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

scsi: core: Fix unremoved procfs host directory regression

Commit fc663711b944 (“scsi: core: Remove the /proc/scsi/${proc_name}
directory earlier”) fixed a bug related to modules loading/unloading, by
adding a call to scsi_proc_hostdir_rm() on scsi_remove_host(). But that led
to a potential duplicate call to the hostdir_rm() routine, since it’s also
called from scsi_host_dev_release(). That triggered a regression report,
which was then fixed by commit be03df3d4bfe (“scsi: core: Fix a procfs host
directory removal regression”). The fix just dropped the hostdir_rm() call
from dev_release().

But it happens that this proc directory is created on scsi_host_alloc(),
and that function “pairs” with scsi_host_dev_release(), while
scsi_remove_host() pairs with scsi_add_host(). In other words, it seems the
reason for removing the proc directory on dev_release() was meant to cover
cases in which a SCSI host structure was allocated, but the call to
scsi_add_host() didn’t happen. And that pattern happens to exist in some
error paths, for example.

Syzkaller causes that by using USB raw gadget device, error’ing on
usb-storage driver, at usb_stor_probe2(). By checking that path, we can see
that the BadDevice label leads to a scsi_host_put() after a SCSI host
allocation, but there’s no call to scsi_add_host() in such path. That leads
to messages like this in dmesg (and a leak of the SCSI host proc
structure):

usb-storage 4-1:87.51: USB Mass Storage device detected
proc_dir_entry ‘scsi/usb-storage’ already registered
WARNING: CPU: 1 PID: 3519 at fs/proc/generic.c:377 proc_register+0x347/0x4e0 fs/proc/generic.c:376

The proper fix seems to still call scsi_proc_hostdir_rm() on dev_release(),
but guard that with the state check for SHOST_CREATED; there is even a
comment in scsi_host_dev_release() detailing that: such conditional is
meant for cases where the SCSI host was allocated but there was no calls to
{add,remove}_host(), like the usb-storage case.

This is what we propose here and with that, the error path of usb-storage
does not trigger the warning anymore.