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Date: Tue, 18 Mar 2025 03:37:38 +0000
From: Al Viro <viro@zeniv.linux.org.uk>
To: James Bottomley <James.Bottomley@hansenpartnership.com>
Cc: linux-efi@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Ard Biesheuvel <ardb@kernel.org>, Ryan Lee <ryan.lee@canonical.com>,
	Malte =?iso-8859-1?Q?Schr=F6der?= <malte.schroeder@tnxip.de>,
	Christian Brauner <brauner@kernel.org>
Subject: Re: [PATCH] efivarfs: fix NULL dereference on resume
Message-ID: <20250318033738.GV2023217@ZenIV>
References: <3e998bf87638a442cbc6864cdcd3d8d9e08ce3e3.camel@HansenPartnership.com>
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Mon, Mar 17, 2025 at 11:06:01PM -0400, James Bottomley wrote:

> +	/* ensure single superblock is alive and pin it */
> +	if (!atomic_inc_not_zero(&s->s_active))
> +		return NOTIFY_DONE;
> +
>  	pr_info("efivarfs: resyncing variable state\n");
>  
> -	/* O_NOATIME is required to prevent oops on NULL mnt */
> +	path.dentry = sfi->sb->s_root;
> +
> +	/*
> +	 * do not add SB_KERNMOUNT which a single superblock could
> +	 * expose to userspace and which also causes MNT_INTERNAL, see
> +	 * below
> +	 */
> +	path.mnt = vfs_kern_mount(&efivarfs_type, 0,
> +				  efivarfs_type.name, NULL);

Umm...  That's probably safe, but not as a long-term solution -
it's too intimately dependent upon fs/super.c internals.
The reasons why you can't run into ->s_umount deadlock here
are non-trivial...