[PATCH] CAPABILITIES: remove undefined caps from all processes

* [PATCH] CAPABILITIES: remove undefined caps from all processes
@ 2014-07-21 20:59 Eric Paris
  2014-07-21 22:27 ` Andy Lutomirski
                   ` (2 more replies)
  0 siblings, 3 replies; 11+ messages in thread
From: Eric Paris @ 2014-07-21 20:59 UTC (permalink / raw)
  To: linux-kernel, linux-security-module
  Cc: Eric Paris, Andrew Vagin, Andrew G. Morgan, Serge E. Hallyn,
	Kees Cook, Steve Grubb, Dan Walsh, stable

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 <eparis@redhat.com>
Cc: Andrew Vagin <avagin@openvz.org>
Cc: Andrew G. Morgan <morgan@kernel.org>
Cc: Serge E. Hallyn <serge.hallyn@canonical.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Steve Grubb <sgrubb@redhat.com>
Cc: Dan Walsh <dwalsh@redhat.com>
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 }})
 # define CAP_FS_SET       ((kernel_cap_t){{ CAP_FS_MASK_B0 \
 				    | CAP_TO_MASK(CAP_LINUX_IMMUTABLE), \
 				    CAP_FS_MASK_B1 } })
-- 
1.9.3


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