[SCSI] megaraid_sas: Sanity check user supplied length before passing it to dma_alloc_coherent()

The ioc->sgl[i].iov_len value is supplied by the ioctl caller, and can be
zero in some cases.  Assume that's valid and continue without error.

Fixes (multiple individual reports of the same problem for quite a while):

http://marc.info/?l=linux-ide&m=128941801715301
http://bugs.debian.org/604627
http://www.mail-archive.com/linux-poweredge@dell.com/msg02575.html

megasas: Failed to alloc kernel SGL buffer for IOCTL

and

[   69.162538] ------------[ cut here ]------------
[   69.162806] kernel BUG at /build/buildd/linux-2.6.32/lib/swiotlb.c:368!
[   69.163134] invalid opcode: 0000 [#1] SMP
[   69.163570] last sysfs file: /sys/devices/system/cpu/cpu3/cache/index2/shared_cpu_map
[   69.163975] CPU 0
[   69.164227] Modules linked in: fbcon tileblit font bitblit softcursor vga16fb vgastate ioatdma radeon ttm drm_kms_helper shpchp drm i2c_algo_bit lp parport floppy pata_jmicron megaraid_sas igb dca
[   69.167419] Pid: 1206, comm: smartctl Tainted: G        W  2.6.32-25-server #45-Ubuntu X8DTN
[   69.167843] RIP: 0010:[<ffffffff812c4dc5>]  [<ffffffff812c4dc5>] map_single+0x255/0x260
[   69.168370] RSP: 0018:ffff88081c0ebc58  EFLAGS: 00010246
[   69.168655] RAX: 000000000003bffc RBX: 00000000ffffffff RCX: 0000000000000002
[   69.169000] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88001dffe000
[   69.169346] RBP: ffff88081c0ebcb8 R08: 0000000000000000 R09: ffff880000030840
[   69.169691] R10: 0000000000100000 R11: 0000000000000000 R12: 0000000000000000
[   69.170036] R13: 00000000ffffffff R14: 0000000000000001 R15: 0000000000200000
[   69.170382] FS:  00007fb8de189720(0000) GS:ffff88001de00000(0000) knlGS:0000000000000000
[   69.170794] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   69.171094] CR2: 00007fb8dd59237c CR3: 000000081a790000 CR4: 00000000000006f0
[   69.171439] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   69.171784] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[   69.172130] Process smartctl (pid: 1206, threadinfo ffff88081c0ea000, task ffff88081a760000)
[   69.194513] Stack:
[   69.205788]  0000000000000034 00000002817e3390 0000000000000000 ffff88081c0ebe00
[   69.217739] <0> 0000000000000000 000000000003bffc 0000000000000000 0000000000000000
[   69.241250] <0> 0000000000000000 00000000ffffffff ffff88081c5b4080 ffff88081c0ebe00
[   69.277310] Call Trace:
[   69.289278]  [<ffffffff812c52ac>] swiotlb_alloc_coherent+0xec/0x130
[   69.301118]  [<ffffffff81038b31>] x86_swiotlb_alloc_coherent+0x61/0x70
[   69.313045]  [<ffffffffa002d0ce>] megasas_mgmt_fw_ioctl+0x1ae/0x690 [megaraid_sas]
[   69.336399]  [<ffffffffa002d748>] megasas_mgmt_ioctl_fw+0x198/0x240 [megaraid_sas]
[   69.359346]  [<ffffffffa002f695>] megasas_mgmt_ioctl+0x35/0x50 [megaraid_sas]
[   69.370902]  [<ffffffff81153b12>] vfs_ioctl+0x22/0xa0
[   69.382322]  [<ffffffff8115da2a>] ? alloc_fd+0x10a/0x150
[   69.393622]  [<ffffffff81153cb1>] do_vfs_ioctl+0x81/0x410
[   69.404696]  [<ffffffff8155cc13>] ? do_page_fault+0x153/0x3b0
[   69.415761]  [<ffffffff811540c1>] sys_ioctl+0x81/0xa0
[   69.426640]  [<ffffffff810121b2>] system_call_fastpath+0x16/0x1b
[   69.437491] Code: fe ff ff 48 8b 3d 74 38 76 00 41 bf 00 00 20 00 e8 51 f5 d7 ff 83 e0 ff 48 05 ff 07 00 00 48 c1 e8 0b 48 89 45 c8 e9 13 fe ff ff <0f> 0b eb fe 0f 1f 80 00 00 00 00 55 48 89 e5 48 83 ec 20 4c 89
[   69.478216] RIP  [<ffffffff812c4dc5>] map_single+0x255/0x260
[   69.489668]  RSP <ffff88081c0ebc58>
[   69.500975] ---[ end trace 6a2181b634e2abc7 ]---

Reported-by: Bokhan Artem <aptem@ngs.ru>
Reported by: Marc-Christian Petersen <m.c.p@gmx.de>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Cc: "Benz, Michael" <Michael.Benz@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>

drivers/scsi/megaraid/megaraid_sas_base.c

@@ -4609,6 +4609,9 @@ megasas_mgmt_fw_ioctl(struct megasas_instance *instance,
 	 * For each user buffer, create a mirror buffer and copy in
 	 */
 	for (i = 0; i < ioc->sge_count; i++) {
+		if (!ioc->sgl[i].iov_len)
+			continue;
+
 		kbuff_arr[i] = dma_alloc_coherent(&instance->pdev->dev,
 						    ioc->sgl[i].iov_len,
 						    &buf_handle, GFP_KERNEL);