Re: [PATCH v2 3/4] seccomp: Audit attempts to modify the actions_logged sysctl

Steve Grubb
2018-05-02T18:18:34Z

On Wednesday, May 2, 2018 11:53:19 AM EDT Tyler Hicks wrote:
> The decision to log a seccomp action will always be subject to the
> value of the kernel.seccomp.actions_logged sysctl, even for processes
> that are being inspected via the audit subsystem, in an upcoming patch.
> Therefore, we need to emit an audit record on attempts at writing to the
> actions_logged sysctl when auditing is enabled.
>
> This patch updates the write handler for the actions_logged sysctl to
> emit an audit record on attempts to write to the sysctl. Successful
> writes to the sysctl will result in a record that includes a normalized
> list of logged actions in the "actions" field and a "res" field equal to
> 0. Unsuccessful writes to the sysctl will result in a record that
> doesn't include the "actions" field and has a "res" field equal to 1.
>
> Not all unsuccessful writes to the sysctl are audited. For example, an
> audit record will not be emitted if an unprivileged process attempts to
> open the sysctl file for reading since that access control check is not
> part of the sysctl's write handler.
>
> Below are some example audit records when writing various strings to the
> actions_logged sysctl.
>
> Writing "not-a-real-action", when the kernel.seccomp.actions_logged
> sysctl previously was "kill_process kill_thread trap errno trace log",
> emits this audit record:
>
> type=CONFIG_CHANGE msg=audit(1525275273.537:130): op=seccomp-logging
> old-actions=kill_process,kill_thread,trap,errno,trace,log res=0
>
> If you then write "kill_process kill_thread errno trace log", this audit
> record is emitted:
>
> type=CONFIG_CHANGE msg=audit(1525275310.208:136): op=seccomp-logging
> actions=kill_process,kill_thread,errno,trace,log
> old-actions=kill_process,kill_thread,trap,errno,trace,log res=1
>
> If you then write the string "log log errno trace kill_process
> kill_thread", which is unordered and contains the log action twice,
> it results in the same actions value as the previous record:
>
> type=CONFIG_CHANGE msg=audit(1525275325.613:142): op=seccomp-logging
> actions=kill_process,kill_thread,errno,trace,log
> old-actions=kill_process,kill_thread,errno,trace,log res=1
>
> No audit records are generated when reading the actions_logged sysctl.

ACK for the format of the records.

