【OLK 5.10】并发创建md设备触发uaf

# 问题现象
```
==================================================================
BUG: KASAN: use-after-free in md_alloc+0xb0/0x660 drivers/md/md.c:5772
Read of size 8 at addr ffff8880927f1048 by task syz-executor.3/11266

CPU: 2 PID: 11266 Comm: syz-executor.3 Tainted: G        W         5.10.0-10135-g9ab9d9e426bc-dirty #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
Call Trace:
 __dump_stack lib/dump_stack.c:82 [inline]
 dump_stack+0xbe/0xfd lib/dump_stack.c:123
 print_address_description.constprop.0+0x19/0x170 mm/kasan/report.c:400
 __kasan_report.cold+0x6c/0x84 mm/kasan/report.c:560
 kasan_report+0x3a/0x50 mm/kasan/report.c:585
 md_alloc+0xb0/0x660 drivers/md/md.c:5772
 md_probe+0x5b/0x70 drivers/md/md.c:5858
 kobj_lookup+0x1d3/0x2e0 drivers/base/map.c:152
 get_gendisk+0x60/0x260 block/genhd.c:1127
 bdev_get_gendisk fs/block_dev.c:1140 [inline]
 __blkdev_get+0xb5/0x1170 fs/block_dev.c:1567
 blkdev_get+0x9b/0x1c0 fs/block_dev.c:1784
 blkdev_open+0x14e/0x190 fs/block_dev.c:1901
 do_dentry_open+0x310/0x8b0 fs/open.c:817
 do_open+0x5b0/0x800 fs/namei.c:3320
 path_openat+0x1c2/0x3b0 fs/namei.c:3438
 do_filp_open+0x134/0x280 fs/namei.c:3465
 do_sys_openat2+0x579/0x790 fs/open.c:1186
 do_sys_open fs/open.c:1202 [inline]
 __do_sys_openat fs/open.c:1218 [inline]
 __se_sys_openat fs/open.c:1213 [inline]
 __x64_sys_openat+0xf3/0x180 fs/open.c:1213
 do_syscall_64+0x30/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x62/0xc7
RIP: 0033:0x45513d
Code: 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fee384de058 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 000000000058c050 RCX: 000000000045513d
RDX: 0000000000000200 RSI: 0000000020000040 RDI: ffffffffffffff9c
RBP: 000000000058c050 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 000000000000006e R14: 0000000000425470 R15: 00007fff476992c0

Allocated by task 11264:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track mm/kasan/common.c:56 [inline]
 set_alloc_info mm/kasan/common.c:512 [inline]
 __kasan_kmalloc mm/kasan/common.c:544 [inline]
 __kasan_kmalloc.constprop.0+0xf0/0x130 mm/kasan/common.c:515
 kmalloc include/linux/slab.h:563 [inline]
 kzalloc include/linux/slab.h:675 [inline]
 mddev_find_or_alloc+0x1d6/0x510 drivers/md/md.c:795
 md_alloc+0x26/0x660 drivers/md/md.c:5750
 md_probe+0x5b/0x70 drivers/md/md.c:5858
 kobj_lookup+0x1d3/0x2e0 drivers/base/map.c:152
 get_gendisk+0x60/0x260 block/genhd.c:1127
 bdev_get_gendisk fs/block_dev.c:1140 [inline]
 __blkdev_get+0xb5/0x1170 fs/block_dev.c:1567
 blkdev_get+0x9b/0x1c0 fs/block_dev.c:1784
 blkdev_open+0x14e/0x190 fs/block_dev.c:1901
 do_dentry_open+0x310/0x8b0 fs/open.c:817
 do_open+0x5b0/0x800 fs/namei.c:3320
 path_openat+0x1c2/0x3b0 fs/namei.c:3438
 do_filp_open+0x134/0x280 fs/namei.c:3465
 do_sys_openat2+0x579/0x790 fs/open.c:1186
 do_sys_open fs/open.c:1202 [inline]
 __do_sys_openat fs/open.c:1218 [inline]
 __se_sys_openat fs/open.c:1213 [inline]
 __x64_sys_openat+0xf3/0x180 fs/open.c:1213
 do_syscall_64+0x30/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x62/0xc7

Freed by task 11264:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track+0x1c/0x30 mm/kasan/common.c:56
 kasan_set_free_info+0x20/0x40 mm/kasan/generic.c:384
 __kasan_slab_free.part.0+0x13f/0x1b0 mm/kasan/common.c:496
 slab_free_hook mm/slub.c:1569 [inline]
 slab_free_freelist_hook mm/slub.c:1608 [inline]
 slab_free mm/slub.c:3179 [inline]
