[PATCH] vhost-scsi: fix T10-PI lifecycle hard BUG after endpoint

[Jia Jia <physicalmtea@gmail.com>]

vhost_scsi_setup_vq_cmds() runs only from VHOST_SCSI_SET_ENDPOINT and
allocates each command's protection scatterlist array (prot_sgl) only
when VIRTIO_SCSI_F_T10_PI is already set at that moment.  Command pools
are not rebuilt later.  vhost_scsi_set_features() may still flip that
bit after the endpoint is live: it updates acked_features and returns
success without reallocating prot_sgl.

That is a feature-versus-resource lifetime mismatch.  The broken order
is:

  VHOST_SET_FEATURES(0)
  VHOST_SCSI_SET_ENDPOINT
      setup_vq_cmds() sees no T10-PI -> prot_sgl stays NULL
  VHOST_SET_FEATURES(VIRTIO_SCSI_F_T10_PI)
      only acked_features changes; command resources stay as above
  submit a T10-PI WRITE with a 129-page protection payload

The I/O path then follows the new feature bit while using the old
command objects.  vhost_scsi_mapal() (static, inlined into
vhost_scsi_handle_vq() here) does:

  sg_alloc_table_chained(table, 129, first_chunk=NULL,
                         nents_first_chunk=inline_sg_cnt)

With first_chunk NULL, __sg_alloc_table() sets curr_max_ents from
nents_first_chunk and calls sg_pool_alloc(129).  sg_pool_index() then
hits:

  BUG_ON(nents > SG_CHUNK_SIZE);   /* 129 > 128 */

The kernel reported the following call trace and register state:

  Call Trace:
   <TASK>
   ? __sg_alloc_table+0x1d8/0x250
   ? __pfx_vhost_run_work_list+0x10/0x10 [vhost]
   sg_alloc_table_chained+0x59/0xf0
   ? __pfx_sg_pool_alloc+0x10/0x10
   ? vhost_scsi_calc_sgls.constprop.0+0x43/0x60 [vhost_scsi]
   vhost_scsi_handle_vq+0xf02/0x1700 [vhost_scsi]
   ? __pfx_vhost_scsi_handle_vq+0x10/0x10 [vhost_scsi]
   vhost_scsi_handle_kick+0x37/0x50 [vhost_scsi]
   vhost_run_work_list+0x8e/0xd0 [vhost]
   vhost_task_fn+0xe1/0x210
   ret_from_fork+0x348/0x540
   </TASK>

  RIP: 0010:0x4
  CR2: 0000000000000004
  RSP: 0018:ffffc90000dbf940 EFLAGS: 00010202
  RAX: ffffffff82396810 RBX: ffff88811dc28b80 RCX: 0000000000000000
  RDX: 0000000000000000 RSI: 0000000000000820 RDI: 0000000000000081

The host worker dies and the machine stops accepting network service
until reset.  The missing object is prot_sgl after a post-endpoint
T10-PI enable introduced by commit bf2d650391be ("vhost-scsi: Allocate
T10 PI structs only when enabled").

I also checked the other feature bits handled around endpoint setup.
Only T10-PI allocates this endpoint-time per-command resource and does
not rebuild it on a later SET_FEATURES.  LOG_ALL uses lazy tvc_log and
already tears log storage down when cleared; HOTPLUG does not allocate
prot_sgl.  So the chosen fix is to refuse T10-PI enable/disable while
an endpoint is active, rather than adding a second rebuild path in
set_features().

Return -EBUSY if vs->vs_tpg is set and the T10-PI bit would change.
Userspace must CLEAR_ENDPOINT, SET_FEATURES, then SET_ENDPOINT again so
setup_vq_cmds() can allocate protection SGLs when needed.  That matches
the existing "build command resources at endpoint" model.

Fixes: bf2d650391be ("vhost-scsi: Allocate T10 PI structs only when enabled")
Signed-off-by: Jia Jia <physicalmtea@gmail.com>
---
 drivers/vhost/scsi.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
index 9a1253b9d8c5..000000000000 100644
--- a/drivers/vhost/scsi.c
+++ b/drivers/vhost/scsi.c
@@ -2219,6 +2219,7 @@ static int vhost_scsi_set_features(struct vhost_scsi *vs, u64 features)
 {
 	struct vhost_virtqueue *vq;
 	bool is_log, was_log;
+	bool is_t10_pi, was_t10_pi;
 	int i;
 
 	if (features & ~VHOST_SCSI_FEATURES)
@@ -2234,6 +2235,13 @@ static int vhost_scsi_set_features(struct vhost_scsi *vs, u64 features)
 	if (!vs->dev.nvqs)
 		goto out;
 
+	is_t10_pi = features & (1ULL << VIRTIO_SCSI_F_T10_PI);
+	was_t10_pi = vhost_has_feature(&vs->vqs[0].vq, VIRTIO_SCSI_F_T10_PI);
+	if (vs->vs_tpg && is_t10_pi != was_t10_pi) {
+		mutex_unlock(&vs->dev.mutex);
+		return -EBUSY;
+	}
+
 	is_log = features & (1 << VHOST_F_LOG_ALL);
 	/*
 	 * All VQs should have same feature.
-- 
2.43.0