[RFC PATCH 00/11] Adding FreeBSD's Capsicum security framework (part 1)

From: David Drysdale
Date: Mon Jun 30 2014 - 06:31:28 EST

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Hi all,

The last couple of versions of FreeBSD (9.x/10.x) have included the
Capsicum security framework [1], which allows security-aware
applications to sandbox themselves in a very fine-grained way. For
example, OpenSSH now (>= 6.5) uses Capsicum in its FreeBSD version to
restrict sshd's credentials checking process, to reduce the chances of
credential leakage.

It would be good to have equivalent functionality in Linux, so I've been
working on getting the Capsicum framework running in the kernel, and I'd
appreciate some feedback/opinions on the general design approach.

I'm attaching a corresponding draft patchset for reference, but
hopefully this cover email can cover the significant features to save
everyone having to look through the code details. (It does mean this is
a long email though -- apologies for that.)

1) Capsicum Capabilities
------------------------

The most significant aspect of Capsicum is associating *rights* with
(some) file descriptors, so that the kernel only allows operations on an
FD if the rights permit it. This allows userspace applications to
sandbox themselves by tightly constraining what's allowed with both
input and outputs; for example, tcpdump might restrict itself so it can
only read from the network FD, and only write to stdout.

[Capsicum also includes 'capability mode', which locks down the
available syscalls so the rights restrictions can't just be bypassed
by opening new file descriptors; I'll describe that separately later.]

The kernel thus needs to police the rights checks for these file
descriptors (referred to as 'Capsicum capabilities', completely
different than POSIX.1e capabilities), and the best place to do this is
at the points where a file descriptor from userspace is converted to a
struct file * within the kernel.

[Policing the rights checks anywhere else, for example at the system
call boundary, isn't a good idea because it opens up the possibility
of time-of-check/time-of-use (TOCTOU) attacks [2] where FDs are
changed (as openat/close/dup2 are allowed in capability mode) between
the 'check' at syscall entry and the 'use' at fget() invocation.]

However, this does lead to quite an invasive change to the kernel --
every invocation of fget() or similar functions (fdget(),
sockfd_lookup(), user_path_at(),...) needs to be annotated with the
rights associated with the specific operations that will be performed on
the struct file. There are ~100 such invocations that need annotation.

My current implementation approach is to use varargs variants of the
fget() functions that include the required rights, varargs-macroed so
that the only impact in a non-Capsicum build is the need to cope with an
ERR_PTR on failure rather than just NULL:

#ifdef CONFIG_SECURITY_CAPSICUM
#define fgetr(fd, ...) _fgetr((fd), __VA_ARGS__, CAP_LIST_END)
/* + Other variants... */
#else
#define fgetr(fd, ...) (fget(fd) ?: ERR_PTR(-EBADF))
/* + Other variants... */
#endif

For example, an existing chunk of code like:

SYSCALL_DEFINE1(fchdir, unsigned int, fd)
{
struct fd f = fdget_raw(fd);
struct inode *inode;
int error = -EBADF;

error = -EBADF;
if (!f.file)
goto out;
...

might become:

SYSCALL_DEFINE1(fchdir, unsigned int, fd)
{
struct fd f = fdgetr_raw(fd, CAP_FCHDIR);
struct inode *inode;
int error = -EBADF;

if (IS_ERR(f.file)) {
error = PTR_ERR(f.file);
goto out;
}
...

In a Capsicum build the fdgetr_raw() function performs rights checks
(and potentially returns a new errno as ERR_PTR(-ENOTCAPABLE)), whereas
in a non-Capsicum build the only change is that fdget_raw() returns
ERR_PTR(-EBADF) rather than just NULL.

2) Capsicum Capabilities Data Structure
---------------------------------------

Internally, the rights associated with a Capsicum capability FD are
stored in a special struct file wrapper. For a normal file, the rights
check inside fget() falls through, but for a capability wrapper the
rights in the wrapper are checked and (if capable) the underlying
wrapped struct file is returned.

[This is approximately the implementation that was present in FreeBSD
9.x. For FreeBSD 10.x, the wrapper file was removed and the rights
associated with a file descriptor are now stored in the fdtable. As
that impacts memory use for all processes, whether Capsicum users or
not, I've stuck with the FreeBSD 9.x approach.]

3) New LSM Hooks
----------------

To actually perform the checking and unwrapping, I've added a couple of
new LSM hooks:
- .file_lookup(), which allows modification of the result of fget().
- .file_install(), which allows for the wrapping of a newly-created file
when that file was created from a Capsicum capability (e.g. via
openat(2) or accept(2)).

However, I'm not sure that adding the functionality via new LSM hooks is
appropriate, because I don't think Capsicum should be a fully-fledged
LSM:
- Capsicum doesn't use any of the existing LSM hooks, so (say) AppArmor
and Capsicum use a disjoint set of hooks.
- Capsicum needs to co-exist with the existing LSMs, and given the
current disjoint use, can do so without revisiting the general
problem of LSM stacking.

