[PATCH 5.15.y] md/md-bitmap: Synchronize bitmap_get_stats() with bitmap lifetime

[Johnny Hao <johnny_haocn@sina.com>]

From: Yu Kuai <yukuai3@huawei.com>

[ Upstream commit 8d28d0ddb986f56920ac97ae704cc3340a699a30 ]

After commit ec6bb299c7c3 ("md/md-bitmap: add 'sync_size' into struct
md_bitmap_stats"), following panic is reported:

Oops: general protection fault, probably for non-canonical address
RIP: 0010:bitmap_get_stats+0x2b/0xa0
Call Trace:
 <TASK>
 md_seq_show+0x2d2/0x5b0
 seq_read_iter+0x2b9/0x470
 seq_read+0x12f/0x180
 proc_reg_read+0x57/0xb0
 vfs_read+0xf6/0x380
 ksys_read+0x6c/0xf0
 do_syscall_64+0x82/0x170
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Root cause is that bitmap_get_stats() can be called at anytime if mddev
is still there, even if bitmap is destroyed, or not fully initialized.
Deferenceing bitmap in this case can crash the kernel. Meanwhile, the
above commit start to deferencing bitmap->storage, make the problem
easier to trigger.

Fix the problem by protecting bitmap_get_stats() with bitmap_info.mutex.

Cc: stable@vger.kernel.org # v6.12+
Fixes: 32a7627cf3a3 ("[PATCH] md: optimised resync using Bitmap based intent logging")
Reported-and-tested-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Closes: https://lore.kernel.org/linux-raid/ca3a91a2-50ae-4f68-b317-abd9889f3907@oracle.com/T/#m6e5086c95201135e4941fe38f9efa76daf9666c5
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250124092055.4050195-1-yukuai1@huaweicloud.com
Signed-off-by: Song Liu <song@kernel.org>
[ The context change is due to the commit 38f287d7e495
("md/md-bitmap: replace md_bitmap_status() with a new helper md_bitmap_get_stats()")
in v6.12 and the commit f9cfe7e7f96a ("md: Fix md_seq_ops() regressions") in v6.8
which are irrelevant to the logic of this patch. ]
Signed-off-by: Johnny Hao <johnny_haocn@sina.com>
---
 drivers/md/md-bitmap.c | 4 ++++
 drivers/md/md.c        | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/drivers/md/md-bitmap.c b/drivers/md/md-bitmap.c
index 03efb3c72980..2789dbbd7eaf 100644
--- a/drivers/md/md-bitmap.c
+++ b/drivers/md/md-bitmap.c
@@ -2033,6 +2033,10 @@ void md_bitmap_status(struct seq_file *seq, struct bitmap *bitmap)
 
 	if (!bitmap)
 		return;
+	if (bitmap->mddev->bitmap_info.external)
+		return;
+	if (!bitmap->storage.sb_page) /* no superblock */
+		return;
 
 	counts = &bitmap->counts;
 
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 332458ad9663..bb6b5360d94b 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -8300,6 +8300,9 @@ static int md_seq_show(struct seq_file *seq, void *v)
 		return 0;
 	}
 
+	/* prevent bitmap to be freed after checking */
+	mutex_lock(&mddev->bitmap_info.mutex);
+
 	spin_lock(&mddev->lock);
 	if (mddev->pers || mddev->raid_disks || !list_empty(&mddev->disks)) {
 		seq_printf(seq, "%s : %sactive", mdname(mddev),
@@ -8371,6 +8374,7 @@ static int md_seq_show(struct seq_file *seq, void *v)
 		seq_printf(seq, "\n");
 	}
 	spin_unlock(&mddev->lock);
+	mutex_unlock(&mddev->bitmap_info.mutex);
 
 	return 0;
 }
-- 
2.34.1