[PATCH v3 1/1] audit: Record fanotify access control decisions

Discussion:

Steve Grubb

2017-09-25 15:19:17 UTC

Hello,

The fanotify interface allows user space daemons to make access
control decisions. Under common criteria requirements, we need to
optionally record decisions based on policy. This patch adds a bit mask,
FAN_AUDIT, that a user space daemon can 'or' into the response decision
which will tell the kernel that it made a decision and record it.

It would be used something like this in user space code:

response.response = FAN_DENY | FAN_AUDIT;
write(fd, &response, sizeof(struct fanotify_response));

When the syscall ends, the audit system will record the decision as a
AUDIT_FANOTIFY auxiliary record to denote that the reason this event
occurred is the result of an access control decision from fanotify
rather than DAC or MAC policy.

A sample event looks like this:

type=PATH msg=audit(1504310584.332:290): item=0 name="./evil-ls"
inode=1319561 dev=fc:03 mode=0100755 ouid=1000 ogid=1000 rdev=00:00
obj=unconfined_u:object_r:user_home_t:s0 nametype=NORMAL
type=CWD msg=audit(1504310584.332:290): cwd="/home/sgrubb"
type=SYSCALL msg=audit(1504310584.332:290): arch=c000003e syscall=2
success=no exit=-1 a0=32cb3fca90 a1=0 a2=43 a3=8 items=1 ppid=901
pid=959 auid=1000 uid=1000 gid=1000 euid=1000 suid=1000
fsuid=1000 egid=1000 sgid=1000 fsgid=1000 tty=pts1 ses=3 comm="bash"
exe="/usr/bin/bash" subj=unconfined_u:unconfined_r:unconfined_t:
s0-s0:c0.c1023 key=(null)
type=FANOTIFY msg=audit(1504310584.332:290): resp=2

Prior to using the audit flag, the developer needs to call
fanotify_init or'ing in FAN_AUDIT_ENABLE to ensure that the kernel
supports auditing. The calling process must also have the CAP_AUDIT_WRITE
capability.

Signed-off-by: sgrubb <***@redhat.com>
---
fs/notify/fanotify/fanotify.c | 8 +++++++-
fs/notify/fanotify/fanotify_user.c | 12 +++++++++++-
include/linux/audit.h | 7 +++++++
include/linux/fsnotify_backend.h | 1 +
include/uapi/linux/audit.h | 1 +
include/uapi/linux/fanotify.h | 5 ++++-
kernel/auditsc.c | 6 ++++++
7 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 2fa99ae..1968d21 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -9,6 +9,7 @@
#include <linux/sched/user.h>
#include <linux/types.h>
#include <linux/wait.h>
+#include <linux/audit.h>

#include "fanotify.h"

@@ -78,7 +79,7 @@ static int fanotify_get_response(struct fsnotify_group *group,
fsnotify_finish_user_wait(iter_info);
out:
/* userspace responded, convert to something usable */
- switch (event->response) {
+ switch (event->response & ~FAN_AUDIT) {
case FAN_ALLOW:
ret = 0;
break;
@@ -86,6 +87,11 @@ static int fanotify_get_response(struct fsnotify_group *group,
default:
ret = -EPERM;
}
+
+ /* Check if the response should be audited */
+ if (event->response & FAN_AUDIT)
+ audit_fanotify(event->response & ~FAN_AUDIT);
+
event->response = 0;

pr_debug("%s: group=%p event=%p about to return ret=%d\n", __func__,
diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 907a481..231db8b 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -179,7 +179,7 @@ static int process_access_response(struct fsnotify_group *group,
* userspace can send a valid response or we will clean it up after the
* timeout
*/
- switch (response) {
+ switch (response & ~FAN_AUDIT) {
case FAN_ALLOW:
case FAN_DENY:
break;
@@ -190,6 +190,9 @@ static int process_access_response(struct fsnotify_group *group,
if (fd < 0)
return -EINVAL;

+ if ((response & FAN_AUDIT) && (group->audit_enabled == 0))
+ return -EINVAL;
+
event = dequeue_event(group, fd);
if (!event)
return -ENOENT;
@@ -805,6 +808,13 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
group->fanotify_data.max_marks = FANOTIFY_DEFAULT_MAX_MARKS;
}

+ if (flags & FAN_ENABLE_AUDIT) {
+ fd = -EPERM;
+ if (!capable(CAP_AUDIT_WRITE))
+ goto out_destroy_group;
+ group->audit_enabled = 1;
+ }
+
fd = anon_inode_getfd("[fanotify]", &fanotify_fops, group, f_flags);
if (fd < 0)
goto out_destroy_group;
diff --git a/include/linux/audit.h b/include/linux/audit.h
index 2150bdc..bf55732 100644
--- a/include/linux/audit.h
+++ b/include/linux/audit.h
@@ -360,6 +360,7 @@ extern int __audit_log_bprm_fcaps(struct linux_binprm *bprm,
extern void __audit_log_capset(const struct cred *new, const struct cred *old);
extern void __audit_mmap_fd(int fd, int flags);
extern void __audit_log_kern_module(char *name);
+extern void __audit_fanotify(unsigned int response);

static inline void audit_ipc_obj(struct kern_ipc_perm *ipcp)
{
@@ -456,6 +457,12 @@ static inline void audit_log_kern_module(char *name)
__audit_log_kern_module(name);
}

