linux-kernel - Re: [PATCH v2] /dev/mem: Revoke mappings when a driver claims the region

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200519184634.GZ16070@bombadil.infradead.org>
Date:   Tue, 19 May 2020 11:46:34 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     gregkh@...uxfoundation.org, Arnd Bergmann <arnd@...db.de>,
        Ingo Molnar <mingo@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Russell King <linux@....linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] /dev/mem: Revoke mappings when a driver claims the
 region

On Tue, May 19, 2020 at 12:03:06AM -0700, Dan Williams wrote:
> +void revoke_devmem(struct resource *res)
> +{
> +	struct inode *inode = READ_ONCE(devmem_inode);
> +
> +	/*
> +	 * Check that the initialization has completed. Losing the race
> +	 * is ok because it means drivers are claiming resources before
> +	 * the fs_initcall level of init and prevent /dev/mem from
> +	 * establishing mappings.
> +	 */
> +	smp_rmb();
> +	if (!inode)
> +		return;

Which wmb() is this pairing with?

> +static int devmem_init_inode(void)
> +{
> +	static struct vfsmount *devmem_vfs_mount;
> +	static int devmem_fs_cnt;
> +	struct inode *inode;
> +	int rc;
> +
> +	rc = simple_pin_fs(&devmem_fs_type, &devmem_vfs_mount, &devmem_fs_cnt);
> +	if (rc < 0) {
> +		pr_err("Cannot mount /dev/mem pseudo filesystem: %d\n", rc);
> +		return rc;
> +	}
> +
> +	inode = alloc_anon_inode(devmem_vfs_mount->mnt_sb);
> +	if (IS_ERR(inode)) {
> +		rc = PTR_ERR(inode);
> +		pr_err("Cannot allocate inode for /dev/mem: %d\n", rc);
> +		simple_release_fs(&devmem_vfs_mount, &devmem_fs_cnt);
> +		return rc;
> +	}
> +
> +	/* publish /dev/mem initialized */
> +	WRITE_ONCE(devmem_inode, inode);
> +	smp_wmb();
> +
> +	return 0;

... is that this one?  I don't see what it's guarding against.  Surely if
it's needed to ensure that the writes to 'inode' have happened before
the write of the inode pointer, the smp_wmb() needs to be before the
WRITE_ONCE, not after it?