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Subject[PATCH][RFC] Light-weight Auditing Framework
This note describes a patch against 2.6.4-rc1-bk2 that provides a
low-overhead system-call auditing framework for Linux that is usable by
LSM components (e.g., SELinux). Comments will be appreciated.

The main goals were to provide system call auditing with 1) as low
overhead as possible, and 2) without duplicating functionality that is
already provided by SELinux (and/or other security infrastructures).
This framework will work "stand-alone", but is not designed to provide,
e.g., CAPP functionality without another security component in place.

There are two main parts, one that is always on (generic logging in
audit.c) and one that you can disable at boot- or run-time
(per-system-call auditing in auditsc.c). The patch includes changes to
security/selinux/avc.c as an example of how system-call auditing can be
integrated with other code that identifies auditable events.

1) Uses a netlink socket for communication with user-space. All
messages are logged via the netlink socket if a user-space daemon
is listening. If not, the messages are logged via printk to the
syslog daemon (by default).
2) Messages can be dropped (optionally) based on message rate or
memory use (this isn't fully integrated into the selinux/avc.c
part of the patch: the avc.c code that currently does this can be
3) When some part of the kernel generates part of an audit record,
the partial record is sent immediately to user-space, AND the
system call "auditable" flag is automatically set for that call
-- thereby producing extra information at syscall exit (if
syscall auditing is enabled).

System-call auditing:
1) At task-creation time, an audit context is allocated and linked
off the task structure.
2) At syscall entry time, if the audit context exists, information
is filled in (syscall number, timestamp; but not arguments).
3) During the system call, calls to getname() and path_lookup() are
intercepted. These routines are called when the kernel is
actually looking up information that will be used to make the
decision about whether the syscall will succeed or fail. An
effort has been made to avoid copying the information that
getname generates, since getname is already making a
kernel-private copy of the information. [Note that storing
copies of all syscall arguments requires complexity and overhead
that arguably isn't needed. With this patch, for example, if
chroot("foo") fails because you are not root, "foo" will not
appear in the audit record because the kernel determined the
syscall cannot proceed before it ever needed to look up "foo".
This approach avoids storing user-supplied information that could
be misleading or unreliable (e.g., due to a cooperative
shared-memory attack) in favor of reporting information actually
used by the kernel.]
4) At syscall exit time, if the "auditable" flag has been set (e.g.,
because SELinux generated an avc record; or some other part of
the kernel detected an auditable event), the syscall-part of the
audit record is generated, including file names and inode numbers
(if available). Some of this information is currently
complementary to the information that selinux/avc.c generates
(e.g., file names and some inode numbers), but some is less
complete (e.g., getname doesn't return a fully-qualified path,
and this patch does not add the overhead of determining one).
[Note that the complete audit record comes to userspace in
pieces, which eliminates the need to store messages for
arbitrarily long periods inside the kernel.]
5) At task-exit time, the audit context is destroyed.

At steps 1, 2, and 4, simple filtering can be done (e.g., a database
role uid might have syscall auditing disabled for performance
reasons). The filtering is simple and could be made more complex.
However, I tried to implement as much filtering as possible without
adding significant overhead (e.g., d_path()). In general, the audit
framework should rely on some other kernel component (e.g., SELinux)
to make the majority of the decisions about what is and is not

A few (obvious) things are not yet implemented:
1) Filtering on syscall personality (i.e., i386 calls on x86_64).
2) Multi-platform syscall support (currently syscall auditing has
only been implemented on i386, x86_64, and ppc64).
3) Documentation.

I have also written very crude user-space tools that can be used to test
the features provided (auditd-0.3.tar.gz). This demo tarball, a
readme.txt (see for how to compile), and the patch (and more recent
patches?) are available from:

arch/i386/kernel/entry.S | 6
arch/i386/kernel/ptrace.c | 8
arch/ppc64/kernel/entry.S | 15
arch/ppc64/kernel/ptrace.c | 27 -
arch/x86_64/ia32/ia32entry.S | 12
arch/x86_64/kernel/entry.S | 21
arch/x86_64/kernel/ptrace.c | 23 -
fs/namei.c | 15
include/asm-i386/thread_info.h | 5
include/asm-ppc64/thread_info.h | 3
include/asm-x86_64/thread_info.h | 5
include/linux/audit.h | 210 +++++++++
include/linux/fs.h | 10
include/linux/netlink.h | 1
include/linux/sched.h | 5
init/Kconfig | 20
kernel/Makefile | 2
kernel/audit.c | 771 ++++++++++++++++++++++++++++++++++
kernel/auditsc.c | 872 +++++++++++++++++++++++++++++++++++++++
kernel/fork.c | 9
security/selinux/avc.c | 146 ++----
security/selinux/include/avc.h | 7
security/selinux/ss/services.c | 2
23 files changed, 2065 insertions(+), 130 deletions(-)

Due to size, the patch is available from;

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