ubifs: Fix oops when remounting with no_bulk_read.

When remounting with the no_bulk_read option,
there is a problem accessing the "bulk_read buffer(bu.buf)"
which has already been freed.

If the bulk_read option is enabled,
ubifs_tnc_bulk_read uses the pre-allocated bu.buf.

While bu.buf is being used by ubifs_tnc_bulk_read,
remounting with no_bulk_read frees bu.buf.

So I added code to check the use of "bu.buf" to avoid this situation.

------
I tested as follows(kernel v3.18) :

Use the script to repeat "no_bulk_read <-> bulk_read"
	remount.sh
	#!/bin/sh
	while true do;
		mount -o remount,no_bulk_read ${MOUNT_POINT};
		sleep 1;
		mount -o remount,bulk_read ${MOUNT_POINT};
		sleep 1;
	done

Perform read operation
	cat ${MOUNT_POINT}/* > /dev/null

The problem is reproduced immediately.

[  234.256845][kernel.0]Internal error: Oops: 17 [#1] PREEMPT ARM
[  234.258557][kernel.0]CPU: 0 PID: 2752 Comm: cat Tainted: G        W  O   3.18.31+ #51
[  234.259531][kernel.0]task: cbff8580 ti: cbd66000 task.ti: cbd66000
[  234.260306][kernel.0]PC is at validate_data_node+0x10/0x264
[  234.260994][kernel.0]LR is at ubifs_tnc_bulk_read+0x388/0x3ec
[  234.261712][kernel.0]pc : [<c01d98fc>]    lr : [<c01dc300>]    psr: 80000013
[  234.261712][kernel.0]sp : cbd67ba0  ip : 00000001  fp : 00000000
[  234.263337][kernel.0]r10: cd3e0260  r9 : c0df2008  r8 : 00000000
[  234.264087][kernel.0]r7 : cd3e0000  r6 : 00000000  r5 : cd3e0278  r4 : cd3e0000
[  234.264999][kernel.0]r3 : 00000003  r2 : cd3e0280  r1 : 00000000  r0 : cd3e0000
[  234.265910][kernel.0]Flags: Nzcv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
[  234.266896][kernel.0]Control: 10c53c7d  Table: 8c40c059  DAC: 00000015
[  234.267711][kernel.0]Process cat (pid: 2752, stack limit = 0xcbd66400)
[  234.268525][kernel.0]Stack: (0xcbd67ba0 to 0xcbd68000)
[  234.269169][kernel.0]7ba0: cd7c3940 c03d8650 0001bfe0 00002ab2 00000000 cbd67c5c cbd67c58 0001bfe0
[  234.270287][kernel.0]7bc0: cd3e0000 00002ab2 0001bfe0 00000014 cbd66000 cd3e0260 00000000 c01d6660
[  234.271403][kernel.0]7be0: 00002ab2 00000000 c82a5800 ffffffff cd3e0298 cd3e0278 00000000 cd3e0000
[  234.272520][kernel.0]7c00: 00000000 00000000 cd3e0260 c01dc300 00002ab2 00000000 60000013 d663affa
[  234.273639][kernel.0]7c20: cd3e01f0 cd3e01f0 60000013 c09397ec 00000000 cd3e0278 00002ab2 00000000
[  234.274755][kernel.0]7c40: cd3e0000 c01dbf48 00000014 00000003 00000160 00000015 00000004 d663affa
[  234.275874][kernel.0]7c60: ccdaa978 cd3e0278 cd3e0000 cf32a5f4 ccdaa820 00000044 cbd66000 cd3e0260
[  234.276992][kernel.0]7c80: 00000003 c01cec84 ccdaa8dc cbd67cc4 cbd67ec0 00000010 ccdaa978 00000000
[  234.278108][kernel.0]7ca0: 0000015e ccdaa8dc 00000000 00000000 cf32a5d0 00000000 0000015f ccdaa8dc
[  234.279228][kernel.0]7cc0: 00000000 c8488300 0009e5a4 0000000e cbd66000 0000015e cf32a5f4 c0113c04
[  234.280346][kernel.0]7ce0: 0000009f 0000003c c00098c4 ffffffff 00001000 00000000 000000ad 00000010
[  234.281463][kernel.0]7d00: 00000038 cd68f580 00000150 c8488360 00000000 cbd67d30 cbd67d70 0000000e
[  234.282579][kernel.0]7d20: 00000010 00000000 c0951874 c0112a9c cf379b60 cf379b84 cf379890 cf3798b4
[  234.283699][kernel.0]7d40: cf379578 cf37959c cf379380 cf3793a4 cf3790b0 cf3790d4 cf378fd8 cf378ffc
[  234.284814][kernel.0]7d60: cf378f48 cf378f6c cf32a5f4 cf32a5d0 00000000 00001000 00000018 00000000
[  234.285932][kernel.0]7d80: 00001000 c0050da4 00000000 00001000 cec04c00 00000000 00001000 c0e11328
