Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754716AbaGUW1M (ORCPT ); Mon, 21 Jul 2014 18:27:12 -0400 Received: from mail-pa0-f44.google.com ([209.85.220.44]:36407 "EHLO mail-pa0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752067AbaGUW1K (ORCPT ); Mon, 21 Jul 2014 18:27:10 -0400 From: Andy Lutomirski X-Google-Original-From: Andy Lutomirski Message-ID: <53CD93BA.1010302@mit.edu> Date: Mon, 21 Jul 2014 15:27:06 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Eric Paris , linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org CC: Andrew Vagin , "Andrew G. Morgan" , "Serge E. Hallyn" , Kees Cook , Steve Grubb , Dan Walsh , stable@kernel.org Subject: Re: [PATCH] CAPABILITIES: remove undefined caps from all processes References: <1405976341-23818-1-git-send-email-eparis@redhat.com> In-Reply-To: <1405976341-23818-1-git-send-email-eparis@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/21/2014 01:59 PM, Eric Paris wrote: > This is effectively a revert of 7b9a7ec565505699f503b4fcf61500dceb36e744 > plus fixing it a different way... > > We found, when trying to run an application from an application which > had dropped privs that the kernel does security checks on undefined > capability bits. This was ESPECIALLY difficult to debug as those > undefined bits are hidden from /proc/$PID/status. > > Consider a root application which drops all capabilities from ALL 4 > capability sets. We assume, since the application is going to set > eff/perm/inh from an array that it will clear not only the defined caps > less than CAP_LAST_CAP, but also the higher 28ish bits which are > undefined future capabilities. > > The BSET gets cleared differently. Instead it is cleared one bit at a > time. The problem here is that in security/commoncap.c::cap_task_prctl() > we actually check the validity of a capability being read. So any task > which attempts to 'read all things set in bset' followed by 'unset all > things set in bset' will not even attempt to unset the undefined bits > higher than CAP_LAST_CAP. > > So the 'parent' will look something like: > CapInh: 0000000000000000 > CapPrm: 0000000000000000 > CapEff: 0000000000000000 > CapBnd: ffffffc000000000 > > All of this 'should' be fine. Given that these are undefined bits that > aren't supposed to have anything to do with permissions. But they do... > > So lets now consider a task which cleared the eff/perm/inh completely > and cleared all of the valid caps in the bset (but not the invalid caps > it couldn't read out of the kernel). We know that this is exactly what > the libcap-ng library does and what the go capabilities library does. > They both leave you in that above situation if you try to clear all of > you capapabilities from all 4 sets. If that root task calls execve() > the child task will pick up all caps not blocked by the bset. The bset > however does not block bits higher than CAP_LAST_CAP. So now the child > task has bits in eff which are not in the parent. These are > 'meaningless' undefined bits, but still bits which the parent doesn't > have. > > The problem is now in cred_cap_issubset() (or any operation which does a > subset test) as the child, while a subset for valid cap bits, is not a > subset for invalid cap bits! So now we set durring commit creds that > the child is not dumpable. Given it is 'more priv' than its parent. It > also means the parent cannot ptrace the child and other stupidity. > > The solution here is 2 things. > 1) stop hiding capability bits in status > we hide those upper bits which meant I couldn't spot this issue > 2) stop giving any task undefined capability bits. it's simple, it you > don't put those invalid bits in CAP_FULL_SET you won't get them in init > and you won't get them in any other task either. > > Signed-off-by: Eric Paris > Cc: Andrew Vagin > Cc: Andrew G. Morgan > Cc: Serge E. Hallyn > Cc: Kees Cook > Cc: Steve Grubb > Cc: Dan Walsh > Cc: stable@kernel.org > --- > fs/proc/array.c | 9 --------- > include/linux/capability.h | 2 +- > 2 files changed, 1 insertion(+), 10 deletions(-) > > diff --git a/fs/proc/array.c b/fs/proc/array.c > index 64db2bc..d882018 100644 > --- a/fs/proc/array.c > +++ b/fs/proc/array.c > @@ -302,10 +302,6 @@ static void render_cap_t(struct seq_file *m, const char *header, > seq_putc(m, '\n'); > } > > -/* Remove non-existent capabilities */ > -#define NORM_CAPS(v) (v.cap[CAP_TO_INDEX(CAP_LAST_CAP)] &= \ > - CAP_TO_MASK(CAP_LAST_CAP + 1) - 1) > - > static inline void task_cap(struct seq_file *m, struct task_struct *p) > { > const struct cred *cred; > @@ -319,11 +315,6 @@ static inline void task_cap(struct seq_file *m, struct task_struct *p) > cap_bset = cred->cap_bset; > rcu_read_unlock(); > > - NORM_CAPS(cap_inheritable); > - NORM_CAPS(cap_permitted); > - NORM_CAPS(cap_effective); > - NORM_CAPS(cap_bset); > - > render_cap_t(m, "CapInh:\t", &cap_inheritable); > render_cap_t(m, "CapPrm:\t", &cap_permitted); > render_cap_t(m, "CapEff:\t", &cap_effective); > diff --git a/include/linux/capability.h b/include/linux/capability.h > index 84b13ad..1c36782 100644 > --- a/include/linux/capability.h > +++ b/include/linux/capability.h > @@ -79,7 +79,7 @@ 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 }}) " - 1", please. Acked-by: Andy Lutomirski --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/