md/md-bitmap: Synchronize bitmap_get_stats() with bitmap lifetime

JIRA: https://issues.redhat.com/browse/RHEL-73514

commit 8d28d0d
Author: Yu Kuai <yukuai3@huawei.com>
Date:   Fri Jan 24 17:20:55 2025 +0800

    md/md-bitmap: Synchronize bitmap_get_stats() with bitmap lifetime

    After commit ec6bb29 ("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: 32a7627 ("[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>

Signed-off-by: Nigel Croxon <ncroxon@redhat.com>

Author: ncroxon

drivers/md/md-bitmap.c

@@ -2355,7 +2355,10 @@ static int bitmap_get_stats(void *data, struct md_bitmap_stats *stats)
 
 	if (!bitmap)
 		return -ENOENT;
-
+	if (bitmap->mddev->bitmap_info.external)
+		return -ENOENT;
+	if (!bitmap->storage.sb_page) /* no superblock */
+		return -EINVAL;
 	sb = kmap_local_page(bitmap->storage.sb_page);
 	stats->sync_size = le64_to_cpu(sb->sync_size);
 	kunmap_local(sb);

drivers/md/md.c

@@ -8402,6 +8402,10 @@ static int md_seq_show(struct seq_file *seq, void *v)
 		return 0;
 
 	spin_unlock(&all_mddevs_lock);
+
+	/* 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 : ", mdname(mddev));
@@ -8477,6 +8481,7 @@ 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);
 	spin_lock(&all_mddevs_lock);
 
 	if (mddev == list_last_entry(&all_mddevs, struct mddev, all_mddevs))