• Eric Paris's avatar
    CAPABILITIES: remove undefined caps from all processes · 76f01555
    Eric Paris authored
    commit 7d8b6c63751cfbbe5eef81a48c22978b3407a3ad upstream.
    
    This is effectively a revert of 7b9a7ec5
    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:
    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: 's avatarEric Paris <eparis@redhat.com>
    Reviewed-by: 's avatarKees Cook <keescook@chromium.org>
    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>
    Signed-off-by: 's avatarJames Morris <james.l.morris@oracle.com>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    76f01555
capability.c 12.4 KB