Of course, if in future an LSM wanted to use one of these new hooks,
it would have to deal with Capsicum being the "fallback" implementation
of the hook -- i.e. the stacking/interaction problem would show up
again. So maybe it would be better to avoid the LSM infrastructure
altogether?

4) New System Calls
-------------------

To allow userspace applications to access the Capsicum capability
functionality, I'm proposing two new system calls: cap_rights_limit(2)
and cap_rights_get(2). I guess these could potentially be implemented
elsewhere (e.g. as fcntl(2) operations?) but the changes seem
significant enough that new syscalls are warranted.

[FreeBSD 10.x actually includes six new syscalls for manipulating the
rights associated with a Capsicum capability -- the capability rights
can police that only specific fcntl(2) or ioctl(2) commands are
allowed, and FreeBSD sets these with distinct syscalls.]

5) New openat(2) O_BENEATH_ONLY Flag
------------------------------------

For Capsicum capabilities that are directory file descriptors, the
Capsicum framework only allows openat(cap_dfd, path, ...) operations to
work for files that are beneath the specified directory (and even that
only when the directory FD has the CAP_LOOKUP right), rejecting paths
that start with "/" or include "..".

As this seemed like functionality that might be more generally useful,
I've implemented it independently as a new O_BENEATH_ONLY flag for
openat(2). The Capsicum code then always triggers the use of that flag
when the dfd is a Capsicum capability.

6) Patchset Notes
-----------------

I've appended the draft patchset (against v3.15) for the implementation
of Capsicum capabilities, in case anyone wants to dive into the details.

However, I should point out that it might include some code that hasn't
been compiled -- I attempted to change every fget() invocation I could
find, even if it was for a build that I can't perform (but I have built
allyesconfig on x86 & ARM).

Regards,

David Drysdale

[1] http://www.cl.cam.ac.uk/research/security/capsicum/papers/2010usenix-security-capsicum-website.pdf
[2] http://www.watson.org/~robert/2007woot/

David Drysdale (11):
fs: add O_BENEATH_ONLY flag to openat(2)
selftests: Add test of O_BENEATH_ONLY & openat(2)
capsicum: rights values and structure definitions
capsicum: implement fgetr() and friends
capsicum: convert callers to use fgetr() etc
capsicum: implement sockfd_lookupr()
capsicum: convert callers to use sockfd_lookupr() etc
capsicum: add new LSM hooks on FD/file conversion
capsicum: implementations of new LSM hooks
capsicum: invocation of new LSM hooks
capsicum: add syscalls to limit FD rights

