Re: [PATCH v2] scsi: sg: Avoid sg device teardown race

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On 20.03.24 16:02, Bart Van Assche wrote:
On 3/20/24 04:08, Alexander Wetzel wrote:
sg_remove_sfp_usercontext() must not use sg_device_destroy() after
calling scsi_device_put().

sg_device_destroy() is accessing the parent scsi device request_queue.
Which will already be set to NULL when the preceding call to
scsi_device_put() removed the last reference to the parent scsi device.

The resulting NULL pointer exception will then crash the kernel.

Link: https://lore.kernel.org/r/20240305150509.23896-1-Alexander@xxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Alexander Wetzel <Alexander@xxxxxxxxxxxxxx>
---
Changes compared to V1:
Reworked the commit message

Alexander
---
  drivers/scsi/sg.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 86210e4dd0d3..80e0d1981191 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -2232,8 +2232,8 @@ sg_remove_sfp_usercontext(struct work_struct *work)
              "sg_remove_sfp: sfp=0x%p\n", sfp));
      kfree(sfp);
-    scsi_device_put(sdp->device);
      kref_put(&sdp->d_ref, sg_device_destroy);
+    scsi_device_put(sdp->device);
      module_put(THIS_MODULE);
  }

Is it guaranteed that the above kref_put() call is the last kref_put()
call on sdp->d_ref? If not, how about inserting code between the
kref_put() call and the scsi_device_put() call that waits until
sg_device_destroy() has finished?


While I'm not familiar with the code, I'm pretty sure kref_put() is removing the last reference to d_ref here. Anything else would be odd, based on my - really sketchy - understanding of the flows.

Also waiting for another process looks wrong. I guess we would then have to delay the call to sg_release().

And at least for me it's always the last d_ref reference.
I changed the section to:

        kref_put(&sdp->d_ref, sg_device_destroy);
        printk("XXXX scsi=%u, dref=%u\n", \
		kref_read(&sdp->device->sdev_gendev.kobj.kref), \
		kref_read(&sdp->d_ref));
        scsi_device_put(sdp->device);

And connected/disconnected my test USB device a few times:
 XXXX scsi=2, dref=0
 XXXX scsi=1, dref=0
 XXXX scsi=2, dref=0
 XXXX scsi=1, dref=0
 XXXX scsi=1, dref=0
 XXXX scsi=1, dref=0
 XXXX scsi=1, dref=0
 XXXX scsi=1, dref=0
 XXXX scsi=1, dref=0
 XXXX scsi=1, dref=0

(scsi=1 are the cases which would cause the NULL pointer exceptions with the unpatched driver.)

Alexander





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