scsi: scsi_debug: Fix out-of-bound read in resp_readcap16()

scsi: scsi_debug: Fix out-of-bound read in resp_readcap16()

The following warning was observed running syzkaller:

[ 3813.830724] sg_write: data in/out 65466/242 bytes for SCSI command 0x9e-- guessing data in;
[ 3813.830724]    program syz-executor not setting count and/or reply_len properly
[ 3813.836956] ==================================================================
[ 3813.839465] BUG: KASAN: stack-out-of-bounds in sg_copy_buffer+0x157/0x1e0
[ 3813.841773] Read of size 4096 at addr ffff8883cf80f540 by task syz-executor/1549
[ 3813.846612] Call Trace:
[ 3813.846995]  dump_stack+0x108/0x15f
[ 3813.847524]  print_address_description+0xa5/0x372
[ 3813.848243]  kasan_report.cold+0x236/0x2a8
[ 3813.849439]  check_memory_region+0x240/0x270
[ 3813.850094]  memcpy+0x30/0x80
[ 3813.850553]  sg_copy_buffer+0x157/0x1e0
[ 3813.853032]  sg_copy_from_buffer+0x13/0x20
[ 3813.853660]  fill_from_dev_buffer+0x135/0x370
[ 3813.854329]  resp_readcap16+0x1ac/0x280
[ 3813.856917]  schedule_resp+0x41f/0x1630
[ 3813.858203]  scsi_debug_queuecommand+0xb32/0x17e0
[ 3813.862699]  scsi_dispatch_cmd+0x330/0x950
[ 3813.863329]  scsi_request_fn+0xd8e/0x1710
[ 3813.863946]  __blk_run_queue+0x10b/0x230
[ 3813.864544]  blk_execute_rq_nowait+0x1d8/0x400
[ 3813.865220]  sg_common_write.isra.0+0xe61/0x2420
[ 3813.871637]  sg_write+0x6c8/0xef0
[ 3813.878853]  __vfs_write+0xe4/0x800
[ 3813.883487]  vfs_write+0x17b/0x530
[ 3813.884008]  ksys_write+0x103/0x270
[ 3813.886268]  __x64_sys_write+0x77/0xc0
[ 3813.886841]  do_syscall_64+0x106/0x360
[ 3813.887415]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

This issue can be reproduced with the following syzkaller log:

r0 = openat(0xffffffffffffff9c, &(0x7f0000000040)='./file0\x00', 0x26e1, 0x0)
r1 = syz_open_procfs(0xffffffffffffffff, &(0x7f0000000000)='fd/3\x00')
open_by_handle_at(r1, &(0x7f00000003c0)=ANY=[@ANYRESHEX], 0x602000)
r2 = syz_open_dev$sg(&(0x7f0000000000), 0x0, 0x40782)
write$binfmt_aout(r2, &(0x7f0000000340)=ANY=[@ANYBLOB="00000000deff000000000000000000000000000000000000000000000000000047f007af9e107a41ec395f1bded7be24277a1501ff6196a83366f4e6362bc0ff2b247f68a972989b094b2da4fb3607fcf611a22dd04310d28c75039d"], 0x126)

In resp_readcap16() we get "int alloc_len" value -1104926854, and then pass
the huge arr_len to fill_from_dev_buffer(), but arr is only 32 bytes. This
leads to OOB in sg_copy_buffer().

To solve this issue, define alloc_len as u32.

Link: https://lore.kernel.org/r/20211013033913.2551004-2-yebin10@huawei.com
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>