From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Fri, 22 Jul 2022 19:45:39 +0900
Subject: mm: memcontrol: fix potential oom_lock recursion deadlock
Git-commit: 68aaee147e597b495622b7c9038e5922c7c61f57
Patch-mainline: v6.0-rc1
References: bsc#1202447

syzbot is reporting GFP_KERNEL allocation with oom_lock held when
reporting memcg OOM [1].  If this allocation triggers the global OOM
situation then the system can livelock because the GFP_KERNEL
allocation with oom_lock held cannot trigger the global OOM killer
because __alloc_pages_may_oom() fails to hold oom_lock.

Fix this problem by removing the allocation from memory_stat_format()
completely, and pass static buffer when calling from memcg OOM path.

Note that the caller holding filesystem lock was the trigger for syzbot
to report this locking dependency.  Doing GFP_KERNEL allocation with
filesystem lock held can deadlock the system even without involving OOM
situation.

Link: https://syzkaller.appspot.com/bug?extid=2d2aeadc6ce1e1f11d45 [1]
Link: https://lkml.kernel.org/r/86afb39f-8c65-bec2-6cfc-c5e3cd600c0b@I-love.SAKURA.ne.jp
Fixes: c8713d0b23123759 ("mm: memcontrol: dump memory.stat during cgroup OOM")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+2d2aeadc6ce1e1f11d45@syzkaller.appspotmail.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[mkoutny: Adjust context]
Acked-by: Michal Koutný <mkoutny@suse.com>
---
 mm/memcontrol.c |   22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1427,14 +1427,12 @@ static inline unsigned long memcg_page_s
 	return memcg_page_state(memcg, item) * memcg_page_state_unit(item);
 }
 
-static char *memory_stat_format(struct mem_cgroup *memcg)
+static void memory_stat_format(struct mem_cgroup *memcg, char *buf, int bufsize)
 {
 	struct seq_buf s;
 	int i;
 
-	seq_buf_init(&s, kmalloc(PAGE_SIZE, GFP_KERNEL), PAGE_SIZE);
-	if (!s.buffer)
-		return NULL;
+	seq_buf_init(&s, buf, bufsize);
 
 	/*
 	 * Provide statistics on the state of the memory subsystem as
@@ -1493,8 +1491,6 @@ static char *memory_stat_format(struct m
 
 	/* The above should easily fit into one page */
 	WARN_ON_ONCE(seq_buf_has_overflowed(&s));
-
-	return s.buffer;
 }
 
 #define K(x) ((x) << (PAGE_SHIFT-10))
@@ -1530,7 +1526,10 @@ void mem_cgroup_print_oom_context(struct
  */
 void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
 {
-	char *buf;
+	/* Use static buffer, for the caller is holding oom_lock. */
+	static char buf[PAGE_SIZE];
+
+	lockdep_assert_held(&oom_lock);
 
 	pr_info("memory: usage %llukB, limit %llukB, failcnt %lu\n",
 		K((u64)page_counter_read(&memcg->memory)),
@@ -1551,11 +1550,8 @@ void mem_cgroup_print_oom_meminfo(struct
 	pr_info("Memory cgroup stats for ");
 	pr_cont_cgroup_path(memcg->css.cgroup);
 	pr_cont(":");
-	buf = memory_stat_format(memcg);
-	if (!buf)
-		return;
+	memory_stat_format(memcg, buf, sizeof(buf));
 	pr_info("%s", buf);
-	kfree(buf);
 }
 
 /*
@@ -6348,11 +6344,11 @@ static int memory_events_local_show(stru
 static int memory_stat_show(struct seq_file *m, void *v)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
-	char *buf;
+	char *buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
 
-	buf = memory_stat_format(memcg);
 	if (!buf)
 		return -ENOMEM;
+	memory_stat_format(memcg, buf, PAGE_SIZE);
 	seq_puts(m, buf);
 	kfree(buf);
 	return 0;