==================================================================
BUG: KASAN: use-after-free in oom_kill_process.cold+0x484/0x9d4 mm/oom_kill.c:978
Read of size 8 at addr ffff8880595f6c40 by task syz-executor1/15817

CPU: 1 PID: 15817 Comm: syz-executor1 Not tainted 5.0.0-rc2+ #29
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1db/0x2d0 lib/dump_stack.c:113
 print_address_description.cold+0x7c/0x20d mm/kasan/report.c:187
 kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:135
 oom_kill_process.cold+0x484/0x9d4 mm/oom_kill.c:978
 out_of_memory+0x885/0x1420 mm/oom_kill.c:1133
 mem_cgroup_out_of_memory+0x160/0x210 mm/memcontrol.c:1393
 mem_cgroup_oom mm/memcontrol.c:1721 [inline]
 try_charge+0xd44/0x19b0 mm/memcontrol.c:2283
 memcg_kmem_charge_memcg+0x7c/0x130 mm/memcontrol.c:2591
 memcg_kmem_charge+0x13b/0x340 mm/memcontrol.c:2624
 __alloc_pages_nodemask+0x7b8/0xdc0 mm/page_alloc.c:4559
 __alloc_pages include/linux/gfp.h:473 [inline]
 __alloc_pages_node include/linux/gfp.h:486 [inline]
 alloc_pages_node include/linux/gfp.h:500 [inline]
 alloc_thread_stack_node kernel/fork.c:246 [inline]
 dup_task_struct kernel/fork.c:849 [inline]
 copy_process+0x847/0x8710 kernel/fork.c:1753
 _do_fork+0x1a9/0x1170 kernel/fork.c:2227
 __do_sys_clone kernel/fork.c:2334 [inline]
 __se_sys_clone kernel/fork.c:2328 [inline]
 __x64_sys_clone+0xbf/0x150 kernel/fork.c:2328
 do_syscall_64+0x1a3/0x800 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457ec9
Code: 6d b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 3b b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f36f091cc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000038
RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 0000000000457ec9
RDX: 9999999999999999 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 000000000073bf00 R08: ffffffffffffffff R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f36f091d6d4
R13: 00000000004be2a0 R14: 00000000004ce760 R15: 00000000ffffffff