Documentation/security/capsicum.txt | 102 ++++++
arch/alpha/include/uapi/asm/fcntl.h | 1 +
arch/alpha/kernel/osf_sys.c | 6 +-
arch/ia64/kernel/perfmon.c | 54 ++--
arch/parisc/hpux/fs.c | 6 +-
arch/parisc/include/uapi/asm/fcntl.h | 1 +
arch/powerpc/kvm/powerpc.c | 4 +-
arch/powerpc/platforms/cell/spu_syscalls.c | 15 +-
arch/powerpc/platforms/cell/spufs/coredump.c | 2 +
arch/sparc/include/uapi/asm/fcntl.h | 1 +
arch/x86/syscalls/syscall_64.tbl | 2 +
drivers/base/dma-buf.c | 6 +-
drivers/block/loop.c | 14 +-
drivers/block/nbd.c | 5 +-
drivers/infiniband/core/ucma.c | 4 +-
drivers/infiniband/core/uverbs_cmd.c | 6 +-
drivers/infiniband/core/uverbs_main.c | 4 +-
drivers/infiniband/hw/usnic/usnic_transport.c | 2 +-
drivers/md/md.c | 8 +-
drivers/scsi/iscsi_tcp.c | 2 +-
drivers/staging/android/sync.c | 2 +-
drivers/staging/lustre/lustre/llite/file.c | 6 +-
drivers/staging/lustre/lustre/lmv/lmv_obd.c | 7 +-
drivers/staging/lustre/lustre/mdc/lproc_mdc.c | 8 +-
drivers/staging/lustre/lustre/mdc/mdc_request.c | 4 +-
drivers/staging/usbip/stub_dev.c | 2 +-
drivers/staging/usbip/vhci_sysfs.c | 2 +-
drivers/vfio/pci/vfio_pci.c | 6 +-
drivers/vfio/pci/vfio_pci_intrs.c | 6 +-
drivers/vfio/vfio.c | 6 +-
drivers/vhost/net.c | 8 +-
drivers/video/fbdev/msm/mdp.c | 4 +-
fs/aio.c | 37 ++-
fs/autofs4/dev-ioctl.c | 16 +-
fs/autofs4/inode.c | 4 +-
fs/btrfs/ioctl.c | 20 +-
fs/btrfs/send.c | 7 +-
fs/cifs/ioctl.c | 6 +-
fs/coda/inode.c | 4 +-
fs/coda/psdev.c | 2 +-
fs/compat.c | 18 +-
fs/compat_ioctl.c | 14 +-
fs/eventfd.c | 17 +-
fs/eventpoll.c | 19 +-
fs/ext4/ioctl.c | 6 +-
fs/fcntl.c | 106 ++++++-
fs/fhandle.c | 6 +-
fs/file.c | 130 ++++++++
fs/fuse/inode.c | 10 +-
fs/ioctl.c | 13 +-
fs/locks.c | 10 +-
fs/namei.c | 307 ++++++++++++++----
fs/ncpfs/inode.c | 5 +-
fs/notify/dnotify/dnotify.c | 2 +
fs/notify/fanotify/fanotify_user.c | 16 +-
fs/notify/inotify/inotify_user.c | 12 +-
fs/ocfs2/cluster/heartbeat.c | 8 +-
fs/open.c | 46 +--
fs/proc/fd.c | 16 +-
fs/proc/namespaces.c | 6 +-
fs/read_write.c | 113 ++++---
fs/readdir.c | 18 +-
fs/select.c | 11 +-
fs/signalfd.c | 6 +-
fs/splice.c | 34 +-
fs/stat.c | 10 +-
fs/statfs.c | 8 +-
fs/sync.c | 21 +-
fs/timerfd.c | 40 ++-
fs/utimes.c | 10 +-
fs/xattr.c | 26 +-
fs/xfs/xfs_ioctl.c | 14 +-
include/linux/capsicum.h | 57 ++++
include/linux/file.h | 136 ++++++++
include/linux/namei.h | 10 +
include/linux/net.h | 16 +
include/linux/security.h | 48 +++
include/linux/syscalls.h | 12 +
include/uapi/asm-generic/errno.h | 3 +
include/uapi/asm-generic/fcntl.h | 4 +
include/uapi/linux/Kbuild | 1 +
include/uapi/linux/capsicum.h | 343 ++++++++++++++++++++
ipc/mqueue.c | 30 +-
kernel/events/core.c | 14 +-
kernel/module.c | 10 +-
kernel/sys.c | 6 +-
kernel/sys_ni.c | 4 +
kernel/taskstats.c | 4 +-
kernel/time/posix-clock.c | 27 +-
mm/fadvise.c | 7 +-
mm/internal.h | 19 ++
mm/memcontrol.c | 12 +-
mm/mmap.c | 7 +-
mm/nommu.c | 9 +-
mm/readahead.c | 6 +-
net/9p/trans_fd.c | 10 +-
net/bluetooth/bnep/sock.c | 2 +-
net/bluetooth/cmtp/sock.c | 2 +-
net/bluetooth/hidp/sock.c | 4 +-
net/compat.c | 4 +-
net/l2tp/l2tp_core.c | 11 +-
net/l2tp/l2tp_core.h | 2 +
net/sched/sch_atm.c | 2 +-
net/socket.c | 207 +++++++++---
net/sunrpc/svcsock.c | 4 +-
security/Kconfig | 15 +
security/Makefile | 2 +-
security/capability.c | 17 +-
security/capsicum-rights.c | 201 ++++++++++++
security/capsicum-rights.h | 10 +
security/capsicum.c | 403 ++++++++++++++++++++++++
security/security.c | 13 +
sound/core/pcm_native.c | 10 +-
tools/testing/selftests/openat/.gitignore | 3 +
tools/testing/selftests/openat/Makefile | 24 ++
tools/testing/selftests/openat/openat.c | 146 +++++++++
virt/kvm/eventfd.c | 6 +-
virt/kvm/vfio.c | 12 +-
118 files changed, 2840 insertions(+), 535 deletions(-)
create mode 100644 Documentation/security/capsicum.txt
create mode 100644 include/linux/capsicum.h
create mode 100644 include/uapi/linux/capsicum.h
create mode 100644 security/capsicum-rights.c
create mode 100644 security/capsicum-rights.h
create mode 100644 security/capsicum.c
create mode 100644 tools/testing/selftests/openat/.gitignore
create mode 100644 tools/testing/selftests/openat/Makefile
create mode 100644 tools/testing/selftests/openat/openat.c

--
2.0.0.526.g5318336

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Next message: David Drysdale: "[PATCH 09/11] capsicum: implementations of new LSM hooks"
Previous message: David Drysdale: "[PATCH 4/5] man-pages: cap_rights_limit.2: limit FD rights for Capsicum"
Next in thread: David Drysdale: "[PATCH 02/11] selftests: Add test of O_BENEATH_ONLY & openat(2)"
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