mm: memcontrol: fix NULL-ptr deref in percpu stats flush

__mem_cgroup_free() can be called on the failure path in
mem_cgroup_alloc().  However memcg_flush_percpu_vmstats() and
memcg_flush_percpu_vmevents() which are called from __mem_cgroup_free()
access the fields of memcg which can potentially be null if called from
failure path from mem_cgroup_alloc().  Indeed syzbot has reported the
following crash:

	kasan: CONFIG_KASAN_INLINE enabled
	kasan: GPF could be caused by NULL-ptr deref or user memory access
	general protection fault: 0000 [#1] PREEMPT SMP KASAN
	CPU: 0 PID: 30393 Comm: syz-executor.1 Not tainted 5.4.0-rc2+ #0
	Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
	RIP: 0010:memcg_flush_percpu_vmstats+0x4ae/0x930 mm/memcontrol.c:3436
	Code: 05 41 89 c0 41 0f b6 04 24 41 38 c7 7c 08 84 c0 0f 85 5d 03 00 00 44 3b 05 33 d5 12 08 0f 83 e2 00 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 0f 85 91 03 00 00 48 8b 85 10 fe ff ff 48 8b b0 90
	RSP: 0018:ffff888095c27980 EFLAGS: 00010206
	RAX: 0000000000000012 RBX: ffff888095c27b28 RCX: ffffc90008192000
	RDX: 0000000000040000 RSI: ffffffff8340fae7 RDI: 0000000000000007
	RBP: ffff888095c27be0 R08: 0000000000000000 R09: ffffed1013f0da33
	R10: ffffed1013f0da32 R11: ffff88809f86d197 R12: fffffbfff138b760
	R13: dffffc0000000000 R14: 0000000000000090 R15: 0000000000000007
	FS:  00007f5027170700(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
	CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
	CR2: 0000000000710158 CR3: 00000000a7b18000 CR4: 00000000001406f0
	DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
	DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
	Call Trace:
	__mem_cgroup_free+0x1a/0x190 mm/memcontrol.c:5021
	mem_cgroup_free mm/memcontrol.c:5033 [inline]
	mem_cgroup_css_alloc+0x3a1/0x1ae0 mm/memcontrol.c:5160
	css_create kernel/cgroup/cgroup.c:5156 [inline]
	cgroup_apply_control_enable+0x44d/0xc40 kernel/cgroup/cgroup.c:3119
	cgroup_mkdir+0x899/0x11b0 kernel/cgroup/cgroup.c:5401
	kernfs_iop_mkdir+0x14d/0x1d0 fs/kernfs/dir.c:1124
	vfs_mkdir+0x42e/0x670 fs/namei.c:3807
	do_mkdirat+0x234/0x2a0 fs/namei.c:3830
	__do_sys_mkdir fs/namei.c:3846 [inline]
	__se_sys_mkdir fs/namei.c:3844 [inline]
	__x64_sys_mkdir+0x5c/0x80 fs/namei.c:3844
	do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290
	entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixing this by moving the flush to mem_cgroup_free as there is no need
to flush anything if we see failure in mem_cgroup_alloc().

Link: http://lkml.kernel.org/r/20191018165231.249872-1-shakeelb@google.com
Fixes: bb65f89b7d ("mm: memcontrol: flush percpu vmevents before releasing memcg")
Fixes: c350a99ea2 ("mm: memcontrol: flush percpu vmstats before releasing memcg")
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Reported-by: syzbot+515d5bcfe179cdf049b2@syzkaller.appspotmail.com
Reviewed-by: Roman Gushchin <guro@fb.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -5014,12 +5014,6 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
 {
 	int node;
 
-	/*
-	 * Flush percpu vmstats and vmevents to guarantee the value correctness
-	 * on parent's and all ancestor levels.
-	 */
-	memcg_flush_percpu_vmstats(memcg, false);
-	memcg_flush_percpu_vmevents(memcg);
 	for_each_node(node)
 		free_mem_cgroup_per_node_info(memcg, node);
 	free_percpu(memcg->vmstats_percpu);
@@ -5030,6 +5024,12 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
 static void mem_cgroup_free(struct mem_cgroup *memcg)
 {
 	memcg_wb_domain_exit(memcg);
+	/*
+	 * Flush percpu vmstats and vmevents to guarantee the value correctness
+	 * on parent's and all ancestor levels.
+	 */
+	memcg_flush_percpu_vmstats(memcg, false);
+	memcg_flush_percpu_vmevents(memcg);
 	__mem_cgroup_free(memcg);
 }