Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933455AbaGWUqn (ORCPT ); Wed, 23 Jul 2014 16:46:43 -0400 Received: from mail-pd0-f179.google.com ([209.85.192.179]:42796 "EHLO mail-pd0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932847AbaGWUqm (ORCPT ); Wed, 23 Jul 2014 16:46:42 -0400 Message-ID: <53D01F2D.8090301@amacapital.net> Date: Wed, 23 Jul 2014 13:46:37 -0700 From: Andy Lutomirski 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@vger.kernel.org Subject: Re: [PATCH] CAPABILITIES: remove undefined caps from all processes References: <1406144186-23002-1-git-send-email-eparis@redhat.com> In-Reply-To: <1406144186-23002-1-git-send-email-eparis@redhat.com> X-Enigmail-Version: 1.6 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 07/23/2014 12:36 PM, Eric Paris wrote: > This is effectively a revert of 7b9a7ec565505699f503b4fcf61500dceb36e744 > plus fixing it a different way... You sent something like this a couple days ago. What changed? --Andy > > 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: > 1) stop hiding capability bits in status > This makes debugging easier! > > 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. > This fixes the cap_issubset() tests and resulting fallout (which > made the init task in a docker container untraceable among other > things) > > 3) mask out undefined bits when sys_capset() is called as it might use > ~0, ~0 to denote 'all capabilities' for backward/forward compatibility. > This lets 'capsh --caps="all=eip" -- -c /bin/bash' run. > > 4) mask out undefined bit when we read a file capability off of disk as > again likely all bits are set in the xattr for forward/backward > compatibility. > This lets 'setcap all+pe /bin/bash; /bin/bash' run > > 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@vger.kernel.org > --- > fs/proc/array.c | 11 +---------- > include/linux/capability.h | 5 ++++- > kernel/audit.c | 2 +- > kernel/capability.c | 4 ++++ > security/commoncap.c | 3 +++ > 5 files changed, 13 insertions(+), 12 deletions(-) > > diff --git a/fs/proc/array.c b/fs/proc/array.c > index 64db2bc..3e1290b 100644 > --- a/fs/proc/array.c > +++ b/fs/proc/array.c > @@ -297,15 +297,11 @@ static void render_cap_t(struct seq_file *m, const char *header, > seq_puts(m, header); > CAP_FOR_EACH_U32(__capi) { > seq_printf(m, "%08x", > - a->cap[(_KERNEL_CAPABILITY_U32S-1) - __capi]); > + a->cap[CAP_LAST_U32 - __capi]); > } > 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..aa93e5e 100644 > --- a/include/linux/capability.h > +++ b/include/linux/capability.h > @@ -78,8 +78,11 @@ extern const kernel_cap_t __cap_init_eff_set; > # error Fix up hand-coded capability macro initializers > #else /* HAND-CODED capability initializers */ > > +#define CAP_LAST_U32 ((_KERNEL_CAPABILITY_U32S) - 1) > +#define CAP_LAST_U32_VALID_MASK (CAP_TO_MASK(CAP_LAST_CAP + 1) -1) > + > # 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_LAST_U32_VALID_MASK }}) > # define CAP_FS_SET ((kernel_cap_t){{ CAP_FS_MASK_B0 \ > | CAP_TO_MASK(CAP_LINUX_IMMUTABLE), \ > CAP_FS_MASK_B1 } }) > diff --git a/kernel/audit.c b/kernel/audit.c > index 3ef2e0e..ba2ff5a 100644 > --- a/kernel/audit.c > +++ b/kernel/audit.c > @@ -1677,7 +1677,7 @@ void audit_log_cap(struct audit_buffer *ab, char *prefix, kernel_cap_t *cap) > audit_log_format(ab, " %s=", prefix); > CAP_FOR_EACH_U32(i) { > audit_log_format(ab, "%08x", > - cap->cap[(_KERNEL_CAPABILITY_U32S-1) - i]); > + cap->cap[CAP_LAST_U32 - i]); > } > } > > diff --git a/kernel/capability.c b/kernel/capability.c > index a5cf13c..989f5bf 100644 > --- a/kernel/capability.c > +++ b/kernel/capability.c > @@ -258,6 +258,10 @@ SYSCALL_DEFINE2(capset, cap_user_header_t, header, const cap_user_data_t, data) > i++; > } > > + effective.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK; > + permitted.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK; > + inheritable.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK; > + > new = prepare_creds(); > if (!new) > return -ENOMEM; > diff --git a/security/commoncap.c b/security/commoncap.c > index b9d613e..963dc59 100644 > --- a/security/commoncap.c > +++ b/security/commoncap.c > @@ -421,6 +421,9 @@ int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data > cpu_caps->inheritable.cap[i] = le32_to_cpu(caps.data[i].inheritable); > } > > + cpu_caps->permitted.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK; > + cpu_caps->inheritable.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK; > + > return 0; > } > > -- 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/