rlimits: allow setrlimit to non-current tasks

Add locking to allow setrlimit accept task parameter other than
current.

Namely, lock tasklist_lock for read and check whether the task
structure has sighand non-null. Do all the signal processing under
that lock still held.

There are some points:
1) security_task_setrlimit is now called with that lock held. This is
   not new, many security_* functions are called with this lock held
   already so it doesn't harm (all this security_* stuff does almost
   the same).
2) task->sighand->siglock (in update_rlimit_cpu) is nested in
   tasklist_lock. This dependence is already existing.
3) tsk->alloc_lock is nested in tasklist_lock. This is OK too, already
   existing dependence.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/kernel/sys.c b/kernel/sys.c
index b5b96e30e0d6..9dbcbbcce153 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1272,6 +1272,7 @@ SYSCALL_DEFINE2(old_getrlimit, unsigned int, resource,
 
 #endif
 
+/* make sure you are allowed to change @tsk limits before calling this */
 int do_setrlimit(struct task_struct *tsk, unsigned int resource,
                 struct rlimit *new_rlim)
 {
@@ -1285,9 +1286,16 @@ int do_setrlimit(struct task_struct *tsk, unsigned int resource,
         if (resource == RLIMIT_NOFILE && new_rlim->rlim_max > sysctl_nr_open)
                 return -EPERM;
 
+        /* protect tsk->signal and tsk->sighand from disappearing */
+        read_lock(&tasklist_lock);
+        if (!tsk->sighand) {
+                retval = -ESRCH;
+                goto out;
+        }
+
         retval = security_task_setrlimit(tsk->group_leader, resource, new_rlim);
         if (retval)
-                return retval;
+                goto out;
 
         if (resource == RLIMIT_CPU && new_rlim->rlim_cur == 0) {
                 /*
@@ -1322,6 +1330,7 @@ int do_setrlimit(struct task_struct *tsk, unsigned int resource,
 
         update_rlimit_cpu(tsk, new_rlim->rlim_cur);
 out:
+        read_unlock(&tasklist_lock);
         return retval;
 }