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Date:   Thu,  5 Aug 2021 18:14:18 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     tytso@....edu, adilger.kernel@...ger.ca, johann@...mcloud.com
Cc:     linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        Pavel Skripkin <paskripkin@...il.com>,
        syzbot+c9ff4822a62eee994ea3@...kaller.appspotmail.com
Subject: [PATCH] ext4: avoid huge mmp update interval value

Syzbot reported task hung bug in ext4_fill_super(). The problem was in
too huge mmp update interval.

Syzkaller reproducer setted s_mmp_update_interval to 39785 seconds. This
update interaval is unreasonable huge and it can cause tasks to hung on
kthread_stop() call, since it will wait until timeout timer expires.

To avoid this sutiation, I've added MIN and MAX constants for kmmp
interval and clamped s_mmp_update_interval within the boundaries

Reported-and-tested-by: syzbot+c9ff4822a62eee994ea3@...kaller.appspotmail.com
Fixes: c5e06d101aaf ("ext4: add support for multiple mount protection")
Signed-off-by: Pavel Skripkin <paskripkin@...il.com>
---

Hi, Ted and ext4 maintainers!

I am not sure about min/max values for interval, so I look forward to
receiving your views on these values and patch in general!



With regards,
Pavel Skripkin

---
 fs/ext4/mmp.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index bc364c119af6..160abee66dce 100644
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -7,6 +7,9 @@
 
 #include "ext4.h"
 
+#define EXT4_KMMP_MAX_INTERVAL		100
+#define EXT4_KMMP_MIN_INTERVAL		5
+
 /* Checksumming functions */
 static __le32 ext4_mmp_csum(struct super_block *sb, struct mmp_struct *mmp)
 {
@@ -140,6 +143,12 @@ static int kmmpd(void *data)
 	unsigned long diff;
 	int retval;
 
+	/* We should avoid unreasonable huge update interval, since it can cause
+	 * task hung bug on umount or on error handling path in ext4_fill_super()
+	 */
+	mmp_update_interval = clamp(mmp_update_interval, EXT4_KMMP_MIN_INTERVAL,
+							 EXT4_KMMP_MAX_INTERVAL);
+
 	mmp_block = le64_to_cpu(es->s_mmp_block);
 	mmp = (struct mmp_struct *)(bh->b_data);
 	mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());
@@ -156,6 +165,9 @@ static int kmmpd(void *data)
 	memcpy(mmp->mmp_nodename, init_utsname()->nodename,
 	       sizeof(mmp->mmp_nodename));
 
+	ext4_msg(sb, KERN_INFO, "Started kmmp thread with update interval = %u\n",
+		 mmp_update_interval);
+
 	while (!kthread_should_stop() && !sb_rdonly(sb)) {
 		if (!ext4_has_feature_mmp(sb)) {
 			ext4_warning(sb, "kmmpd being stopped since MMP feature"
-- 
2.32.0

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