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@wetzel-home.de Cc: stable@vger.kernel.org Signed-off-by: Alexander Wetzel Alexander@wetzel-home.de
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