[  234.287049][kernel.0]7da0: 00000000 00001000 cbd66000 00000000 00001000 c0012a60 00000000 00001000
[  234.288166][kernel.0]7dc0: cbd67dd4 00000000 00001000 80000013 00000000 00001000 cd68f580 00000000
[  234.289285][kernel.0]7de0: 00001000 c915d600 00000000 00001000 cbd67e48 00000000 00001000 00000018
[  234.290402][kernel.0]7e00: 00000000 00001000 00000000 00000000 00001000 c915d768 c915d768 c0113550
[  234.291522][kernel.0]7e20: cd68f580 cbd67e48 cd68f580 cb6713c0 00010000 000ac5a4 00000000 001fc5a4
[  234.292637][kernel.0]7e40: 00000000 c8488300 cbd67ec0 00eb0000 cd68f580 c0113ee4 00000000 cbd67ec0
[  234.293754][kernel.0]7e60: cd68f580 c8488300 cbd67ec0 00eb0000 cd68f580 00150000 c8488300 00eb0000
[  234.294874][kernel.0]7e80: 00010000 c0112fd0 00000000 cbd67ec0 cd68f580 00150000 00000000 cd68f580
[  234.295991][kernel.0]7ea0: cbd67ef0 c011308c 00000000 00000002 cd768850 00010000 00000000 c01133fc
[  234.297110][kernel.0]7ec0: 00150000 00000000 cbd67f50 00000000 00000000 cb6713c0 01000000 cbd67f48
[  234.298226][kernel.0]7ee0: cbd67f50 c8488300 00000000 c0113204 00010000 01000000 00000000 cb6713c0
[  234.299342][kernel.0]7f00: 00150000 00000000 cbd67f50 00000000 00000000 00000000 00000000 00000000
[  234.300462][kernel.0]7f20: cbd67f50 01000000 01000000 cb6713c0 c8488300 c00ebba8 01000000 00000000
[  234.301577][kernel.0]7f40: c8488300 cb6713c0 00000000 00000000 00000000 00000000 ccdaa820 00000000
[  234.302697][kernel.0]7f60: 00000000 01000000 00000003 00000001 cbd66000 00000000 00000001 c00ec678
[  234.303813][kernel.0]7f80: 00000000 00000200 00000000 01000000 01000000 00000000 00000000 000000ef
[  234.304933][kernel.0]7fa0: c000e904 c000e780 01000000 00000000 00000001 00000003 00000000 01000000
[  234.306049][kernel.0]7fc0: 01000000 00000000 00000000 000000ef 00000001 00000003 01000000 00000001
[  234.307165][kernel.0]7fe0: 00000000 beafb78c 0000ad08 00128d1c 60000010 00000001 00000000 00000000
[  234.308292][kernel.0][<c01d98fc>] (validate_data_node) from [<c01dc300>] (ubifs_tnc_bulk_read+0x388/0x3ec)
[  234.309493][kernel.0][<c01dc300>] (ubifs_tnc_bulk_read) from [<c01cec84>] (ubifs_readpage+0x1dc/0x46c)
[  234.310656][kernel.0][<c01cec84>] (ubifs_readpage) from [<c0113c04>] (__generic_file_splice_read+0x29c/0x4cc)
[  234.311890][kernel.0][<c0113c04>] (__generic_file_splice_read) from [<c0113ee4>] (generic_file_splice_read+0xb0/0xf4)
[  234.313214][kernel.0][<c0113ee4>] (generic_file_splice_read) from [<c0112fd0>] (do_splice_to+0x68/0x7c)
[  234.314386][kernel.0][<c0112fd0>] (do_splice_to) from [<c011308c>] (splice_direct_to_actor+0xa8/0x190)
[  234.315544][kernel.0][<c011308c>] (splice_direct_to_actor) from [<c0113204>] (do_splice_direct+0x90/0xb8)
[  234.316741][kernel.0][<c0113204>] (do_splice_direct) from [<c00ebba8>] (do_sendfile+0x17c/0x2b8)
[  234.317838][kernel.0][<c00ebba8>] (do_sendfile) from [<c00ec678>] (SyS_sendfile64+0xc4/0xcc)
[  234.318890][kernel.0][<c00ec678>] (SyS_sendfile64) from [<c000e780>] (ret_fast_syscall+0x0/0x38)
[  234.319983][kernel.0]Code: e92d47f0 e24dd050 e59f9228 e1a04000 (e5d18014)
Signed-off-by: Nkaram.lee <karam.lee@lge.com>
Signed-off-by: NRichard Weinberger <richard@nod.at>

fs/ubifs/super.c

@@ -1876,8 +1876,10 @@ static int ubifs_remount_fs(struct super_block *sb, int *flags, char *data)
 		bu_init(c);
 	else {
 		dbg_gen("disable bulk-read");
+		mutex_lock(&c->bu_mutex);
 		kfree(c->bu.buf);
 		c->bu.buf = NULL;
+		mutex_unlock(&c->bu_mutex);
 	}
 
 	ubifs_assert(c->lst.taken_empty_lebs > 0);