NetBSD/share/man/man9/veriexec.9

.\" $NetBSD: veriexec.9,v 1.4 2006/08/11 19:17:47 christos Exp $
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.\" Copyright 2006 Elad Efrat <elad@NetBSD.org>
.\" Copyright 2006 Brett Lymn <blymn@NetBSD.org>
.\"
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.\" by Brett Lymn and Elad Efrat
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.Dd August 10, 2006
.Dt VERIEXEC 9
.Os
.Sh NAME
.Nm veriexec
.Nd in-kernel file integrity subsystem KPI
.Sh SYNOPSIS
.In sys/verified_exec.h
.Sh DESCRIPTION
.Nm
is the KPI for Veriexec,
the
.Nx
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.
.Pp
This manual divides the
.Nm
KPI to four main areas: core, fingerprint related, table management, and hook
handlers.
.Ss Data-structures
.Nm
uses several data-structures to manage its data:
.Bl -tag -width "123456"
.It Ft "struct veriexec_fp_ops"
Describes a hashing algorithm, for example MD5, SHA1, RMD160.
Contains the following members:
.Bl -tag -width "123456"
.It Ft "char type[VERIEXEC_TYPE_MAXLEN]"
Name of the hashing algorithm.
.It Ft size_t hash_len
Length, in bytes,
.It Ft size_t context_size
Size, in bytes, of the calculation context.
.It Ft VERIEXEC_INIT_FN Ar init
.It Ft VERIEXEC_UPDATE_FN Ar update
.It Ft VERIEXEC_FINAL_FN Ar final
Routines used to calculate the fingerprint.
.El
.Pp
.Nm
provides a macro,
.Fn VERIEXEC_OPINIT ,
to ease initialization of a
.Ft "struct veriexec_fp_ops"
(see below).
.It Ft "struct veriexec_file_entry"
Describes Veriexec data associated with a single file.
Contains the following members:
.Bl -tag -width "123456"
.It Ft u_char Ar type
The entry type, indicating what valid access types are allowed for this file.
Can be a binary-OR'd combination of the following:
.Bl -tag -width "123456"
.It Dv VERIEXEC_DIRECT
The file can be executed directly.
.It Dv VERIEXEC_INDIRECT
The file can be executed indirectly, for example, as a script interpreter.
.It Dv VERIEXEC_FILE
The file can be opened, for example for reading and/or writing.
.El
.It Ft u_char Ar status
The evaluation status.
Can be one of the following:
.Bl -tag -width "123456"
.It Dv FINGERPRINT_NOTEVAL
Fingerprint hasn't been evaluated yet.
.It Dv FINGERPRINT_VALID
The fingerprint was found to be valid.
.It Dv FINGERPRINT_NOMATCH
There was a fingerprint mismatch.
.El
.It Ft u_char Ar page_fp_status
Unused.
.It Ft "u_char *" Ar fp
The fingerprint for the file.
.It Ft "void *" Ar page_fp
.It Ft size_t Ar npages
.It Ft size_t Ar last_page_size
Unused.
.It Ft "struct veriexec_fp_ops *" Ar ops
Algorithm used to calculate the fingerprint for this file.
.El
.It Ft "struct veriexec_table_entry"
Describes Veriexec data associated with a mount.
Contains the following members:
.Bl -tag -width "123456"
.It Ft uint64_t Ar vte_count
Number of Veriexec entries (monitored files) on the mount.
.It Ft "const struct sysctlnode *" Ar vte_node
The
.Xr sysctl 9
node for the mount, with information about the number of monitored files,
file-system type, and mount point.
.El
.El
.Ss Core Routines
.Bl -tag -width "123456"
.It Ft "struct veriexec_file_entry *vfe" Fn veriexec_lookup "struct vnode *vp"
Lookup private Veriexec data associated with
.Ar vp .
Returns
.Dv NULL
if file is not monitored by Veriexec.
.It Ft int Fn veriexec_verify "struct lwp *l" "struct vnode *vp" \
"u_char *name" "int flag" "struct veriexec_file_entry **vfep"
Verifies the digital fingerprint of
.Ar vp .
.Ar name
is the filename, and
.Ar flag
is the access flag.
The access flag can be one of:
.Bl -tag -width "123456"
.It Dv VERIEXEC_DIRECT
The file was executed directly via
.Xr execve 2 .
.It Dv VERIEXEC_INDIRECT
The file was executed indirectly, most likely as an interpreter for a script.
.It Dv VERIEXEC_FILE
The file was opened, most chances are by
.Xr open 2 .
.El
.Pp
.Ar l
is the LWP for the request context.
.Pp
An optional argument,
.Ar vfep ,
is a pointer to return the Veriexec file entry, if one was found.
.It Ft void Fn veriexec_report "const u_char *msg" "u_char *name" \
"struct lwp *l" "int flags"
Log a Veriexec message in
.Ar msg
for file
.Ar name .
.Ar l
is optionally the LWP context in question.
.Pp
.Ar flags
control how the message will be formatted, where will it be logged to, and
whether the system should panic after posting it.
Flags can be binary-OR'd together.
Available flags include:
.Bl -tag -width "123456"
.It Dv REPORT_ALWAYS
The message should always be printed.
This is a synonym for 0.
.It Dv REPORT_VERBOSE
The message should be printed only if Veriexec is in verbose mode.
.It Dv REPORT_DEBUG
The message should be printed only if Veriexec is in debug verbosity.
