Re: [PATCH v3 1/2] capabilities: Ambient capabilities

On Tue, Jun 9, 2015 at 4:09 PM, Kees Cook [off-list ref] wrote:

On Wed, May 27, 2015 at 4:47 PM, Andy Lutomirski [off-list ref] wrote:

quoted

Credit where credit is due: this idea comes from Christoph Lameter
with a lot of valuable input from Serge Hallyn.  This patch is
heavily based on Christoph's patch.

===== The status quo =====

On Linux, there are a number of capabilities defined by the kernel.
To perform various privileged tasks, processes can wield
capabilities that they hold.

Each task has four capability masks: effective (pE), permitted (pP),
inheritable (pI), and a bounding set (X).  When the kernel checks
for a capability, it checks pE.  The other capability masks serve to
modify what capabilities can be in pE.

Any task can remove capabilities from pE, pP, or pI at any time.  If
a task has a capability in pP, it can add that capability to pE
and/or pI.  If a task has CAP_SETPCAP, then it can add any
capability to pI, and it can remove capabilities from X.

Tasks are not the only things that can have capabilities; files can
also have capabilities.  A file can have no capabilty information at
all [1].  If a file has capability information, then it has a
permitted mask (fP) and an inheritable mask (fI) as well as a single
effective bit (fE) [2].  File capabilities modify the capabilities
of tasks that execve(2) them.

A task that successfully calls execve has its capabilities modified
for the file ultimately being excecuted (i.e. the binary itself if
that binary is ELF or for the interpreter if the binary is a
script.) [3] In the capability evolution rules, for each mask Z, pZ
represents the old value and pZ' represents the new value.  The
rules are:

  pP' = (X & fP) | (pI & fI)
  pI' = pI
  pE' = (fE ? pP' : 0)
  X is unchanged

For setuid binaries, fP, fI, and fE are modified by a moderately
complicated set of rules that emulate POSIX behavior.  Similarly, if
euid == 0 or ruid == 0, then fP, fI, and fE are modified differently
(primary, fP and fI usually end up being the full set).  For nonroot
users executing binaries with neither setuid nor file caps, fI and
fP are empty and fE is false.

As an extra complication, if you execute a process as nonroot and fE
is set, then the "secure exec" rules are in effect: AT_SECURE gets
set, LD_PRELOAD doesn't work, etc.

This is rather messy.  We've learned that making any changes is
dangerous, though: if a new kernel version allows an unprivileged
program to change its security state in a way that persists cross
execution of a setuid program or a program with file caps, this
persistent state is surprisingly likely to allow setuid or
file-capped programs to be exploited for privilege escalation.

===== The problem =====

Capability inheritance is basically useless.

If you aren't root and you execute an ordinary binary, fI is zero,
so your capabilities have no effect whatsoever on pP'.  This means
that you can't usefully execute a helper process or a shell command
with elevated capabilities if you aren't root.

On current kernels, you can sort of work around this by setting fI
to the full set for most or all non-setuid executable files.  This
causes pP' = pI for nonroot, and inheritance works.  No one does
this because it's a PITA and it isn't even supported on most
filesystems.

If you try this, you'll discover that every nonroot program ends up
with secure exec rules, breaking many things.

This is a problem that has bitten many people who have tried to use
capabilities for anything useful.

===== The proposed change =====

This patch adds a fifth capability mask called the ambient mask
(pA).  pA does what most people expect pI to do.

pA obeys the invariant that no bit can ever be set in pA if it is
not set in both pP and pI.  Dropping a bit from pP or pI drops that
bit from pA.  This ensures that existing programs that try to drop
capabilities still do so, with a complication.  Because capability
inheritance is so broken, setting KEEPCAPS, using setresuid to
switch to nonroot uids, and then calling execve effectively drops
capabilities.  Therefore, setresuid from root to nonroot
conditionally clears pA unless SECBIT_NO_SETUID_FIXUP is set.
Processes that don't like this can re-add bits to pA afterwards.

The capability evolution rules are changed:

  pA' = (file caps or setuid or setgid ? 0 : pA)
  pP' = (X & fP) | (pI & fI) | pA'
  pI' = pI
  pE' = (fE ? pP' : pA')
  X is unchanged

If you are nonroot but you have a capability, you can add it to pA.
If you do so, your children get that capability in pA, pP, and pE.
For example, you can set pA = CAP_NET_BIND_SERVICE, and your
children can automatically bind low-numbered ports.  Hallelujah!

Chrome OS could use this right now. :)

quoted

Unprivileged users can create user namespaces, map themselves to a
nonzero uid, and create both privileged (relative to their
namespace) and unprivileged process trees.  This is currently more
or less impossible.  Hallelujah!

You cannot use pA to try to subvert a setuid, setgid, or file-capped
program: if you execute any such program, pA gets cleared and the
resulting evolution rules are unchanged by this patch.

Users with nonzero pA are unlikely to unintentionally leak that
capability.  If they run programs that try to drop privileges,
dropping privileges will still work.

It's worth noting that the degree of paranoia in this patch could
possibly be reduced without causing serious problems.  Specifically,
if we allowed pA to persist across executing non-pA-aware setuid
binaries and across setresuid, then, naively, the only capabilities
that could leak as a result would be the capabilities in pA, and any
attacker *already* has those capabilities.  This would make me
nervous, though -- setuid binaries that tried to privilege-separate
might fail to do so, and putting CAP_DAC_READ_SEARCH or
CAP_DAC_OVERRIDE into pA could have unexpected side effects.
(Whether these unexpected side effects would be exploitable is an
open question.)  I've therefore taken the more paranoid route.  We
can revisit this later.

I think this is correct. Stuff using file caps, or set*id bits are
fundamentally using a different privilege management model. Keeping pA
separate makes a lot of sense to me.

quoted

An alternative would be to require PR_SET_NO_NEW_PRIVS before
setting ambient capabilities.  I think that this would be annoying
and would make granting otherwise unprivileged users minor ambient
capabilities (CAP_NET_BIND_SERVICE or CAP_NET_RAW for example) much
less useful than it is with this patch.

Agreed: we should keep nnp out of this.

quoted

===== Footnotes =====

[1] Files that are missing the "security.capability" xattr or that
have unrecognized values for that xattr end up with has_cap set to
false.  The code that does that appears to be complicated for no
good reason.

Would it make more sense to have has_cap true, but have it lack any actual caps?

I assume you're referring to the case where we fail to parse the
xattr.  If so, I don't really know if or when this happens.  Should
that be addressed separately from this patch set?

quoted

[2] The libcap capability mask parsers and formatters are
dangerously misleading and the documentation is flat-out wrong.  fE
is *not* a mask; it's a single bit.  This has probably confused
every single person who has tried to use file capabilities.

Sounds like it would be a valuable documentation patch.

I'll try.  Let's get the current thing done first.

quoted

[3] Linux very confusingly processes both the script and the
interpreter if applicable, for reasons that elude me.  The results
from thinking about a script's file capabilities and/or setuid bits
are mostly discarded.

I wonder if this is important enough to fix?

Not sure.

However, the fact that AFAICT LSM due to a script (as opposed to an
interpreter) is preserved sounds rather dangerous to me.  I'm not sure
whether we can safely fix that at this point.

--Andy