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VERIEXEC(9) Kernel Developer's Manual VERIEXEC(9)

veriexec — in-kernel file integrity subsystem KPI

#include <sys/verified_exec.h>

void
veriexec_init(void);

bool
veriexec_lookup(struct vnode *vp);

int
veriexec_verify(struct lwp *l, struct vnode *vp, const u_char *name, int flag, bool *found);

void
veriexec_purge(struct vnode *vp);

int
veriexec_fpops_add(const char *fp_type, size_t hash_len, size_t ctx_size, veriexec_fpop_init_t init, veriexec_fpop_update_t update, veriexec_fpop_final_t final);

int
veriexec_file_add(struct lwp *l, prop_dictionary_t dict);

int
veriexec_file_delete(struct lwp *l, struct vnode *vp);

int
veriexec_table_delete(struct lwp *l, struct mount *mp);

int
veriexec_flush(struct lwp *l);

int
veriexec_openchk(struct lwp *l, struct vnode *vp, const char *path, int fmode);

int
veriexec_renamechk(struct lwp *l, struct vnode *fromvp, const char *fromname, struct vnode *tovp, const char *toname);

int
veriexec_removechk(struct lwp *l, struct vnode *vp, const char *name);

int
veriexec_unmountchk(struct mount *mp);

int
veriexec_convert(struct vnode *vp, prop_dictionary_t rdict);

int
veriexec_dump(struct lwp *l, prop_array_t rarray);

veriexec is the KPI for Veriexec, the NetBSD in-kernel file integrity subsystem. It is responsible for managing the supported hashing algorithms, fingerprint calculation and comparison, file monitoring tables, and relevant hooks to enforce the Veriexec policy.

(void)
Initialize the Veriexec subsystem. Called only once during system startup.
(vp)
Check if vp is monitored by Veriexec. Returns true if it is, or false otherwise.
(l, vp, name, flag, found)
Verifies the digital fingerprint of vp. name is the filename, and flag is the access flag. The access flag can be one of:
The file was executed directly via execve(2).
The file was executed indirectly, either as an interpreter for a script or mapped to an executable memory region.
The file was opened for reading/writing.

l is the LWP for the request context.

An optional argument, found, is a pointer to a boolean indicating whether an entry for the file was found in the Veriexec tables.

(vp)
Purge the file entry for vp. This invalidates the fingerprint so it will be evaluated next time the file is accessed.
(fp_type, hash_len, ctx_size, init, update, final)
Add support for fingerprinting algorithm fp_type with binary hash length hash_len and calculation context size ctx_size to Veriexec. init, update, and final are the routines used to initialize, update, and finalize a calculation context.

(l, dict)
Add a Veriexec entry for the file described by dict.

dict is expected to have the following:

Type Purpose
file string filename
entry-type uint8 entry type flags (see veriexec(4))
fp-type string fingerprint hashing algorithm
fp data the fingerprint
(l, vp)
Remove Veriexec entry for vp.
(l, mp)
Remove Veriexec table for mount-point mp.
(l)
Delete all Veriexec tables.

(l, vp, path, fmode)
Called when a file is opened.

l is the LWP opening the file, vp is a vnode for the file being opened as returned from namei(9). If NULL, the file is being created. path is the pathname for the file (not necessarily a full path), and fmode are the mode bits with which the file was opened.

(l, fromvp, fromname, tovp, toname)
Called when a file is renamed.

fromvp and fromname are the vnode and filename of the file being renamed. tovp and toname are the vnode and filename of the target file. l is the LWP renaming the file.

Depending on the strict level, veriexec will either track changes appropriately or prevent the rename.

(l, vp, name)
Called when a file is removed.

vp is the vnode of the file being removed, and name is the filename. l is the LWP removing the file,

Depending on the strict level, veriexec will either clean-up after the file or prevent its removal.

(mp)
Checks if the current strict level allows mp to be unmounted.

veriexec_convert(vp, rdict)
Convert Veriexec entry for vp to human-readable proplib(3) dictionary, rdict, with the following elements:
Type Purpose
entry-type uint8 entry type flags (see veriexec(4))
status uint8 entry status (see below)
fp-type string fingerprint hashing algorithm
fp data the fingerprint

The “status” can be one of the following:

Meaning
FINGERPRINT_NOTEVAL not evaluated
FINGERPRINT_VALID fingerprint match
FINGERPRINT_MISMATCH fingerprint mismatch

If no entry was found, ENOENT is returned. Otherwise, zero.

(l, rarray)
Fill rarray with entries for all files monitored by Veriexec that have a filename associated with them.

Each element in rarray is a dictionary with the same elements as filled by (), with an additional field, “file”, containing the filename.

Purpose
src/sys/dev/veriexec.c driver for userland communication
src/sys/sys/verified_exec.h shared (userland/kernel) header file
src/sys/kern/kern_veriexec.c subsystem code
src/sys/kern/vfs_syscalls.c rename, remove, and unmount policies
src/sys/kern/vfs_vnops.c regular file access policy

proplib(3), sysctl(3), veriexec(4), security(7), sysctl(8), veriexecctl(8), veriexecgen(8), fileassoc(9)

Brett Lymn <blymn@NetBSD.org>
Elad Efrat <elad@NetBSD.org>

There are two known issues with Veriexec that should be considered when using it.

There is an issue providing protection for files residing on mounts from remote hosts. Because access to the file-system does not necessarily go through veriexec, there is no way to track on-disk changes. While it is possible to minimize the effect by evaluating the file's fingerprint on each access without caching the result, a problem arises when a file is overwritten after its fingerprint has been evaluated and it is running on the local host.

An attacker could potentially overwrite the file contents in the remote host at that point, and force a flush on the local host, resulting in paging in of the files from the disk, introducing malicious code into a supposedly safe address space.

There is a fix for this issue, however due to dependencies on other work that is still in progress it has not been committed yet.

Due to VFS limitations, veriexec cannot track the same on-disk file across multiple layers of overlay file-systems. Therefore, you cannot expect changes to files on overlay mounts will be detected simply because the underlying mount is monitored by veriexec.

A workaround for this issue is listing all files, under all mounts, you want monitored in the signature file.

December 9, 2015 NetBSD-9.2