Re: [PATCH] [RFC] Make it easier to harden /proc/

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday 16 March 2011 21:08:16 Richard Weinberger wrote:
> Am Mittwoch 16 März 2011, 20:55:49 schrieb Kees Cook:
> > On Wed, Mar 16, 2011 at 08:31:47PM +0100, Richard Weinberger wrote:
> > > When containers like LXC are used a unprivileged and jailed
> > > root user can still write to critical files in /proc/.
> > > E.g: /proc/sys/kernel/{sysrq, panic, panic_on_oops, ... }
> > > 
> > > This new restricted attribute makes it possible to protect such
> > > files. When restricted is set to true root needs CAP_SYS_ADMIN
> > > to into the file.
> > 
> > I was thinking about this too. I'd prefer more fine-grained control
> > in this area, since some sysctl entries aren't strictly controlled by
> > CAP_SYS_ADMIN (e.g. mmap_min_addr is already checking CAP_SYS_RAWIO).
> > 
> > How about this instead?
> 
> Good Idea.
> May we should also consider a per-directory restriction.
> Every file in /proc/sys/{kernel/, vm/, fs/, dev/} needs a protection.
> It would be much easier to set the protection on the parent directory
> instead of protecting file by file...

How does this interact with the per-namespace sysctls that Eric
Biederman added a few years ago?

I had expected that any dangerous sysctl would not be visible in
an unpriviledge container anyway.

	Arnd

> > Signed-off-by: Kees Cook <kees.cook@canonical.com>
> > ---
> > diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> > index 8eb2522..5c5cfab 100644
> > --- a/fs/proc/proc_sysctl.c
> > +++ b/fs/proc/proc_sysctl.c
> > @@ -149,6 +149,10 @@ static ssize_t proc_sys_call_handler(struct file
> > *filp, void __user *buf, if (sysctl_perm(head->root, table, write ?
> > MAY_WRITE : MAY_READ)) goto out;
> > 
> > +	if (write && !cap_isclear(table->write_caps) &&
> > +            !cap_issubset(table->write_caps,
> > current_cred()->cap_permitted)) +		goto out;
> > +
> >  	/* if that can happen at all, it should be -EINVAL, not -EISDIR */
> >  	error = -EINVAL;
> >  	if (!table->proc_handler)
> > diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> > index 11684d9..4e05493 100644
> > --- a/include/linux/sysctl.h
> > +++ b/include/linux/sysctl.h
> > @@ -1018,6 +1018,7 @@ struct ctl_table
> >  	void *data;
> >  	int maxlen;
> >  	mode_t mode;
> > +	kernel_cap_t write_caps;	/* Capabilities required to write */
> >  	struct ctl_table *child;
> >  	struct ctl_table *parent;	/* Automatically set */
> >  	proc_handler *proc_handler;	/* Callback for text formatting */
>