Messages in this thread | | | From | (Eric W. Biederman) | Date | Wed, 04 May 2016 11:57:56 -0500 | Subject | Re: [PATCH] rlimit: locking tidy ups |
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Cc'd Oleg as he tends to be deeply involved with this class of locking.
Mateusz Guzik <mguzik@redhat.com> writes:
> rlimits are stored in task->signal and are guaranteed to remain valid as > long as the task struct is valid. All modifications are protected by > locking task->group_leader. Additionally changes to RLIMIT_CPU need > task->sighand. > > do_prlimit takes tasklist_lock, which as a side effect gurantees stable > ->sighand however, there is no need to take the lock for any limit other > than RLIMIT_CPU and even then we can get away with locking sighand itself. > > proc_pid_limits takes ->sighand lock prior to accessing rlimits, but it > serves no purpose as it does not prevent modifications. > > Both functions effectively always perform ->sighand != NULL check, but it > is only of concern when RLIMIT_CPU is being set. ->sighand is only cleared > when the process is reaped, so a dedicated check only makes it less likely > to access limits of a dead process. > > As such, eliminate the unneeded check and: > - do_prlimit: stop taking tasklist_lock at all and only lock sighand when > necessary > - proc_pid_limits: lock group leader in order to obtain a stable copy > > Signed-off-by: Mateusz Guzik <mguzik@redhat.com> > --- > fs/proc/base.c | 6 ++---- > kernel/sys.c | 22 ++++++++++++++-------- > kernel/time/posix-cpu-timers.c | 3 +-- > security/selinux/hooks.c | 4 +++- > 4 files changed, 20 insertions(+), 15 deletions(-) > > diff --git a/fs/proc/base.c b/fs/proc/base.c > index 704ae63..3d4963e 100644 > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -618,14 +618,12 @@ static int proc_pid_limits(struct seq_file *m, struct pid_namespace *ns, > struct pid *pid, struct task_struct *task) > { > unsigned int i; > - unsigned long flags; > > struct rlimit rlim[RLIM_NLIMITS]; > > - if (!lock_task_sighand(task, &flags)) > - return 0; > + task_lock(task->group_leader); > memcpy(rlim, task->signal->rlim, sizeof(struct rlimit) * RLIM_NLIMITS); > - unlock_task_sighand(task, &flags); > + task_unlock(task->group_leader); > > /* > * print the file header > diff --git a/kernel/sys.c b/kernel/sys.c > index 89d5be4..1c8a67d 100644 > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -1361,7 +1361,9 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource, > struct rlimit *new_rlim, struct rlimit *old_rlim) > { > struct rlimit *rlim; > + unsigned long flags; > int retval = 0; > + int sighand_locked = 0; > > if (resource >= RLIM_NLIMITS) > return -EINVAL; > @@ -1373,15 +1375,17 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource, > return -EPERM; > } > > - /* protect tsk->signal and tsk->sighand from disappearing */ > - read_lock(&tasklist_lock); > - if (!tsk->sighand) { > - retval = -ESRCH; > - goto out; > + task_lock(tsk->group_leader); > + if (new_rlim && resource == RLIMIT_CPU && > + new_rlim->rlim_cur != RLIM_INFINITY) { > + if (!lock_task_sighand(tsk, &flags)) { > + retval = -ESRCH; > + goto out; > + } > + sighand_locked = 1; > } > > rlim = tsk->signal->rlim + resource; > - task_lock(tsk->group_leader); > if (new_rlim) { > /* Keep the capable check against init_user_ns until > cgroups can contain all limits */ > @@ -1407,7 +1411,6 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource, > if (new_rlim) > *rlim = *new_rlim; > } > - task_unlock(tsk->group_leader); > > /* > * RLIMIT_CPU handling. Note that the kernel fails to return an error > @@ -1418,8 +1421,11 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource, > if (!retval && new_rlim && resource == RLIMIT_CPU && > new_rlim->rlim_cur != RLIM_INFINITY) > update_rlimit_cpu(tsk, new_rlim->rlim_cur); > + > + if (sighand_locked) > + unlock_task_sighand(tsk, &flags); > out: > - read_unlock(&tasklist_lock); > + task_unlock(tsk->group_leader); > return retval; > } > > diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c > index 1cafba8..fc38417 100644 > --- a/kernel/time/posix-cpu-timers.c > +++ b/kernel/time/posix-cpu-timers.c > @@ -23,9 +23,8 @@ void update_rlimit_cpu(struct task_struct *task, unsigned long rlim_new) > { > cputime_t cputime = secs_to_cputime(rlim_new); > > - spin_lock_irq(&task->sighand->siglock); > + lockdep_assert_held(&task->sighand->siglock); > set_process_cpu_timer(task, CPUCLOCK_PROF, &cputime, NULL); > - spin_unlock_irq(&task->sighand->siglock); > } > > static int check_clock(const clockid_t which_clock) > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index a86d537..d74b91a 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -2506,8 +2506,10 @@ static void selinux_bprm_committing_creds(struct linux_binprm *bprm) > initrlim = init_task.signal->rlim + i; > rlim->rlim_cur = min(rlim->rlim_max, initrlim->rlim_cur); > } > - task_unlock(current); > + spin_lock_irq(¤t->sighand->siglock); > update_rlimit_cpu(current, rlimit(RLIMIT_CPU)); > + spin_unlock_irq(¤t->sighand->siglock); > + task_unlock(current); > } > }
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