mm, oom: remove 3% bonus for CAP_SYS_ADMIN processes

Since the 2.6 kernel, the oom killer has slightly biased away from
CAP_SYS_ADMIN processes by discounting some of its memory usage in
comparison to other processes.

This has always been implicit and nothing exactly relies on the
behavior.

Gaurav notices that __task_cred() can dereference a potentially freed
pointer if the task under consideration is exiting because a reference
to the task_struct is not held.

Remove the CAP_SYS_ADMIN bias so that all processes are treated equally.

If any CAP_SYS_ADMIN process would like to be biased against, it is
always allowed to adjust /proc/pid/oom_score_adj.

Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1803071548510.6996@chino.kir.corp.google.com
Signed-off-by: David Rientjes <rientjes@google.com>
Reported-by: Gaurav Kohli <gkohli@codeaurora.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -226,13 +226,6 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
 		mm_pgtables_bytes(p->mm) / PAGE_SIZE;
 	task_unlock(p);
 
-	/*
-	 * Root processes get 3% bonus, just like the __vm_enough_memory()
-	 * implementation used by LSMs.
-	 */
-	if (has_capability_noaudit(p, CAP_SYS_ADMIN))
-		points -= (points * 3) / 100;
-
 	/* Normalize to oom_score_adj units */
 	adj *= totalpages / 1000;
 	points += adj;