[PATCH] rlimit: locking tidy ups

[Mateusz Guzik]

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@xxxxxxxxxx>
---
 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(&current->sighand->siglock);
 		update_rlimit_cpu(current, rlimit(RLIMIT_CPU));
+		spin_unlock_irq(&current->sighand->siglock);
+		task_unlock(current);
 	}
 }
 
-- 
1.8.3.1