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Subject[PATCH 1/2] mm: fix null-ptr-deref in kswapd_is_running()
Date
The kswapd_run/stop() will set pgdat->kswapd to NULL, which
could race with kswapd_is_running() in kcompactd(),

kswapd_run/stop() kcompactd()
kswapd_is_running()
if (pgdat->kswapd) // load non-NULL pgdat->kswapd
pgdat->kswapd = NULL
task_is_running(pgdat->kswapd) // Null pointer derefence

The KASAN report the null-ptr-deref shown below,

vmscan: Failed to start kswapd on node 0
...
BUG: KASAN: null-ptr-deref in kcompactd+0x440/0x504
Read of size 8 at addr 0000000000000024 by task kcompactd0/37

CPU: 0 PID: 37 Comm: kcompactd0 Kdump: loaded Tainted: G OE 5.10.60 #1
Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
Call trace:
dump_backtrace+0x0/0x394
show_stack+0x34/0x4c
dump_stack+0x158/0x1e4
__kasan_report+0x138/0x140
kasan_report+0x44/0xdc
__asan_load8+0x94/0xd0
kcompactd+0x440/0x504
kthread+0x1a4/0x1f0
ret_from_fork+0x10/0x18

For race between kswapd_run() and kcompactd(), adding a temporary value
when create a kthread, and only set it to pgdat->kswapd if kthread_run()
return successful task_struct to fix the issue.

For race between kswapd_stop() and kcompactd(), let's call kcompactd_stop()
before kswapd_stop() to fix the issue.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
mm/memory_hotplug.c | 2 +-
mm/vmscan.c | 8 +++++---
2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index fad6d1f2262a..2fd45ccbce45 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1940,8 +1940,8 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,

node_states_clear_node(node, &arg);
if (arg.status_change_nid >= 0) {
- kswapd_stop(node);
kcompactd_stop(node);
+ kswapd_stop(node);
}

writeback_set_ratelimit();
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b2b1431352dc..08c6497f76c3 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4642,16 +4642,18 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
void kswapd_run(int nid)
{
pg_data_t *pgdat = NODE_DATA(nid);
+ struct task_struct *t;

if (pgdat->kswapd)
return;

- pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
- if (IS_ERR(pgdat->kswapd)) {
+ t = kthread_run(kswapd, pgdat, "kswapd%d", nid);
+ if (IS_ERR(t)) {
/* failure at boot is fatal */
BUG_ON(system_state < SYSTEM_RUNNING);
pr_err("Failed to start kswapd on node %d\n", nid);
- pgdat->kswapd = NULL;
+ } else {
+ pgdat->kswapd = t;
}
}

--
2.35.3
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