[PATCH] xen/scsiback: unbind the IRQ before draining in-flight requests
From: Yehyeong Lee
Date: Mon Nov 09 2026 - 07:34:47 EST
scsiback_disconnect() drains the in-flight count before stopping the only
thing that can raise it. scsiback_cmd_exec() (xen-scsiback.c:426) runs in
the backend's threaded IRQ handler, so a request can be admitted just after
wait_event() has seen zero, and its response is written later from target
core context by scsiback_send_response() (:347) - after the ring has been
unmapped, and on the scsiback_remove() path (:1268-1280) after kfree(info).
xen-blkback and xen-netback stop their request processing first:
kthread_stop() before reading ring->inflight
(xen-blkback/xenbus.c:290-304), and the kthreads and napi before
unbinding and unmapping (xen-netback/interface.c:673-700).
Unbind and clear info->irq before the drain: nothing is admitted once the
handler is gone, and the ring stays mapped until the drain finishes. The
notify at the end of scsiback_send_response() is then a no-op, because
notify_remote_via_irq() resolves irq 0 through evtchn_from_irq() to port 0,
which VALID_EVTCHN() rejects (events_base.c:539-545, :411-421).
Reproduced in a nested-Xen lab with KASAN. Reaching the window needed two
test-only usleep_range() delays, one after wait_event() returns and one at
the top of scsiback_do_resp_with_sense() once the teardown has started.
With both off neither arm produced anything in 4 rounds each; with both on
the unpatched driver crashed in 6 of 6 valid cells, the patched in 0 of 7:
[ 168.546709] BUG: KASAN: vmalloc-out-of-bounds in scsiback_send_response+0x516/0x5d0
[ 168.551012] Write of size 4 at addr ffffc900000230a4 by task kworker/0:1/11
[ 168.554757]
[ 168.555908] CPU: 0 UID: 0 PID: 11 Comm: kworker/0:1 Not tainted 7.3.0-kCTLblk2-15482-g21a7f44fd076-dirty #10 PREEMPT(full)
[ 168.555914] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[ 168.555918] Workqueue: target_completion target_complete_ok_work
[ 168.555939] Call Trace:
[ 168.555946] <TASK>
[ 168.555949] dump_stack_lvl+0x64/0x80
[ 168.555958] print_report+0xd0/0x630
[ 168.555966] ? __pfx__raw_spin_lock_irqsave+0x10/0x10
[ 168.555978] ? scsiback_send_response+0x516/0x5d0
[ 168.555983] kasan_report+0xce/0x100
[ 168.555988] ? scsiback_send_response+0x516/0x5d0
[ 168.555992] scsiback_send_response+0x516/0x5d0
[ 168.555997] ? __pfx_scsiback_send_response+0x10/0x10
[ 168.556001] scsiback_do_resp_with_sense+0x87/0x230
[ 168.556007] scsiback_cmd_done+0xc0/0x290
[ 168.556011] ? update_cfs_rq_load_avg+0x5a/0x560
[ 168.556023] scsiback_queue_status+0xd3/0x100
[ 168.556028] target_complete_ok_work+0x467/0xc30
[ 168.556032] ? __pfx_target_complete_ok_work+0x10/0x10
[ 168.556036] ? put_prev_task_fair+0x44d/0xa30
[ 168.556039] ? finish_task_switch.isra.0+0x1d9/0xa70
[ 168.556047] process_one_work+0x697/0x1060
[ 168.556052] ? __pfx___schedule+0x10/0x10
[ 168.556058] ? __pfx_process_one_work+0x10/0x10
[ 168.556061] ? _raw_spin_lock_irq+0x86/0xe0
[ 168.556065] ? __pfx_target_complete_ok_work+0x10/0x10
[ 168.556069] ? assign_work+0x11d/0x370
[ 168.556074] worker_thread+0x45b/0xd10
[ 168.556078] ? __pfx_worker_thread+0x10/0x10
[ 168.556081] ? __pfx_worker_thread+0x10/0x10
[ 168.556083] kthread+0x2c8/0x3b0
[ 168.556090] ? recalc_sigpending+0x15c/0x1e0
[ 168.556094] ? __pfx_kthread+0x10/0x10
[ 168.556098] ret_from_fork+0x373/0x5b0
[ 168.556104] ? __pfx_ret_from_fork+0x10/0x10
[ 168.556107] ? __switch_to+0x3c4/0xb40
[ 168.556115] ? __pfx_kthread+0x10/0x10
[ 168.556120] ret_from_fork_asm+0x1a/0x30
[ 168.556127] </TASK>
[ 168.556129]
[ 168.639704] The buggy address belongs to a vmalloc virtual mapping
[ 168.642324] Memory state around the buggy address:
[ 168.644439] ffffc90000022f80: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
[ 168.647698] ffffc90000023000: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
[ 168.650951] >ffffc90000023080: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
[ 168.669199] ^
[ 168.671056] ffffc90000023100: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
[ 168.674050] ffffc90000023180: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
