vhost-scsi: reject feature changes after endpoint

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

vhost-scsi: reject feature changes after endpoint

vhost_scsi_setup_vq_cmds() runs from VHOST_SCSI_SET_ENDPOINT and allocates
each command's protection scatterlist array (prot_sgl) according to the
acknowledged VIRTIO_SCSI_F_T10_PI bit. The command pools are not rebuilt
when VHOST_SET_FEATURES changes that bit later.

Although virtio feature bits must not change after feature negotiation,
vhost_scsi_set_features() currently accepts such a request after the
endpoint is active and updates acked_features. Enabling T10-PI after
endpoint setup therefore leaves prot_sgl NULL while the I/O path follows
the new feature bit.

For a 129-page protection payload, vhost_scsi_mapal() passes the missing
first chunk to sg_alloc_table_chained():

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

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 = 0x4
RSP: 0018:ffffc90000dbf940 EFLAGS: 00010202
RAX: ffffffff82396810 RBX: ffff88811dc28b80 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000820 RDI: 0000000000000081

VHOST_F_LOG_ALL is a vhost-specific runtime feature and remains the only
exception.

Reject changes to any feature other than VHOST_F_LOG_ALL while the
endpoint is active. This preserves the existing runtime log toggle while
preventing feature-dependent command resources and data-path state from
becoming inconsistent. Userspace must clear the endpoint before changing
any other negotiated feature and set the endpoint up again afterward.