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This is some background information about the Linux Auditing Framework. LICENSE ======= The audit daemon is released as GPL'd code. The audit daemon's libraries libaudit.* and libauparse.* are released under LGPL so that it may be linked with 3rd party software. BUILDING ======== See the Install(.tmp) file. USAGE ===== See the man pages for audit, auditctl, audit.rules, ausearch, and aureport. DISCUSSION ========== Original lkml thread(s): https://marc.info/?t=107815888100001&r=1&w=2 https://marc.info/?t=107901570800002&r=1&w=2 There is a linux audit mail list where any question whether kernel design, setup and configuration, or usage can be discussed: http://www.redhat.com/mailman/listinfo/linux-audit DESIGN INFO (Very old) ===================== 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. Logging: 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 eliminated). 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 auditable.
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