block/iscsi:fix heap-buffer-overflow in iscsi_aio_ioctl_cb

There is an overflow, the source 'datain.data[2]' is 100 bytes,
 but the 'ss' is 252 bytes.This may cause a security issue because
 we can access a lot of unrelated memory data.

The len for sbp copy data should take the minimum of mx_sb_len and
 sb_len_wr, not the maximum.

If we use iscsi device for VM backend storage, ASAN show stack:

READ of size 252 at 0xfffd149dcfc4 thread T0
    #0 0xaaad433d0d34 in __asan_memcpy (aarch64-softmmu/qemu-system-aarch64+0x2cb0d34)
    #1 0xaaad45f9d6d0 in iscsi_aio_ioctl_cb /qemu/block/iscsi.c:996:9
    #2 0xfffd1af0e2dc  (/usr/lib64/iscsi/libiscsi.so.8+0xe2dc)
    #3 0xfffd1af0d174  (/usr/lib64/iscsi/libiscsi.so.8+0xd174)
    #4 0xfffd1af19fac  (/usr/lib64/iscsi/libiscsi.so.8+0x19fac)
    qemu#5 0xaaad45f9acc8 in iscsi_process_read /qemu/block/iscsi.c:403:5
    qemu#6 0xaaad4623733c in aio_dispatch_handler /qemu/util/aio-posix.c:467:9
    qemu#7 0xaaad4622f350 in aio_dispatch_handlers /qemu/util/aio-posix.c:510:20
    qemu#8 0xaaad4622f350 in aio_dispatch /qemu/util/aio-posix.c:520
    qemu#9 0xaaad46215944 in aio_ctx_dispatch /qemu/util/async.c:298:5
    qemu#10 0xfffd1bed12f4 in g_main_context_dispatch (/lib64/libglib-2.0.so.0+0x512f4)
    qemu#11 0xaaad46227de0 in glib_pollfds_poll /qemu/util/main-loop.c:219:9
    qemu#12 0xaaad46227de0 in os_host_main_loop_wait /qemu/util/main-loop.c:242
    qemu#13 0xaaad46227de0 in main_loop_wait /qemu/util/main-loop.c:518
    qemu#14 0xaaad43d9d60c in qemu_main_loop /qemu/softmmu/vl.c:1662:9
    qemu#15 0xaaad4607a5b0 in main /qemu/softmmu/main.c:49:5
    qemu#16 0xfffd1a460b9c in __libc_start_main (/lib64/libc.so.6+0x20b9c)
    qemu#17 0xaaad43320740 in _start (aarch64-softmmu/qemu-system-aarch64+0x2c00740)

0xfffd149dcfc4 is located 0 bytes to the right of 100-byte region [0xfffd149dcf60,0xfffd149dcfc4)
allocated by thread T0 here:
    #0 0xaaad433d1e70 in __interceptor_malloc (aarch64-softmmu/qemu-system-aarch64+0x2cb1e70)
    #1 0xfffd1af0e254  (/usr/lib64/iscsi/libiscsi.so.8+0xe254)
    #2 0xfffd1af0d174  (/usr/lib64/iscsi/libiscsi.so.8+0xd174)
    #3 0xfffd1af19fac  (/usr/lib64/iscsi/libiscsi.so.8+0x19fac)
    #4 0xaaad45f9acc8 in iscsi_process_read /qemu/block/iscsi.c:403:5
    qemu#5 0xaaad4623733c in aio_dispatch_handler /qemu/util/aio-posix.c:467:9
    qemu#6 0xaaad4622f350 in aio_dispatch_handlers /qemu/util/aio-posix.c:510:20
    qemu#7 0xaaad4622f350 in aio_dispatch /qemu/util/aio-posix.c:520
    qemu#8 0xaaad46215944 in aio_ctx_dispatch /qemu/util/async.c:298:5
    qemu#9 0xfffd1bed12f4 in g_main_context_dispatch (/lib64/libglib-2.0.so.0+0x512f4)
    qemu#10 0xaaad46227de0 in glib_pollfds_poll /qemu/util/main-loop.c:219:9
    qemu#11 0xaaad46227de0 in os_host_main_loop_wait /qemu/util/main-loop.c:242
    qemu#12 0xaaad46227de0 in main_loop_wait /qemu/util/main-loop.c:518
    qemu#13 0xaaad43d9d60c in qemu_main_loop /qemu/softmmu/vl.c:1662:9
    qemu#14 0xaaad4607a5b0 in main /qemu/softmmu/main.c:49:5
    qemu#15 0xfffd1a460b9c in __libc_start_main (/lib64/libc.so.6+0x20b9c)
    qemu#16 0xaaad43320740 in _start (aarch64-softmmu/qemu-system-aarch64+0x2c00740)

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20200418062602.10776-1-kuhn.chenqun@huawei.com
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

block/iscsi.c

@@ -991,8 +991,7 @@ iscsi_aio_ioctl_cb(struct iscsi_context *iscsi, int status,
         acb->ioh->driver_status |= SG_ERR_DRIVER_SENSE;
 
         acb->ioh->sb_len_wr = acb->task->datain.size - 2;
-        ss = (acb->ioh->mx_sb_len >= acb->ioh->sb_len_wr) ?
-             acb->ioh->mx_sb_len : acb->ioh->sb_len_wr;
+        ss = MIN(acb->ioh->mx_sb_len, acb->ioh->sb_len_wr);
         memcpy(acb->ioh->sbp, &acb->task->datain.data[2], ss);
     }