.It Dv REPORT_ALARM
The message is an alarm.
If
.Ar l
is not
.Dv NULL ,
user and process ids will be printed.
The log will be sent to the
.Em LOG_ALERT
syslog facility.
.It Dv REPORT_PANIC
The system should
.Xr panic 9
after posting the message.
.El
.El
.Ss Fingerprint Related Routines
These routines manage the supported fingerprinting algorithms, as well as
fingerprint calculation and comparison.
.Bl -tag -width "123456"
.It Ft void Fn veriexec_init_fp_ops "void"
Initialize the supported fingerprinting algorithms database.
Should be called only once during system startup.
.It Ft Fn veriexec_add_fp_ops "struct veriexec_fp_ops *ops"
Add fingerprinting ops
.Ar ops .
.Pp
.Nm
provides a macro to initialize
.Ar ops ,
called
.Fn VERIEXEC_OPINIT .
It takes seven parameters: a pointer to a
.Ft "struct veriexec_fp_ops" ,
a
.Ft "const char *"
describing the algorithm name (will be copied), the byte length of a binary
representation of a fingerprint as a
.Ft size_t ,
the size of the fingerprint calculation context (usually the
.Em _CTX
types),
and pointers to the initialization, update, and final routines, used to
calculate the fingerprint.
.It Ft "struct veriexec_fp_ops *" Fn veriexec_find_ops "u_char *name"
Lookup fingerprinting ops for algorithm
.Ar name .
.It Ft int Fn veriexec_fp_calc "struct lwp *l" "struct vnode *vp" \
"u_char *name" "struct veriexec_file_entry *vfe"
Calculate fingerprint and store evaluation for
.Ar vp .
.Ar name
is the filename,
.Ar vfe
is the Veriexec-private data for the file.
.It Ft int Fn veriexec_fp_cmp "struct veriexec_fp_ops *ops" "u_char *fp1" \
"u_char *fp2"
Compare two fingerprints in
.Ar fp1
and
.Ar fp2
using the common fingerprint ops in
.Ar ops .
.Pp
Two fingerprints must have been generated by the same algorithm.
.It Ft void Fn veriexec_purge "struct veriexec_file_entry *vfe"
Purge the file entry
.Ar vfe .
This invalidates the fingerprint so it will be evaluated next time the file
is accessed.
.El
.Ss Table Management Routines
.Bl -tag -width "123456"
.It Ft int Fn veriexec_hashadd "struct vnode *vp" \
"struct veriexec_file_entry *vfe"
Add a Veriexec entry for
.Ar vp
with data in
.Ar vfe .
.It Ft "struct veriexec_table_entry *vte" Fn veriexec_tblfind "struct vnode *vp"
Lookup table data for the mount
.Ar vp
is on.
.\" .It Ft Fn veriexec_newtable
.\" .It Ft Fn veriexec_load
.\" .It Ft Fn veriexec_delete
.\" .It Ft Fn veriexec_query
.El
.Ss Hook Handlers
Below are the routines called from code where Veriexec policy enforcement is
required.
.Bl -tag -width "123456"
.It Ft int Fn veriexec_renamechk "struct vnode *fromvp" \
"const char *fromname" "struct vnode *tovp" "const char *toname" \
"struct lwp *l"
Called when a file is renamed.
.Pp
.Ar fromvp
and
.Ar fromname
are the vnode and filename of the file being renamed.
.Ar tovp
and
.Ar toname
are the vnode and filename of the target file.
.Ar l
is the LWP renaming the file.
.Pp
Depending on the strict level,
.Nm
will either track changes appropriately or prevent the rename.
.It Ft int Fn veriexec_removechk "struct vnode *vp" "const char *name" \
"struct lwp *l"
Called when a file is removed.
.Pp
.Ar vp
is the vnode of the file being removed, and
.Ar name
is the filename.
.Ar l
is the LWP removing the file,
.Pp
Depending on the strict level,
.Nm
will either clean-up after the file or prevent its removal.
.It Ft int Fn veriexec_clear "void *data" "int what"
Passed as the
.Xr fileassoc 9
clean-up callback routine,
.Fn veriexec_clear
is responsible for garbage collection of unnecessary
.Nm
objects.
It is not meant to be called directly.
.It Ft int Fn veriexec_rawchk "struct vnode *vp"
Enforce raw disk access policy.
.Ar vp
is the vnode of the mount-point for the disk, if any.
.El
.Sh FILES
.Pa src/sys/dev/verified_exec.c
.Pa src/sys/kern/kern_verifiedexec.c
.Pa src/sys/sys/verified_exec.h
.Sh SEE ALSO
.Xr sysctl 3 ,
.Xr veriexec 4 ,
.Xr sysctl 8 ,
.Xr veriexecctl 8 ,
.Xr fileassoc 9
.Sh AUTHORS
.An Brett Lymn Aq blymn@NetBSD.org
.An Elad Efrat Aq elad@NetBSD.org
.Sh CAVEATS
There are two known issues with Veriexec that should be considered when
using it.
.Ss Remote File-systems
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
.Nm ,
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.
.Pp
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.
.Pp
There is a fix for this issue, however due to dependencies on other work
that is still in progress it has not been commited yet.
.Ss Layered File-systems
Due to VFS limitations,
.Nm
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
.Nm .
.Pp
A workaround for this issue is listing all files, under all mounts, you want
monitored in the signature file.