
 
+	if (cap_info->index > 0 &&
+	    (cap_info->header.headersize == 0 ||
+	     cap_info->count < cap_info->total_size)) {
+		pr_err("capsule upload not complete\n");
+		efi_free_all_buff_pages(cap_info);
+	}
+
 	kfree(cap_info->pages);
 	kfree(cap_info->phys);
 	kfree(file->private_data);
@@ -324,7 +308,6 @@ static const struct file_operations efi_capsule_fops = {
 	.owner = THIS_MODULE,
 	.open = efi_capsule_open,
 	.write = efi_capsule_write,
-	.flush = efi_capsule_flush,
 	.release = efi_capsule_release,
 	.llseek = no_llseek,
 };