Re: [patch 07/10] unprivileged mounts: add sysctl tunable for"safe" property

[Serge E. Hallyn]

Quoting Miklos Szeredi (miklos@xxxxxxxxxx):
> > > > > Maybe sysctls just need to check capabilities, instead of uids.  I
> > > > > think that would make a lot of sense anyway.
> > > > 
> > > > Would it be as simple as tagging the inodes with capability sets?  One
> > > > set for writing, or one each for reading and writing?
> > > 
> > > Yes, or something even simpler, like mapping the owner permission bits
> > > to CAP_SYS_ADMIN.  There seem to be very few different permissions
> > > under /proc/sys:
> > > 
> > > --w-------
> > > -r--r--r--
> > > -rw-------
> > > -rw-r--r--
> > > 
> > > As long as the group and other bits are always the same, and we accept
> > > that the owner bits really mean CAP_SYS_ADMIN and not something else,
> > 
> > But I would assume some things under /proc/sys/net/ipv4 or
> > /proc/sys/net/ath0 require CAP_NET_ADMIN rather than CAP_SYS_ADMIN?
> 
> I guess so.  I'm not very familiar with the different capabilities :)
> 
> How about this patch then: a hybrid solution between just relying on
> permission bits, and specifying separate capability sets for read and
> write in addition to the permission bits.
> 
> Untested, the 'cap' field obviously still needs to be filled in where
> appropriate.
> 
> Miklos
> ----
> 
> Index: linux/include/linux/sysctl.h
> ===================================================================
> --- linux.orig/include/linux/sysctl.h	2008-02-04 12:29:01.000000000 +0100
> +++ linux/include/linux/sysctl.h	2008-02-07 15:19:06.000000000 +0100
> @@ -1041,6 +1041,7 @@ struct ctl_table 
>  	void *data;
>  	int maxlen;
>  	mode_t mode;
> +	int cap;			/* Capability needed to read/write */
>  	struct ctl_table *child;
>  	struct ctl_table *parent;	/* Automatically set */
>  	proc_handler *proc_handler;	/* Callback for text formatting */
> Index: linux/kernel/sysctl.c
> ===================================================================
> --- linux.orig/kernel/sysctl.c	2008-02-05 22:17:05.000000000 +0100
> +++ linux/kernel/sysctl.c	2008-02-07 15:30:45.000000000 +0100
> @@ -1527,14 +1527,26 @@ out:
>   * some sysctl variables are readonly even to root.
>   */
> 
> -static int test_perm(int mode, int op)
> +static int test_perm(struct ctl_table *table, int op)
>  {
> -	if (!current->euid)
> -		mode >>= 6;
> -	else if (in_egroup_p(0))
> -		mode >>= 3;
> +	int cap = table->cap;
> +	mode_t mode = table->mode;
> +
> +	if (!cap)
> +		cap = CAP_SYS_ADMIN;
> +
> +	if ((op & MAY_READ) && !(mode & S_IRUGO))
> +		return -EACCES;
> +
> +	if ((op & MAY_WRITE) && !(mode & S_IWUGO))
> +		return -EACCES;
> +
> +	if (capable(cap))
> +		return 0;
> +
>  	if ((mode & op & 0007) == op)
>  		return 0;
> +
>  	return -EACCES;

I like how simple it appears to be :)

At first I missed the fact that owning uid is always 0 so I thought the
uid processing wasn't quite enough.  But since it's always 0, the only
question is whether there are any /proc/sys files whose users currently
depend on being setgid 0 and setgid non-0 with no capabilities.

On my laptop, 'find /proc/sys -type f -perm -020' gives me no results,
so that is promising.

So this certainly seems like a good first step.  In fact, combined with
/proc/sys/ being partially remounted per container like /proc/sys/net is
doing, we may not even need to do anything with CAP_NS_OVERRIDE.

thanks,
-serge

>  }
> 
> @@ -1544,7 +1556,7 @@ int sysctl_perm(struct ctl_table *table,
>  	error = security_sysctl(table, op);
>  	if (error)
>  		return error;
> -	return test_perm(table->mode, op);
> +	return test_perm(table, op);
>  }
> 
>  #ifdef CONFIG_SYSCTL_SYSCALL
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