+static inline void audit_fanotify(unsigned int response)
+{
+ if (!audit_dummy_context())
+ __audit_fanotify(response);
+}
+
extern int audit_n_rules;
extern int audit_signals;
#else /* CONFIG_AUDITSYSCALL */
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index c6c6931..8f7ea68 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -193,6 +193,7 @@ struct fsnotify_group {
} fanotify_data;
#endif /* CONFIG_FANOTIFY */
};
+ unsigned int audit_enabled;
};

/* when calling fsnotify tell it if the data is a path or inode */
diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
index 0714a66..221f8b7 100644
--- a/include/uapi/linux/audit.h
+++ b/include/uapi/linux/audit.h
@@ -112,6 +112,7 @@
#define AUDIT_FEATURE_CHANGE 1328 /* audit log listing feature changes */
#define AUDIT_REPLACE 1329 /* Replace auditd if this packet unanswerd */
#define AUDIT_KERN_MODULE 1330 /* Kernel Module events */
+#define AUDIT_FANOTIFY 1331 /* Fanotify access decision */

#define AUDIT_AVC 1400 /* SE Linux avc denial or grant */
#define AUDIT_SELINUX_ERR 1401 /* Internal SE Linux Errors */
diff --git a/include/uapi/linux/fanotify.h b/include/uapi/linux/fanotify.h
index 030508d..46bb431 100644
--- a/include/uapi/linux/fanotify.h
+++ b/include/uapi/linux/fanotify.h
@@ -35,10 +35,11 @@

#define FAN_UNLIMITED_QUEUE 0x00000010
#define FAN_UNLIMITED_MARKS 0x00000020
+#define FAN_ENABLE_AUDIT 0x00000040

#define FAN_ALL_INIT_FLAGS (FAN_CLOEXEC | FAN_NONBLOCK | \
FAN_ALL_CLASS_BITS | FAN_UNLIMITED_QUEUE |\
- FAN_UNLIMITED_MARKS)
+ FAN_UNLIMITED_MARKS | FAN_ENABLE_AUDIT)

/* flags used for fanotify_modify_mark() */
#define FAN_MARK_ADD 0x00000001
@@ -99,6 +100,8 @@ struct fanotify_response {
/* Legit userspace responses to a _PERM event */
#define FAN_ALLOW 0x01
#define FAN_DENY 0x02
+#define FAN_AUDIT 0x10 /* Bit mask to create audit record for result */
+
/* No fd set in event */
#define FAN_NOFD -1

diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index 3260ba2..e046de8 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -2390,6 +2390,12 @@ void __audit_log_kern_module(char *name)
context->type = AUDIT_KERN_MODULE;
}

+void __audit_fanotify(unsigned int response)
+{
+ audit_log(current->audit_context, GFP_KERNEL,
+ AUDIT_FANOTIFY, "resp=%u", response);
+}
+
static void audit_log_task(struct audit_buffer *ab)
{
kuid_t auid, uid;

--
2.9.5

Amir Goldstein

2017-09-25 18:49:19 UTC

Permalink

Post by Steve Grubb
Hello,
The fanotify interface allows user space daemons to make access
control decisions. Under common criteria requirements, we need to
optionally record decisions based on policy. This patch adds a bit mask,
FAN_AUDIT, that a user space daemon can 'or' into the response decision
which will tell the kernel that it made a decision and record it.
response.response = FAN_DENY | FAN_AUDIT;
write(fd, &response, sizeof(struct fanotify_response));
When the syscall ends, the audit system will record the decision as a
AUDIT_FANOTIFY auxiliary record to denote that the reason this event
occurred is the result of an access control decision from fanotify
rather than DAC or MAC policy.
type=PATH msg=audit(1504310584.332:290): item=0 name="./evil-ls"
inode=1319561 dev=fc:03 mode=0100755 ouid=1000 ogid=1000 rdev=00:00
obj=unconfined_u:object_r:user_home_t:s0 nametype=NORMAL
type=CWD msg=audit(1504310584.332:290): cwd="/home/sgrubb"
type=SYSCALL msg=audit(1504310584.332:290): arch=c000003e syscall=2
success=no exit=-1 a0=32cb3fca90 a1=0 a2=43 a3=8 items=1 ppid=901
pid=959 auid=1000 uid=1000 gid=1000 euid=1000 suid=1000
fsuid=1000 egid=1000 sgid=1000 fsgid=1000 tty=pts1 ses=3 comm="bash"
s0-s0:c0.c1023 key=(null)
type=FANOTIFY msg=audit(1504310584.332:290): resp=2
Prior to using the audit flag, the developer needs to call
fanotify_init or'ing in FAN_AUDIT_ENABLE to ensure that the kernel
supports auditing. The calling process must also have the CAP_AUDIT_WRITE
capability.

