linux-kernel - Re: [PATCH] proc: don't show nonexistent capabilities

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Date:	Fri, 5 Oct 2012 08:54:49 -0700
From:	"Andrew G. Morgan" <morgan@...nel.org>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	Cyrill Gorcunov <gorcunov@...nvz.org>, criu@...nvz.org,
	linux-kernel@...r.kernel.org,
	Serge Hallyn <serge.hallyn@...onical.com>,
	linux-security-module@...r.kernel.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrew Vagin <avagin@...nvz.org>
Subject: Re: [PATCH] proc: don't show nonexistent capabilities

I like the sentiment but have you considered the implications for a
default system root user trying to set all=eip ? Existing code can do
this because all bits are accessible by default. If you set the
bounding set to something less than ~0, then any code that currently
does this will start to fail in the common case.

Cheers

Andrew

On Oct 5, 2012 7:13 AM, "Serge E. Hallyn" <serge@...lyn.com> wrote:
>
> Quoting Andrew Vagin (avagin@...nvz.org):
> > Without this patch it is really hard to interpret a bounding set,
> > if CAP_LAST_CAP is unknown for a current kernel.
>
> To be clear, note that you *can* figure it out using
> prctl(PR_CAPBSET_DROP).  But this is a nice improvement.
>
> > Non-existant capabilities can not be deleted from a bounding set
> > with help of prctl.
> >
> > E.g.: Here are two examples without/with this patch.
> > CapBnd:       ffffffe0fdecffff
> > CapBnd:       00000000fdecffff
> >
> > I suggest to hide non-existent capabilities. Here is two reasons.
> > * It's logically and easier for using.
> > * It helps to checkpoint-restore capabilities of tasks, because tasks
> > can be restored on another kernel, where CAP_LAST_CAP is bigger.
> >
> > Cc: Serge Hallyn <serge.hallyn@...onical.com>
>
> Thanks, Andrew.  I saw your other email about having run LTP, I didn't
> see any problems from userspace either.  Great idea!
>
> Reviewed-by: Serge Hallyn <serge.hallyn@...onical.com>
>
> Still it's been like that for so long that, just to be safe, I'm cc:ing
> Andrew Morgan - can you see any problems with this?
>
> > Cc: Pavel Emelyanov <xemul@...allels.com>
> > Signed-off-by: Andrew Vagin <avagin@...nvz.org>
> > ---
> >  include/linux/capability.h |    3 ++-
> >  1 files changed, 2 insertions(+), 1 deletions(-)
> >
> > diff --git a/include/linux/capability.h b/include/linux/capability.h
> > index d10b7ed..1642778 100644
> > --- a/include/linux/capability.h
> > +++ b/include/linux/capability.h
> > @@ -420,7 +420,8 @@ extern const kernel_cap_t __cap_init_eff_set;
> >  #else /* HAND-CODED capability initializers */
> >
> >  # define CAP_EMPTY_SET    ((kernel_cap_t){{ 0, 0 }})
> > -# define CAP_FULL_SET     ((kernel_cap_t){{ ~0, ~0 }})
> > +# define CAP_FULL_SET     ((kernel_cap_t){{ ~0, \
> > +                                     CAP_TO_MASK(CAP_LAST_CAP + 1) - 1 } })
> >  # define CAP_FS_SET       ((kernel_cap_t){{ CAP_FS_MASK_B0 \
> >                                   | CAP_TO_MASK(CAP_LINUX_IMMUTABLE), \
> >                                   CAP_FS_MASK_B1 } })
> > --
> > 1.7.1
> >
> > --
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