[PATCH] scsi: scsi_debug: sanity check block descriptor length in resp_mode_select

[George Kennedy <george.kennedy@oracle.com>]

In resp_mode_select() sanity check the block descriptor len to avoid UAF.

BUG: KASAN: use-after-free in resp_mode_select+0xa4c/0xb40 drivers/scsi/scsi_debug.c:2509
Read of size 1 at addr ffff888026670f50 by task scsicmd/15032

CPU: 1 PID: 15032 Comm: scsicmd Not tainted 5.15.0-01d0625 #15
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
Call Trace:
 <TASK>
 dump_stack_lvl+0x89/0xb5 lib/dump_stack.c:107
 print_address_description.constprop.9+0x28/0x160 mm/kasan/report.c:257
 kasan_report.cold.14+0x7d/0x117 mm/kasan/report.c:443
 __asan_report_load1_noabort+0x14/0x20 mm/kasan/report_generic.c:306
 resp_mode_select+0xa4c/0xb40 drivers/scsi/scsi_debug.c:2509
 schedule_resp+0x4af/0x1a10 drivers/scsi/scsi_debug.c:5483
 scsi_debug_queuecommand+0x8c9/0x1e70 drivers/scsi/scsi_debug.c:7537
 scsi_queue_rq+0x16b4/0x2d10 drivers/scsi/scsi_lib.c:1521
 blk_mq_dispatch_rq_list+0xb9b/0x2700 block/blk-mq.c:1640
 __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:1762
 __blk_mq_delay_run_hw_queue+0x4f8/0x5c0 block/blk-mq.c:1839
 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:63
 sg_common_write.isra.18+0xeb3/0x2000 drivers/scsi/sg.c:837
 sg_new_write.isra.19+0x570/0x8c0 drivers/scsi/sg.c:775
 sg_ioctl_common+0x14d6/0x2710 drivers/scsi/sg.c:941
 sg_ioctl+0xa2/0x180 drivers/scsi/sg.c:1166
 __x64_sys_ioctl+0x19d/0x220 fs/ioctl.c:52
 do_syscall_64+0x3a/0x80 arch/x86/entry/common.c:50
 entry_SYSCALL_64_after_hwframe+0x44/0xae arch/x86/entry/entry_64.S:113

Reported-by: syzkaller <syzkaller@googlegroups.com>
Signed-off-by: George Kennedy <george.kennedy@oracle.com>
---
 drivers/scsi/scsi_debug.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index 1d0278d..51e3b57 100644
--- a/drivers/scsi/scsi_debug.c
+++ b/drivers/scsi/scsi_debug.c
@@ -2499,11 +2499,11 @@ static int resp_mode_select(struct scsi_cmnd *scp,
 			    __func__, param_len, res);
 	md_len = mselect6 ? (arr[0] + 1) : (get_unaligned_be16(arr + 0) + 2);
 	bd_len = mselect6 ? arr[3] : get_unaligned_be16(arr + 6);
-	if (md_len > 2) {
+	off = bd_len + (mselect6 ? 4 : 8);
+	if (md_len > 2 || off >= res) {
 		mk_sense_invalid_fld(scp, SDEB_IN_DATA, 0, -1);
 		return check_condition_result;
 	}
-	off = bd_len + (mselect6 ? 4 : 8);
 	mpage = arr[off] & 0x3f;
 	ps = !!(arr[off] & 0x80);
 	if (ps) {
-- 
1.8.3.1