Sorry, found more nits...

Post by Steve Grubb
@@ -805,6 +808,13 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
group->fanotify_data.max_marks = FANOTIFY_DEFAULT_MAX_MARKS;
}
+ if (flags & FAN_ENABLE_AUDIT) {
+ fd = -EPERM;
+ if (!capable(CAP_AUDIT_WRITE))
+ goto out_destroy_group;
+ group->audit_enabled = 1;
+ }
+

App should not be able to enable audit if audit is not configured in kernel.
So there should be code that returns EINVAL for flag FAN_ENABLE_AUDIT
if CONFIG_AUDITSYSCALL is not defined.
Same deal as flag FAN_ALL_PERM_EVENTS and kernel config
CONFIG_FANOTIFY_ACCESS_PERMISSIONS in fanotify_mark().
Perhaps it is better not to extend FAN_ALL_INIT_FLAGS and explicitly
test (flags & ~(FAN_ALL_INIT_FLAGS | FAN_ENABLE_AUDIT)) in fanotify_init()
under ifdef, as done in fanotify_mark()?

Post by Steve Grubb
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index c6c6931..8f7ea68 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -193,6 +193,7 @@ struct fsnotify_group {
} fanotify_data;
#endif /* CONFIG_FANOTIFY */
};
+ unsigned int audit_enabled;

This one should be bool and it belongs inside fanotify_data union member.
Also, you need to translate it back to FAN_ENABLE_AUDIT in
fanotify_show_fdinfo().

Amir.

Steve Grubb

2017-09-25 20:37:47 UTC

Permalink

Post by Amir Goldstein

Sorry, found more nits...

Post by Steve Grubb
@@ -805,6 +808,13 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags,
unsigned int, event_f_flags)>
group->fanotify_data.max_marks =
FANOTIFY_DEFAULT_MAX_MARKS;
}
+ if (flags & FAN_ENABLE_AUDIT) {
+ fd = -EPERM;
+ if (!capable(CAP_AUDIT_WRITE))
+ goto out_destroy_group;
+ group->audit_enabled = 1;
+ }
+

App should not be able to enable audit if audit is not configured in kernel.
So there should be code that returns EINVAL for flag FAN_ENABLE_AUDIT if
CONFIG_AUDITSYSCALL is not defined.
Same deal as flag FAN_ALL_PERM_EVENTS and kernel config
CONFIG_FANOTIFY_ACCESS_PERMISSIONS in fanotify_mark().
Perhaps it is better not to extend FAN_ALL_INIT_FLAGS and explicitly
test (flags & ~(FAN_ALL_INIT_FLAGS | FAN_ENABLE_AUDIT)) in fanotify_init()
under ifdef, as done in fanotify_mark()?

OK. How about this, I do the (flags & ~(FAN_ALL_INIT_FLAGS |
FAN_ENABLE_AUDIT)) with an ifdef. This will cause -EINVAL if FAN_ENABLE_AUDIT
is attempted and CONFIG_AUDITSYSCALL is not defined. So, the code initially
discussed above for audit_enabled would not need to have an #else. I can
surround the "if" block with a CONFIG_AUDITSYSCALL so that it compiles out if
audit is not being used. This will cause audit_enabled to always be false when
audit is not compiled in.

Post by Amir Goldstein

Post by Steve Grubb
diff --git a/include/linux/fsnotify_backend.h
b/include/linux/fsnotify_backend.h index c6c6931..8f7ea68 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -193,6 +193,7 @@ struct fsnotify_group {
} fanotify_data;
#endif /* CONFIG_FANOTIFY */
};
+ unsigned int audit_enabled;

This one should be bool and it belongs inside fanotify_data union member.
Also, you need to translate it back to FAN_ENABLE_AUDIT in
fanotify_show_fdinfo().

OK. Will fix this.

-Steve

Amir Goldstein

2017-09-26 04:10:24 UTC

Permalink

...

Post by Steve Grubb

Post by Amir Goldstein

Post by Steve Grubb
+ if (flags & FAN_ENABLE_AUDIT) {
+ fd = -EPERM;
+ if (!capable(CAP_AUDIT_WRITE))
+ goto out_destroy_group;
+ group->audit_enabled = 1;
+ }
+

App should not be able to enable audit if audit is not configured in kernel.
So there should be code that returns EINVAL for flag FAN_ENABLE_AUDIT if
CONFIG_AUDITSYSCALL is not defined.
Same deal as flag FAN_ALL_PERM_EVENTS and kernel config
CONFIG_FANOTIFY_ACCESS_PERMISSIONS in fanotify_mark().
Perhaps it is better not to extend FAN_ALL_INIT_FLAGS and explicitly
test (flags & ~(FAN_ALL_INIT_FLAGS | FAN_ENABLE_AUDIT)) in fanotify_init()
under ifdef, as done in fanotify_mark()?

Sure. only there is no need for wrapping the if block with ifdef as the flag
FAN_ENABLE_AUDIT is already not possible due to the first check, so by
rule of "use bare minimum ifdefs" there is no need for the second ifdef.

Amir.