 kfree+0xce/0x860 mm/slub.c:4179
 mddev_free drivers/md/md.c:5735 [inline]
 md_alloc+0x4f6/0x660 drivers/md/md.c:5850
 md_probe+0x5b/0x70 drivers/md/md.c:5858
 kobj_lookup+0x1d3/0x2e0 drivers/base/map.c:152
 get_gendisk+0x60/0x260 block/genhd.c:1127
 bdev_get_gendisk fs/block_dev.c:1140 [inline]
 __blkdev_get+0xb5/0x1170 fs/block_dev.c:1567
 blkdev_get+0x9b/0x1c0 fs/block_dev.c:1784
 blkdev_open+0x14e/0x190 fs/block_dev.c:1901
 do_dentry_open+0x310/0x8b0 fs/open.c:817
 do_open+0x5b0/0x800 fs/namei.c:3320
 path_openat+0x1c2/0x3b0 fs/namei.c:3438
 do_filp_open+0x134/0x280 fs/namei.c:3465
 do_sys_openat2+0x579/0x790 fs/open.c:1186
 do_sys_open fs/open.c:1202 [inline]
 __do_sys_openat fs/open.c:1218 [inline]
 __se_sys_openat fs/open.c:1213 [inline]
 __x64_sys_openat+0xf3/0x180 fs/open.c:1213
 do_syscall_64+0x30/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x62/0xc7

The buggy address belongs to the object at ffff8880927f1000
 which belongs to the cache kmalloc-4k of size 4096
The buggy address is located 72 bytes inside of
 4096-byte region [ffff8880927f1000, ffff8880927f2000)
The buggy address belongs to the page:
page:ffffea000249fc00 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x927f0
head:ffffea000249fc00 order:3 compound_mapcount:0 compound_pincount:0
flags: 0xfffffc0010200(slab|head|node=0|zone=1|lastcpupid=0x1fffff)
raw: 000fffffc0010200 ffffea000240c408 ffffea0002798408 ffff88810004f500
raw: 0000000000000000 0000000000020002 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8880927f0f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8880927f0f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8880927f1000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

^
 ffff8880927f1080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8880927f1100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================
```
# 复现补丁：
```
diff --git a/drivers/md/md.c b/drivers/md/md.c
index b0007e936b0f..f2c87061a153 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -5753,10 +5753,15 @@ static int md_alloc(dev_t dev, char *name)
        int shift;
        int unit;
        int error;
+       static int alloc_seq = 1;
+
+       printk("delay in %d alloc\n", alloc_seq++);
+       mdelay(3000);

if (!mddev)
                return -ENODEV;

+       printk("use mddev %d\n", alloc_seq);
        partitioned = (MAJOR(mddev->unit) != MD_MAJOR);
        shift = partitioned ? MdpMinorShift : 0;
        unit = MINOR(mddev->unit) >> shift;
@@ -5819,9 +5824,11 @@ static int md_alloc(dev_t dev, char *name)
        disk->flags |= GENHD_FL_EXT_DEVT;
        disk->events |= DISK_EVENT_MEDIA_CHANGE;
        mddev->gendisk = disk;
-       error = add_disk_safe(disk);
-       if (error)
+//     error = add_disk_safe(disk);
+       if (error) {
+               printk("add disk fail\n");
                goto out_put_disk;
+       }

kobject_init(&mddev->kobj, &md_ktype);
        error = kobject_add(&mddev->kobj, &disk_to_dev(disk)->kobj, "%s", "md");
@@ -5848,6 +5855,7 @@ static int md_alloc(dev_t dev, char *name)
        blk_cleanup_disk(disk);
 out_free_mddev:
        mddev_free(mddev);
+       printk("freed mddev\n");
        mutex_unlock(&disks_mutex);
        return error;
 }
```

# 复现步骤：
```
echo md0 > /sys/module/md_mod/parameters/new_array &
mdadm --create /dev/md0 -l1 -n2 /dev/sda /dev/sdb
```

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