Re: [PATCH 3/3] virt: Add sev_secret module to expose confidential computing secrets

[Andrew Scull]

On Mon, Aug 09, 2021 at 07:01:57PM +0000, Dov Murik wrote:
> The new sev_secret module exposes the confidential computing (coco)
> secret area via securityfs interface.
> 
> When the module is loaded (and securityfs is mounted, typically under
> /sys/kernel/security), a "coco/sev_secret" directory is created in
> securityfs.  In it, a file is created for each secret entry.  The name
> of each such file is the GUID of the secret entry, and its content is
> the secret data.
> 
> This allows applications running in a confidential computing setting to
> read secrets provided by the guest owner via a secure secret injection
> mechanism (such as AMD SEV's LAUNCH_SECRET command).
> 
> Removing (unlinking) files in the "coco/sev_secret" directory will zero
> out the secret in memory, and remove the filesystem entry.  If the
> module is removed and loaded again, that secret will not appear in the
> filesystem.

We've also been looking into a similar secret mechanism recently in the
context of Android and protected KVM [1]. Our secrets would come from a
different source, likely described as a reserved-memory node in the DT,
but would need to be exposed to userspace in the same way as the SEV
secrets. Originally I tried using a character device, but this approach
with securityfs feels neater to me.

We're also looking to pass secrets from the bootloader to Linux, outside
of any virtualization or confidential compute context (at least a far as
I have understood the meaning of the term). Again, this feels like it
would be exposed to userspace in the same way.

It would be good to be able to share the parts that would be common. I
expect that would mean the operations for a secret file and for a
directory of secrets at a minimum. But it might also influence the paths
in securityfs; I see, looking back, that the "coco" directory was added
since the RFC but would a generalized "secret" subsystem make sense? Or
would it be preferable for each case to define their own path?

[1] -- https://lwn.net/Articles/836693/

> +static int sev_secret_unlink(struct inode *dir, struct dentry *dentry)
> +{
> +	struct sev_secret *s = sev_secret_get();
> +	struct inode *inode = d_inode(dentry);
> +	struct secret_entry *e = (struct secret_entry *)inode->i_private;
> +	int i;
> +
> +	if (e) {
> +		/* Zero out the secret data */
> +		memzero_explicit(e->data, secret_entry_data_len(e));

Would there be a benefit in flushing these zeros?

> +		e->guid = NULL_GUID;
> +	}
> +
> +	inode->i_private = NULL;
> +
> +	for (i = 0; i < SEV_SECRET_NUM_FILES; i++)
> +		if (s->fs_files[i] == dentry)
> +			s->fs_files[i] = NULL;
> +
> +	/*
> +	 * securityfs_remove tries to lock the directory's inode, but we reach
> +	 * the unlink callback when it's already locked
> +	 */
> +	inode_unlock(dir);
> +	securityfs_remove(dentry);
> +	inode_lock(dir);
> +
> +	return 0;
> +}