[ 168.676974] ==================================================================
[ 168.682117] BUG: unable to handle page fault for address: ffffc900000230a4
[ 168.684784] #PF: supervisor write access in kernel mode
[ 168.686797] #PF: error_code(0x0002) - not-present page
[ 168.688797] PGD 100000067 P4D 100000067 PUD 10019c067 PMD 1001a1067 PTE 0
[ 168.691319] Oops: Oops: 0002 [#1] SMP KASAN NOPTI
[ 168.693246] CPU: 0 UID: 0 PID: 11 Comm: kworker/0:1 Tainted: G B 7.3.0-kCTLblk2-15482-g21a7f44fd076-dirty #10 PREEMPT(full)
[ 168.699142] Tainted: [B]=BAD_PAGE
[ 168.700896] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[ 168.705056] Workqueue: target_completion target_complete_ok_work
[ 168.707701] RIP: 0010:scsiback_send_response+0x1b6/0x5d0
[ 168.710147] Code: 48 89 f8 83 e0 07 83 c0 03 40 38 f0 7c 09 40 84 f6 0f 85 60 03 00 00 48 89 d8 48 c1 e0 06 48 29 d8 48 c1 e0 02 48 8d 7c 01 40 <89> 94 01 a4 00 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1
[ 168.718883] RSP: 0018:ffffc900000bfaa0 EFLAGS: 00010046
[ 168.721447] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffc90000023000
[ 168.724785] RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffffc90000023040
[ 168.727913] RBP: ffff88810a4a7238 R08: 0000000000000246 R09: fffffbfff0de2638
[ 168.730938] R10: ffffffff86f131c7 R11: ffff888100077320 R12: 1ffff92000017f59
[ 168.733902] R13: 0000000000000000 R14: 0000000000000000 R15: ffff88810a6de200
[ 168.736874] FS: 0000000000000000(0000) GS:ffff8881b034d000(0000) knlGS:0000000000000000
[ 168.740171] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 168.742490] CR2: ffffc900000230a4 CR3: 0000000103271005 CR4: 0000000000770ef0
[ 168.745350] PKRU: 55555554
[ 168.746577] Call Trace:
[ 168.747772] <TASK>
[ 168.748872] ? __pfx_scsiback_send_response+0x10/0x10
[ 168.751022] scsiback_do_resp_with_sense+0x87/0x230
[ 168.753109] scsiback_cmd_done+0xc0/0x290
[ 168.754847] ? update_cfs_rq_load_avg+0x5a/0x560
[ 168.756782] scsiback_queue_status+0xd3/0x100
[ 168.758630] target_complete_ok_work+0x467/0xc30
[ 168.760556] ? __pfx_target_complete_ok_work+0x10/0x10
[ 168.762644] ? put_prev_task_fair+0x44d/0xa30
[ 168.764512] ? finish_task_switch.isra.0+0x1d9/0xa70
[ 168.766584] process_one_work+0x697/0x1060
[ 168.768282] ? __pfx___schedule+0x10/0x10
[ 168.769973] ? __pfx_process_one_work+0x10/0x10
[ 168.771880] ? _raw_spin_lock_irq+0x86/0xe0
[ 168.773599] ? __pfx_target_complete_ok_work+0x10/0x10
[ 168.775721] ? assign_work+0x11d/0x370
[ 168.777324] worker_thread+0x45b/0xd10
[ 168.778899] ? __pfx_worker_thread+0x10/0x10
[ 168.780648] ? __pfx_worker_thread+0x10/0x10
[ 168.782415] kthread+0x2c8/0x3b0
[ 168.783877] ? recalc_sigpending+0x15c/0x1e0
[ 168.785697] ? __pfx_kthread+0x10/0x10
[ 168.787218] ret_from_fork+0x373/0x5b0
[ 168.788752] ? __pfx_ret_from_fork+0x10/0x10
[ 168.790500] ? __switch_to+0x3c4/0xb40
[ 168.792029] ? __pfx_kthread+0x10/0x10
[ 168.793555] ret_from_fork_asm+0x1a/0x30
[ 168.795180] </TASK>
[ 168.796205] Modules linked in:
[ 168.797596] CR2: ffffc900000230a4
[ 168.799084] ---[ end trace 0000000000000000 ]---
Fixes: d9d660f6e562 ("xen-scsiback: Add Xen PV SCSI backend driver")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Yehyeong Lee <yhlee@xxxxxxxxxxxxxxxxxx>
---
Reported to the Xen security team first; xen-scsiback is not security supported, so posting here as suggested.
drivers/xen/xen-scsiback.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/drivers/xen/xen-scsiback.c b/drivers/xen/xen-scsiback.c
index c7036e0e41bda1..647a3c7e2a2b39 100644
--- a/drivers/xen/xen-scsiback.c
+++ b/drivers/xen/xen-scsiback.c
@@ -603,11 +603,13 @@ static int scsiback_gnttab_data_map(struct vscsiif_request *ring_req,
static void scsiback_disconnect(struct vscsibk_info *info)
{
+ /* Stop admitting requests before draining the in-flight ones. */
+ unbind_from_irqhandler(info->irq, info);
+ info->irq = 0;
+
wait_event(info->waiting_to_free,
atomic_read(&info->nr_unreplied_reqs) == 0);
- unbind_from_irqhandler(info->irq, info);
- info->irq = 0;
xenbus_unmap_ring_vfree(info->dev, info->ring.sring);
}
base-commit: 7475ea8b58616b33c04afb976cf0fbaad9dd63fb
--
2.43.0