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

Serge E. Hallyn
2008-02-07T16:58:20Z

Quoting Miklos Szeredi (miklos@szeredi.hu):
> > > > > 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