rlimits: make sure ->rlim_max never grows in sys_setrlimit

Mostly preparation for Jiri's changes, but probably makes sense anyway.

sys_setrlimit() checks new_rlim.rlim_max <= old_rlim->rlim_max, but when
it takes task_lock() old_rlim->rlim_max can be already lowered. Move this
check under task_lock().

Currently this is not important, we can only race with our sub-thread,
this means the application is stupid. But when we change the code to allow
the update of !current task's limits, it becomes important to make sure
->rlim_max can be lowered "reliably" even if we race with the application
doing sys_setrlimit().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

1 files changed, 8 insertions, 7 deletions

diff --git a/kernel/sys.c b/kernel/sys.c
index f5183b08adf..f2b2d7aa381 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1283,10 +1283,6 @@ SYSCALL_DEFINE2(setrlimit, unsigned int, resource, struct rlimit __user *, rlim)
                 return -EFAULT;
         if (new_rlim.rlim_cur > new_rlim.rlim_max)
                 return -EINVAL;
-        old_rlim = current->signal->rlim + resource;
-        if ((new_rlim.rlim_max > old_rlim->rlim_max) &&
-            !capable(CAP_SYS_RESOURCE))
-                return -EPERM;
         if (resource == RLIMIT_NOFILE && new_rlim.rlim_max > sysctl_nr_open)
                 return -EPERM;
 
@@ -1304,11 +1300,16 @@ SYSCALL_DEFINE2(setrlimit, unsigned int, resource, struct rlimit __user *, rlim)
                 new_rlim.rlim_cur = 1;
         }
 
+        old_rlim = current->signal->rlim + resource;
         task_lock(current->group_leader);
-        *old_rlim = new_rlim;
+        if (new_rlim.rlim_max > old_rlim->rlim_max &&
+                        !capable(CAP_SYS_RESOURCE))
+                retval = -EPERM;
+        else
+                *old_rlim = new_rlim;
         task_unlock(current->group_leader);
 
-        if (resource != RLIMIT_CPU)
+        if (retval || resource != RLIMIT_CPU)
                 goto out;
 
         /*
@@ -1322,7 +1323,7 @@ SYSCALL_DEFINE2(setrlimit, unsigned int, resource, struct rlimit __user *, rlim)
 
         update_rlimit_cpu(current, new_rlim.rlim_cur);
 out:
-        return 0;
+        return retval;
 }
 
 /*