dm snapshot: disallow the COW and origin devices from being identical

Otherwise loading a "snapshot" table using the same device for the
origin and COW devices, e.g.:

echo "0 20971520 snapshot 253:3 253:3 P 8" | dmsetup create snap

will trigger:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000098
[ 1958.979934] IP: [<ffffffffa040efba>] dm_exception_store_set_chunk_size+0x7a/0x110 [dm_snapshot]
[ 1958.989655] PGD 0
[ 1958.991903] Oops: 0000 [#1] SMP
...
[ 1959.059647] CPU: 9 PID: 3556 Comm: dmsetup Tainted: G IO 4.5.0-rc5.snitm+ #150
...
[ 1959.083517] task: ffff8800b9660c80 ti: ffff88032a954000 task.ti: ffff88032a954000
[ 1959.091865] RIP: 0010:[<ffffffffa040efba>] [<ffffffffa040efba>] dm_exception_store_set_chunk_size+0x7a/0x110 [dm_snapshot]
[ 1959.104295] RSP: 0018:ffff88032a957b30 EFLAGS: 00010246
[ 1959.110219] RAX: 0000000000000000 RBX: 0000000000000008 RCX: 0000000000000001
[ 1959.118180] RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffff880329334a00
[ 1959.126141] RBP: ffff88032a957b50 R08: 0000000000000000 R09: 0000000000000001
[ 1959.134102] R10: 000000000000000a R11: f000000000000000 R12: ffff880330884d80
[ 1959.142061] R13: 0000000000000008 R14: ffffc90001c13088 R15: ffff880330884d80
[ 1959.150021] FS: 00007f8926ba3840(0000) GS:ffff880333440000(0000) knlGS:0000000000000000
[ 1959.159047] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1959.165456] CR2: 0000000000000098 CR3: 000000032f48b000 CR4: 00000000000006e0
[ 1959.173415] Stack:
[ 1959.175656] ffffc90001c13040 ffff880329334a00 ffff880330884ed0 ffff88032a957bdc
[ 1959.183946] ffff88032a957bb8 ffffffffa040f225 ffff880329334a30 ffff880300000000
[ 1959.192233] ffffffffa04133e0 ffff880329334b30 0000000830884d58 00000000569c58cf
[ 1959.200521] Call Trace:
[ 1959.203248] [<ffffffffa040f225>] dm_exception_store_create+0x1d5/0x240 [dm_snapshot]
[ 1959.211986] [<ffffffffa040d310>] snapshot_ctr+0x140/0x630 [dm_snapshot]
[ 1959.219469] [<ffffffffa0005c44>] ? dm_split_args+0x64/0x150 [dm_mod]
[ 1959.226656] [<ffffffffa0005ea7>] dm_table_add_target+0x177/0x440 [dm_mod]
[ 1959.234328] [<ffffffffa0009203>] table_load+0x143/0x370 [dm_mod]
[ 1959.241129] [<ffffffffa00090c0>] ? retrieve_status+0x1b0/0x1b0 [dm_mod]
[ 1959.248607] [<ffffffffa0009e35>] ctl_ioctl+0x255/0x4d0 [dm_mod]
[ 1959.255307] [<ffffffff813304e2>] ? memzero_explicit+0x12/0x20
[ 1959.261816] [<ffffffffa000a0c3>] dm_ctl_ioctl+0x13/0x20 [dm_mod]
[ 1959.268615] [<ffffffff81215eb6>] do_vfs_ioctl+0xa6/0x5c0
[ 1959.274637] [<ffffffff81120d2f>] ? __audit_syscall_entry+0xaf/0x100
[ 1959.281726] [<ffffffff81003176>] ? do_audit_syscall_entry+0x66/0x70
[ 1959.288814] [<ffffffff81216449>] SyS_ioctl+0x79/0x90
[ 1959.294450] [<ffffffff8167e4ae>] entry_SYSCALL_64_fastpath+0x12/0x71
...
[ 1959.323277] RIP [<ffffffffa040efba>] dm_exception_store_set_chunk_size+0x7a/0x110 [dm_snapshot]
[ 1959.333090] RSP <ffff88032a957b30>
[ 1959.336978] CR2: 0000000000000098
[ 1959.344121] ---[ end trace b049991ccad1169e ]---

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1195899
Cc: [e-mail]
Signed-off-by: Ding Xiang <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-snap.c

···
 	int i;
 	int r = -EINVAL;
 	char *origin_path, *cow_path;
+	dev_t origin_dev, cow_dev;
 	unsigned args_used, num_flush_bios = 1;
 	fmode_t origin_mode = FMODE_READ;
 
···
 		ti->error = "Cannot get origin device";
 		goto bad_origin;
 	}
+	origin_dev = s->origin->bdev->bd_dev;
 
 	cow_path = argv[0];
 	argv++;
 	argc--;
+
+	cow_dev = dm_get_dev_t(cow_path);
+	if (cow_dev && cow_dev == origin_dev) {
+		ti->error = "COW device cannot be the same as origin device";
+		r = -EINVAL;
+		goto bad_cow;
+	}
 
 	r = dm_get_device(ti, cow_path, dm_table_get_mode(ti->table), &s->cow);
 	if (r) {

drivers/md/dm-table.c

···
 }
 
 /*
+ * Convert the path to a device
+ */
+dev_t dm_get_dev_t(const char *path)
+{
+	dev_t uninitialized_var(dev);
+	struct block_device *bdev;
+
+	bdev = lookup_bdev(path);
+	if (IS_ERR(bdev))
+		dev = name_to_dev_t(path);
+	else {
+		dev = bdev->bd_dev;
+		bdput(bdev);
+	}
+
+	return dev;
+}
+EXPORT_SYMBOL_GPL(dm_get_dev_t);
+
+/*
  * Add a device to the list, or just increment the usage count if
  * it's already present.
  */
···
 		  struct dm_dev **result)
 {
 	int r;
-	dev_t uninitialized_var(dev);
+	dev_t dev;
 	struct dm_dev_internal *dd;
 	struct dm_table *t = ti->table;
-	struct block_device *bdev;
 
 	BUG_ON(!t);
 
-	/* convert the path to a device */
-	bdev = lookup_bdev(path);
-	if (IS_ERR(bdev)) {
-		dev = name_to_dev_t(path);
-		if (!dev)
-			return -ENODEV;
-	} else {
-		dev = bdev->bd_dev;
-		bdput(bdev);
-	}
+	dev = dm_get_dev_t(path);
+	if (!dev)
+		return -ENODEV;
 
 	dd = find_device(&t->devices, dev);
 	if (!dd) {

include/linux/device-mapper.h

···
 	char name[16];
 };
 
+dev_t dm_get_dev_t(const char *path);
+
 /*
  * Constructors should call these functions to ensure destination devices
  * are opened/closed correctly.