[Devel] Re: [PATCH 4/9][cr][v2]: Restore file_owner info

[Oren Laadan]

On 05/18/2010 11:07 PM, Sukadev Bhattiprolu wrote:
> Restore the file-owner information for each 'struct file'.  This is
> essentially is like a new fcntl(F_SETOWN) and fcntl(F_SETSIG) calls,
> except that the pid, uid, euid and signum values are read from the
> checkpoint image.
> 
> Changelog[v2]:
> 	- [Matt Helsley, Serge Hallyn]: Don't trust uids in checkpoint image.
> 	  (added CAP_KILL check)
> 	- Check that signal number read from the checkpoint image is valid.
> 	  (not sure it is required, since its an incomplete check for tampering)
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
> ---

[...]

>  
> +static int restore_file_owner(struct ckpt_ctx *ctx, struct ckpt_hdr_file *h,
> +		struct file *file)
> +{
> +	int ret;
> +	struct pid *pid;
> +	uid_t uid, euid;
> +
> +	uid = h->f_owner_uid;
> +	euid = h->f_owner_euid;
> +
> +	ckpt_debug("restore_file_owner(): uid %u, euid %u, pid %d, type %d\n",
> +			uid, euid, h->f_owner_pid, h->f_owner_pid_type);
> +	/*
> +	 * We can't trust the uids in the checkpoint image and normally need
> +	 * CAP_KILL. But if the uids match our ids, should be fine since we
> +	 * have access to the file.
> +	 *
> +	 * TODO: Move this check to __f_setown() ?
> +	 */
> +	ret = -EACCES;
> +	if (!capable(CAP_KILL) &&
> +			(uid != current_uid() || euid != current_euid())) {
> +		ckpt_err(ctx, ret, "image uids [%d, %d] don't match current "
> +				"process uids [%d, %d] and no CAP_KILL\n",
> +				uid, euid, current_uid(), current_euid());
> +		return ret;
> +	}
> +
> +	ret = -EINVAL;
> +	if (!valid_signal(h->f_owner_signum)) {
> +		ckpt_err(ctx, ret, "Invalid signum %d\n", h->f_owner_signum);
> +		return ret;
> +	}
> +	file->f_owner.signum = h->f_owner_signum;
> +
> +	rcu_read_lock();
> +	pid = find_vpid(h->f_owner_pid);

What if this fails - the pid is invalid/non-existent ?

> +	/*
> +	 * TODO: Do we need to force==1 or can it be 0 ? 'force' is used to
> +	 * 	 modify the owner, if one is already set. Can it be set when
> +	 * 	 we restart an application ?
> +	 */
> +	ret = __f_setown(file, pid, h->f_owner_pid_type, uid, euid, 1);
> +	rcu_read_unlock();

I wonder if this would be a problem in terms of security on a
non-container restart (e.g. not in a new pid-ns): one could set
any pid as owner and any signal to be sent, and cause an arbitrary
signal to be sent to an arbitrary process ?

> +
> +	if (ret < 0)
> +		ckpt_err(ctx, ret, "__fsetown_uid() failed\n");
> +
> +	return ret;
> +}
> +
>  #define CKPT_SETFL_MASK  \
>  	(O_APPEND | O_NONBLOCK | O_NDELAY | FASYNC | O_DIRECT | O_NOATIME)
>  
> @@ -648,6 +699,10 @@ int restore_file_common(struct ckpt_ctx *ctx, struct file *file,
>  	if (ret < 0)
>  		return ret;
>  
> +	ret = restore_file_owner(ctx, h, file);
> +	if (ret < 0)
> +		return ret;
> +
>  	/*
>  	 * Normally f_mode is set by open, and modified only via
>  	 * fcntl(), so its value now should match that at checkpoint.

Oren.
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