Re: [PATCH v8 17/27] scsi-multipath: revalidate paths upon device unblock

[John Garry <john.garry@linux.dev>]

On 9/7/26 12:56, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> - [High] Concurrent invocation of device block/unblock during initialization dereferences a NULL scsi_mpath_head.
> - [High] TOCTOU race in scsi_device_block and scsi_internal_device_unblock accessing multipath state outside state_mutex, causing a NULL pointer dereference during device teardown.
> --
> 
> commit d9e004c51ebcdd24da98702a7fb3f90971c90665
> Author: John Garry <john.garry@linux.dev>
> 
>      scsi-multipath: revalidate paths upon device unblock
> 
> This patch adds a revalidation step for multipath devices when a scsi_device
> is unblocked, clearing paths and requeuing any queued bios.
> 
>> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
>> index 56200a7f5c748..1a6f5e1fd80cb 100644
>> --- a/drivers/scsi/scsi_lib.c
>> +++ b/drivers/scsi/scsi_lib.c
>> @@ -3093,6 +3093,8 @@ static int scsi_internal_device_unblock(struct scsi_device *sdev,
>>   	mutex_lock(&sdev->state_mutex);
>>   	ret = scsi_internal_device_unblock_nowait(sdev, new_state);
>>   	mutex_unlock(&sdev->state_mutex);
>> +	if ((ret == 0) && sdev->scsi_mpath_dev)
>> +		scsi_mpath_revalidate_paths(sdev->scsi_mpath_dev);
> 

See response for v7

> [Severity: High]
> Are there race conditions involving multipath state modification outside of
> the sdev->state_mutex?
> 
> It appears a concurrent __scsi_remove_device() could acquire state_mutex,
> transition the device to SDEV_CANCEL or SDEV_DEL, and call
> scsi_mpath_remove_device(). This sets scsi_mpath_dev->scsi_mpath_head to
> NULL. When scsi_internal_device_unblock() resumes, it could call
> scsi_mpath_revalidate_paths() which then attempts to use the NULL pointer.
> 
> Is it also possible for this code to execute on a partially initialized
> multipath device?
> 
> During device discovery, scsi_mpath_dev_alloc() assigns sdev->scsi_mpath_dev
> but initially leaves scsi_mpath_dev->scsi_mpath_head as NULL while
> performing a blocking INQUIRY. If a transport event (like a link flap)
> concurrently triggers scsi_target_block() and scsi_target_unblock(),
> scsi_internal_device_unblock() could be called and operate on the still-NULL
> scsi_mpath_head.
> 
> The same unprotected call pattern appears to exist in scsi_device_block(),
> which was introduced in a prior patch in this series.
> 
> [ ... ]
>> diff --git a/drivers/scsi/scsi_multipath.c b/drivers/scsi/scsi_multipath.c
>> index b16fe6a55fa73..456351fee4b98 100644
>> --- a/drivers/scsi/scsi_multipath.c
>> +++ b/drivers/scsi/scsi_multipath.c
>> @@ -238,6 +238,14 @@ static int scsi_multipath_sdev_init(struct scsi_device *sdev)
>>   	return 0;
>>   }
>>   
>> +void scsi_mpath_revalidate_paths(struct scsi_mpath_device *scsi_mpath_dev)
>> +{
>> +       struct scsi_mpath_head *scsi_mpath_head = scsi_mpath_dev->scsi_mpath_head;
>> +       struct mpath_head *mpath_head = &scsi_mpath_head->mpath_head;
> 
> [Severity: High]
> If scsi_mpath_head is NULL due to the races mentioned above during unblock
> or block operations, will this unconditionally dereference the NULL pointer
> when calculating the offset for mpath_head?
>