[PATCH v2] audit: Only use the syscall slowpath when syscall audit rules exist

This toggles TIF_SYSCALL_AUDIT as needed when rules change instead of leaving it set whenever rules might be set in the future. This reduces syscall latency from >60ns to closer to 40ns on my laptop. Cc: Oleg Nesterov <oleg(a)redhat.com&gt; Cc: Steve Grubb <sgrubb(a)redhat.com&gt; Cc: Eric Paris <eparis(a)redhat.com&gt; Signed-off-by: Andy Lutomirski <luto(a)amacapital.net&gt; --- Changes from v1: - For new tasks, set flags in a new audit_sync_flags callback instead of in audit_alloc (thanks, Oleg). - Rework locking. - Use irqsave/irqrestore to avoid having to think about who else might have taken spinlocks. include/linux/audit.h | 9 +++++-- kernel/auditfilter.c | 4 ++-- kernel/auditsc.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++--- 3 files changed, 71 insertions(+), 7 deletions(-) diff --git a/include/linux/audit.h b/include/linux/audit.h index a406419..534ba85 100644 --- a/include/linux/audit.h +++ b/include/linux/audit.h @@ -298,7 +298,9 @@ static inline void audit_mmap_fd(int fd, int flags) __audit_mmap_fd(fd, flags); } -extern int audit_n_rules; +extern void audit_inc_n_rules(void); +extern void audit_dec_n_rules(void); + extern int audit_signals; #else /* CONFIG_AUDITSYSCALL */ static inline int audit_alloc(struct task_struct *task) @@ -404,7 +406,10 @@ static inline void audit_mmap_fd(int fd, int flags) { } static inline void audit_ptrace(struct task_struct *t) { } -#define audit_n_rules 0 +static inline void audit_inc_n_rules(void) +{ } +static inline void audit_dec_n_rules(void) +{ } #define audit_signals 0 #endif /* CONFIG_AUDITSYSCALL */ diff --git a/kernel/auditfilter.c b/kernel/auditfilter.c index 51f3fd4..0ce531d 100644 --- a/kernel/auditfilter.c +++ b/kernel/auditfilter.c @@ -903,7 +903,7 @@ static inline int audit_add_rule(struct audit_entry *entry) } #ifdef CONFIG_AUDITSYSCALL if (!dont_count) - audit_n_rules++; + audit_inc_n_rules(); if (!audit_match_signal(entry)) audit_signals++; @@ -955,7 +955,7 @@ static inline int audit_del_rule(struct audit_entry *entry) #ifdef CONFIG_AUDITSYSCALL if (!dont_count) - audit_n_rules--; + audit_dec_n_rules(); if (!audit_match_signal(entry)) audit_signals--; diff --git a/kernel/auditsc.c b/kernel/auditsc.c index 90594c9..363d0bb 100644 --- a/kernel/auditsc.c +++ b/kernel/auditsc.c @@ -79,8 +79,15 @@ /* no execve audit message should be longer than this (userspace limits) */ #define MAX_EXECVE_AUDIT_LEN 7500 -/* number of audit rules */ -int audit_n_rules; +/* + * number of audit rules + * + * n_rules_lock protects audit_n_rules. It also prevents audit_alloc from + * racing against changes to audit_n_rules: to change someone else's + * audit_context or TIF_SYSCALL_AUDIT, you need write_lock(&n_rules_lock). + */ +static DEFINE_RWLOCK(n_rules_lock); +static int audit_n_rules; /* determines whether we collect data for signals sent */ int audit_signals; @@ -911,6 +918,47 @@ static inline struct audit_context *audit_alloc_context(enum audit_state state) return context; } +void audit_inc_n_rules() +{ + struct task_struct *p, *g; + + write_lock(&n_rules_lock); + + if (audit_n_rules++ != 0) + goto out; /* The overall state isn't changing. */ + + read_lock(&tasklist_lock); + do_each_thread(g, p) { + if (p->audit_context) + set_tsk_thread_flag(p, TIF_SYSCALL_AUDIT); + } while_each_thread(g, p); + read_unlock(&tasklist_lock); + +out: + write_unlock(&n_rules_lock); +} + +void audit_dec_n_rules() +{ + struct task_struct *p, *g; + + write_lock(&n_rules_lock); + --audit_n_rules; + BUG_ON(audit_n_rules < 0); + + if (audit_n_rules != 0) + goto out; /* The overall state isn't changing. */ + + read_lock(&tasklist_lock); + do_each_thread(g, p) { + clear_tsk_thread_flag(p, TIF_SYSCALL_AUDIT); + } while_each_thread(g, p); + read_unlock(&tasklist_lock); + +out: + write_unlock(&n_rules_lock); +} + /** * audit_alloc - allocate an audit context block for a task * @tsk: task @@ -931,6 +979,11 @@ int audit_alloc(struct task_struct *tsk) state = audit_filter_task(tsk, &key); if (state == AUDIT_DISABLED) { + /* + * There is no relevant race against inc/dec_n_rules + * here -- inc won't set TIF_SYSCALL_AUDIT, and, if dec + * clears it redundantly, there's no harm done. + */ clear_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT); return 0; } @@ -942,8 +995,14 @@ int audit_alloc(struct task_struct *tsk) } context->filterkey = key; + read_lock(&n_rules_lock); tsk->audit_context = context; - set_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT); + if (audit_n_rules) + set_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT); + else + clear_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT); + read_unlock(&n_rules_lock); + return 0; } -- 1.8.5.3