-Steve

> Suggested-by: Steve Grubb <sgrubb@redhat.com>
> Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
> ---
> include/linux/audit.h | 5 +++++
> kernel/auditsc.c | 25 +++++++++++++++++++++++++
> kernel/seccomp.c | 51
> ++++++++++++++++++++++++++++++++++++++++++--------- 3 files changed, 72
> insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/audit.h b/include/linux/audit.h
> index 75d5b03..d4e35e7 100644
> --- a/include/linux/audit.h
> +++ b/include/linux/audit.h
> @@ -233,6 +233,8 @@ extern void __audit_inode_child(struct inode *parent,
> const struct dentry *dentry,
> const unsigned char type);
> extern void __audit_seccomp(unsigned long syscall, long signr, int code);
> +extern void audit_seccomp_actions_logged(const char *names,
> + const char *old_names, int res);
> extern void __audit_ptrace(struct task_struct *t);
>
> static inline bool audit_dummy_context(void)
> @@ -502,6 +504,9 @@ static inline void __audit_seccomp(unsigned long
> syscall, long signr, int code) { }
> static inline void audit_seccomp(unsigned long syscall, long signr, int
> code) { }
> +static inline void audit_seccomp_actions_logged(const char *names,
> + const char *old_names, int res)
> +{ }
> static inline int auditsc_get_stamp(struct audit_context *ctx,
> struct timespec64 *t, unsigned int *serial)
> {
> diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> index 4e0a4ac..5a0b770 100644
> --- a/kernel/auditsc.c
> +++ b/kernel/auditsc.c
> @@ -2478,6 +2478,31 @@ void __audit_seccomp(unsigned long syscall, long
> signr, int code) audit_log_end(ab);
> }
>
> +void audit_seccomp_actions_logged(const char *names, const char
> *old_names, + int res)
> +{
> + struct audit_buffer *ab;
> +
> + if (!audit_enabled)
> + return;
> +
> + ab = audit_log_start(current->audit_context, GFP_KERNEL,
> + AUDIT_CONFIG_CHANGE);
> + if (unlikely(!ab))
> + return;
> +
> + audit_log_format(ab, "op=seccomp-logging");
> +
> + if (names)
> + audit_log_format(ab, " actions=%s", names);
> +
> + if (old_names)
> + audit_log_format(ab, " old-actions=%s", old_names);
> +
> + audit_log_format(ab, " res=%d", res);
> + audit_log_end(ab);
> +}
> +
> struct list_head *audit_killed_trees(void)
> {
> struct audit_context *ctx = current->audit_context;
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index b36ac1e..da78835 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -1219,11 +1219,10 @@ static int read_actions_logged(struct ctl_table
> *ro_table, void __user *buffer, }
>
> static int write_actions_logged(struct ctl_table *ro_table, void __user
> *buffer, - size_t *lenp, loff_t *ppos)
> + size_t *lenp, loff_t *ppos, u32 *actions_logged)
> {
> char names[sizeof(seccomp_actions_avail)];
> struct ctl_table table;
> - u32 actions_logged;
> int ret;
>
> if (!capable(CAP_SYS_ADMIN))
> @@ -1238,24 +1237,58 @@ static int write_actions_logged(struct ctl_table
> *ro_table, void __user *buffer, if (ret)
> return ret;
>
> - if (!seccomp_actions_logged_from_names(&actions_logged, table.data))
> + if (!seccomp_actions_logged_from_names(actions_logged, table.data))
> return -EINVAL;
>
> - if (actions_logged & SECCOMP_LOG_ALLOW)
> + if (*actions_logged & SECCOMP_LOG_ALLOW)
> return -EINVAL;
>
> - seccomp_actions_logged = actions_logged;
> + seccomp_actions_logged = *actions_logged;
> return 0;
> }
>
> +static void audit_actions_logged(u32 actions_logged, u32
> old_actions_logged, + int ret)
> +{
> + char names[sizeof(seccomp_actions_avail)];
> + char old_names[sizeof(seccomp_actions_avail)];
> + const char *new = names;
> + const char *old = old_names;
> +
> + if (!audit_enabled)
> + return;
> +
> + memset(names, 0, sizeof(names));
> + memset(old_names, 0, sizeof(old_names));
> +
> + if (ret || !seccomp_names_from_actions_logged(names, sizeof(names),
> + actions_logged, ","))
> + new = NULL;
> +
> + if (!seccomp_names_from_actions_logged(old_names, sizeof(old_names),
> + old_actions_logged, ","))
> + old = NULL;
> +
> + return audit_seccomp_actions_logged(new, old, !ret);
> +}
> +
> static int seccomp_actions_logged_handler(struct ctl_table *ro_table, int
> write, void __user *buffer, size_t *lenp,
> loff_t *ppos)
> {
> - if (write)
> - return write_actions_logged(ro_table, buffer, lenp, ppos);
> - else
> - return read_actions_logged(ro_table, buffer, lenp, ppos);
> + int ret;
> +
> + if (write) {
> + u32 actions_logged = 0;
> + u32 old_actions_logged = seccomp_actions_logged;
> +
> + ret = write_actions_logged(ro_table, buffer, lenp, ppos,
> + &actions_logged);
> + audit_actions_logged(actions_logged, old_actions_logged, ret);
> + } else
> + ret = read_actions_logged(ro_table, buffer, lenp, ppos);
> +
> + return ret;
> }
>
> static struct ctl_path seccomp_sysctl_path[] = {