scsi: scsi_debug: Don't call kcalloc() if size arg is zero

In the Linux kernel, the following vulnerability has been resolved:

scsi: scsi_debug: Don't call kcalloc() if size arg is zero

If the size arg to kcalloc() is zero, it returns ZERO_SIZE_PTR. Because of

that, for a following NULL pointer check to work on the returned pointer,

kcalloc() must not be called with the size arg equal to zero. Return early

without error before the kcalloc() call if size arg is zero.

BUG: KASAN: null-ptr-deref in memcpy include/linux/fortify-string.h:191 [inline]

BUG: KASAN: null-ptr-deref in sg_copy_buffer+0x138/0x240 lib/scatterlist.c:974

Write of size 4 at addr 0000000000000010 by task syz-executor.1/22789

CPU: 1 PID: 22789 Comm: syz-executor.1 Not tainted 5.15.0-syzk #1

Hardware name: Red Hat KVM, BIOS 1.13.0-2

Call Trace:

__dump_stack lib/dump_stack.c:88 [inline]

dump_stack_lvl+0x89/0xb5 lib/dump_stack.c:106

__kasan_report mm/kasan/report.c:446 [inline]

kasan_report.cold.14+0x112/0x117 mm/kasan/report.c:459

check_region_inline mm/kasan/generic.c:183 [inline]

kasan_check_range+0x1a3/0x210 mm/kasan/generic.c:189

memcpy+0x3b/0x60 mm/kasan/shadow.c:66

memcpy include/linux/fortify-string.h:191 [inline]

sg_copy_buffer+0x138/0x240 lib/scatterlist.c:974

do_dout_fetch drivers/scsi/scsi_debug.c:2954 [inline]

do_dout_fetch drivers/scsi/scsi_debug.c:2946 [inline]

resp_verify+0x49e/0x930 drivers/scsi/scsi_debug.c:4276

schedule_resp+0x4d8/0x1a70 drivers/scsi/scsi_debug.c:5478

scsi_debug_queuecommand+0x8c9/0x1ec0 drivers/scsi/scsi_debug.c:7533

scsi_dispatch_cmd drivers/scsi/scsi_lib.c:1520 [inline]

scsi_queue_rq+0x16b0/0x2d40 drivers/scsi/scsi_lib.c:1699

blk_mq_dispatch_rq_list+0xb9b/0x2700 block/blk-mq.c:1639

__blk_mq_sched_dispatch_requests+0x28f/0x590 block/blk-mq-sched.c:325

blk_mq_sched_dispatch_requests+0x105/0x190 block/blk-mq-sched.c:358

__blk_mq_run_hw_queue+0xe5/0x150 block/blk-mq.c:1761

__blk_mq_delay_run_hw_queue+0x4f8/0x5c0 block/blk-mq.c:1838

blk_mq_run_hw_queue+0x18d/0x350 block/blk-mq.c:1891

blk_mq_sched_insert_request+0x3db/0x4e0 block/blk-mq-sched.c:474

blk_execute_rq_nowait+0x16b/0x1c0 block/blk-exec.c:62

blk_execute_rq+0xdb/0x360 block/blk-exec.c:102

sg_scsi_ioctl drivers/scsi/scsi_ioctl.c:621 [inline]

scsi_ioctl+0x8bb/0x15c0 drivers/scsi/scsi_ioctl.c:930

sg_ioctl_common+0x172d/0x2710 drivers/scsi/sg.c:1112

sg_ioctl+0xa2/0x180 drivers/scsi/sg.c:1165

vfs_ioctl fs/ioctl.c:51 [inline]

__do_sys_ioctl fs/ioctl.c:874 [inline]

__se_sys_ioctl fs/ioctl.c:860 [inline]

__x64_sys_ioctl+0x19d/0x220 fs/ioctl.c:860

do_syscall_x64 arch/x86/entry/common.c:50 [inline]

do_syscall_64+0x3a/0x80 arch/x86/entry/common.c:80

entry_SYSCALL_64_after_hwframe+0x44/0xae