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posix-cpu-timers: Use pids not tasks in lookup

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The current posix-cpu-timer code uses pids when holding persistent
references in timers.  However the lookups from clock_id_t still return
tasks that need to be converted into pids for use.

This results in usage being pid->task->pid and that can race with
release_task and de_thread.  This can lead to some not wrong but
surprising results.  Surprising enough that Oleg and I both thought
there were some bugs in the code for a while.

This set of changes modifies the code to just lookup, verify, and return
pids from the clockid_t lookups to remove those potentialy troublesome
races.

Eric W. Biederman (3):
      posix-cpu-timers: Extend rcu_read_lock removing task_struct references
      posix-cpu-timers: Replace cpu_timer_pid_type with clock_pid_type
      posix-cpu-timers: Replace __get_task_for_clock with pid_for_clock

 kernel/time/posix-cpu-timers.c | 102 ++++++++++++++++++-----------------------
 1 file changed, 45 insertions(+), 57 deletions(-)

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2020-04-30 06:43:44 -05:00
commit 2dd8083f6d
1 changed files with 51 additions and 63 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

114
kernel/time/posix-cpu-timers.c
View File

@ -47,80 +47,65 @@ void update_rlimit_cpu(struct task_struct *task, unsigned long rlim_new)
/*
* Functions for validating access to tasks.
*/
static struct task_struct *lookup_task(const pid_t pid, bool thread,
bool gettime)
static struct pid *pid_for_clock(const clockid_t clock, bool gettime)
{
struct task_struct *p;
const bool thread = !!CPUCLOCK_PERTHREAD(clock);
const pid_t upid = CPUCLOCK_PID(clock);
struct pid *pid;
if (CPUCLOCK_WHICH(clock) >= CPUCLOCK_MAX)
return NULL;
/*
* If the encoded PID is 0, then the timer is targeted at current
* or the process to which current belongs.
*/
if (upid == 0)
return thread ? task_pid(current) : task_tgid(current);
pid = find_vpid(upid);
if (!pid)
return thread ? current : current->group_leader;
p = find_task_by_vpid(pid);
if (!p)
return p;
if (thread)
return same_thread_group(p, current) ? p : NULL;
/*
* For clock_gettime(PROCESS) the task does not need to be
* the actual group leader. task->signal gives
* access to the group's clock.
*/
if (gettime && (p == current))
return p;
/*
* For processes require that p is group leader.
*/
return thread_group_leader(p) ? p : NULL;
}
static struct task_struct *__get_task_for_clock(const clockid_t clock,
bool getref, bool gettime)
{
const bool thread = !!CPUCLOCK_PERTHREAD(clock);
const pid_t pid = CPUCLOCK_PID(clock);
struct task_struct *p;
if (CPUCLOCK_WHICH(clock) >= CPUCLOCK_MAX)
return NULL;
rcu_read_lock();
p = lookup_task(pid, thread, gettime);
if (p && getref)
get_task_struct(p);
rcu_read_unlock();
return p;
}
if (thread) {
struct task_struct *tsk = pid_task(pid, PIDTYPE_PID);
return (tsk && same_thread_group(tsk, current)) ? pid : NULL;
}
static inline struct task_struct *get_task_for_clock(const clockid_t clock)
{
return __get_task_for_clock(clock, true, false);
}
/*
* For clock_gettime(PROCESS) allow finding the process by
* with the pid of the current task. The code needs the tgid
* of the process so that pid_task(pid, PIDTYPE_TGID) can be
* used to find the process.
*/
if (gettime && (pid == task_pid(current)))
return task_tgid(current);
static inline struct task_struct *get_task_for_clock_get(const clockid_t clock)
{
return __get_task_for_clock(clock, true, true);
/*
* For processes require that pid identifies a process.
*/
return pid_has_task(pid, PIDTYPE_TGID) ? pid : NULL;
}
static inline int validate_clock_permissions(const clockid_t clock)
{
return __get_task_for_clock(clock, false, false) ? 0 : -EINVAL;
int ret;
rcu_read_lock();
ret = pid_for_clock(clock, false) ? 0 : -EINVAL;
rcu_read_unlock();
return ret;
}
static inline enum pid_type cpu_timer_pid_type(struct k_itimer *timer)
static inline enum pid_type clock_pid_type(const clockid_t clock)
{
return CPUCLOCK_PERTHREAD(timer->it_clock) ? PIDTYPE_PID : PIDTYPE_TGID;
return CPUCLOCK_PERTHREAD(clock) ? PIDTYPE_PID : PIDTYPE_TGID;
}
static inline struct task_struct *cpu_timer_task_rcu(struct k_itimer *timer)
{
return pid_task(timer->it.cpu.pid, cpu_timer_pid_type(timer));
return pid_task(timer->it.cpu.pid, clock_pid_type(timer->it_clock));
}
/*
@ -368,15 +353,18 @@ static int posix_cpu_clock_get(const clockid_t clock, struct timespec64 *tp)
struct task_struct *tsk;
u64 t;
tsk = get_task_for_clock_get(clock);
if (!tsk)
rcu_read_lock();
tsk = pid_task(pid_for_clock(clock, true), clock_pid_type(clock));
if (!tsk) {
rcu_read_unlock();
return -EINVAL;
}
if (CPUCLOCK_PERTHREAD(clock))
t = cpu_clock_sample(clkid, tsk);
else
t = cpu_clock_sample_group(clkid, tsk, false);
put_task_struct(tsk);
rcu_read_unlock();
*tp = ns_to_timespec64(t);
return 0;
@ -389,19 +377,19 @@ static int posix_cpu_clock_get(const clockid_t clock, struct timespec64 *tp)
*/
static int posix_cpu_timer_create(struct k_itimer *new_timer)
{
struct task_struct *p = get_task_for_clock(new_timer->it_clock);
struct pid *pid;
if (!p)
rcu_read_lock();
pid = pid_for_clock(new_timer->it_clock, false);
if (!pid) {
rcu_read_unlock();
return -EINVAL;
}
new_timer->kclock = &clock_posix_cpu;
timerqueue_init(&new_timer->it.cpu.node);
new_timer->it.cpu.pid = get_task_pid(p, cpu_timer_pid_type(new_timer));
/*
* get_task_for_clock() took a reference on @p. Drop it as the timer
* holds a reference on the pid of @p.
*/
put_task_struct(p);
new_timer->it.cpu.pid = get_pid(pid);
rcu_read_unlock();
return 0;
}