Allocated by task 15809:
 save_stack+0x45/0xd0 mm/kasan/common.c:73
 set_track mm/kasan/common.c:85 [inline]
 __kasan_kmalloc mm/kasan/common.c:496 [inline]
 __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:469
 kasan_kmalloc mm/kasan/common.c:504 [inline]
 kasan_slab_alloc+0xf/0x20 mm/kasan/common.c:411
 kmem_cache_alloc_node+0x144/0x710 mm/slab.c:3633
 alloc_task_struct_node kernel/fork.c:158 [inline]
 dup_task_struct kernel/fork.c:845 [inline]
 copy_process+0x405b/0x8710 kernel/fork.c:1753
 _do_fork+0x1a9/0x1170 kernel/fork.c:2227
 __do_sys_clone kernel/fork.c:2334 [inline]
 __se_sys_clone kernel/fork.c:2328 [inline]
 __x64_sys_clone+0xbf/0x150 kernel/fork.c:2328
 do_syscall_64+0x1a3/0x800 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 15817:
 save_stack+0x45/0xd0 mm/kasan/common.c:73
 set_track mm/kasan/common.c:85 [inline]
 __kasan_slab_free+0x102/0x150 mm/kasan/common.c:458
 kasan_slab_free+0xe/0x10 mm/kasan/common.c:466
 __cache_free mm/slab.c:3487 [inline]
 kmem_cache_free+0x86/0x260 mm/slab.c:3749
 free_task_struct kernel/fork.c:163 [inline]
 free_task+0x170/0x1f0 kernel/fork.c:458
 __put_task_struct+0x2e0/0x630 kernel/fork.c:731
 put_task_struct+0x4b/0x60 include/linux/sched/task.h:98
 oom_kill_process.cold+0x93a/0x9d4 mm/oom_kill.c:990
 out_of_memory+0x885/0x1420 mm/oom_kill.c:1133
 mem_cgroup_out_of_memory+0x160/0x210 mm/memcontrol.c:1393
 mem_cgroup_oom mm/memcontrol.c:1721 [inline]
 try_charge+0xd44/0x19b0 mm/memcontrol.c:2283
 memcg_kmem_charge_memcg+0x7c/0x130 mm/memcontrol.c:2591
 memcg_kmem_charge+0x13b/0x340 mm/memcontrol.c:2624
 __alloc_pages_nodemask+0x7b8/0xdc0 mm/page_alloc.c:4559
 __alloc_pages include/linux/gfp.h:473 [inline]
 __alloc_pages_node include/linux/gfp.h:486 [inline]
 alloc_pages_node include/linux/gfp.h:500 [inline]
 alloc_thread_stack_node kernel/fork.c:246 [inline]
 dup_task_struct kernel/fork.c:849 [inline]
 copy_process+0x847/0x8710 kernel/fork.c:1753
 _do_fork+0x1a9/0x1170 kernel/fork.c:2227
 __do_sys_clone kernel/fork.c:2334 [inline]
 __se_sys_clone kernel/fork.c:2328 [inline]
 __x64_sys_clone+0xbf/0x150 kernel/fork.c:2328
 do_syscall_64+0x1a3/0x800 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff8880595f6540
 which belongs to the cache task_struct(33:syz1) of size 6080
The buggy address is located 1792 bytes inside of
 6080-byte region [ffff8880595f6540, ffff8880595f7d00)
The buggy address belongs to the page:
page:ffffea0001657d80 count:1 mapcount:0 mapping:ffff888091f65840 index:0x0 compound_mapcount: 0
flags: 0x1fffc0000010200(slab|head)
raw: 01fffc0000010200 ffffea00028b3288 ffffea0002612788 ffff888091f65840
raw: 0000000000000000 ffff8880595f6540 0000000100000001 ffff888057fe2b00
page dumped because: kasan: bad access detected
page->mem_cgroup:ffff888057fe2b00

--------------------

On looking further it seems like the process selected to be oom-killed
has exited even before reaching read_lock(&tasklist_lock) in
oom_kill_process(). More specifically the tsk->usage is 1 which is due
to get_task_struct() in oom_evaluate_task() and the put_task_struct
within for_each_thread() frees the tsk and for_each_thread() tries to
access the tsk. The easiest fix is to do get/put across the
for_each_thread() on the selected task.

Now the next question is should we continue with the oom-kill as the
previously selected task has exited? However before adding more
complexity and heuristics, let's answer why we even look at the
children of oom-kill selected task? The select_bad_process() has already
selected the worst process in the system/memcg. Due to race, the
selected process might not be the worst at the kill time but does that
matter matter? The userspace can play with oom_score_adj to prefer
children to be killed before the parent. I looked at the history but it
seems like this is there before git history.

Signed-off-by: Shakeel Butt <shakeelb@google.com>
---
 mm/oom_kill.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 0930b4365be7..1a007dae1e8f 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -981,6 +981,13 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 	 * still freeing memory.
 	 */
 	read_lock(&tasklist_lock);
+
+	/*
+	 * The task 'p' might have already exited before reaching here. The
+	 * put_task_struct() will free task_struct 'p' while the loop still try
+	 * to access the field of 'p', so, get an extra reference.
+	 */
+	get_task_struct(p);
 	for_each_thread(p, t) {
 		list_for_each_entry(child, &t->children, sibling) {
 			unsigned int child_points;
@@ -1000,6 +1007,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 			}
 		}
 	}
+	put_task_struct(p);
 	read_unlock(&tasklist_lock);
 
 	/*
