[PATCH] block: break circular locks in blk_request_module
Desmond Cheong Zhi Xi
desmondcheongzx at gmail.com
Thu Jun 17 08:23:36 PDT 2021
On 17/6/21 7:51 pm, Christoph Hellwig wrote:
> On Thu, Jun 17, 2021 at 05:20:16PM +0800, Desmond Cheong Zhi Xi wrote:
>> mutex_lock(&major_names_lock);
>> for (n = &major_names[major_to_index(major)]; *n; n = &(*n)->next) {
>> if ((*n)->major == major && (*n)->probe) {
>> - (*n)->probe(devt);
>> + probe = (*n)->probe;
>> mutex_unlock(&major_names_lock);
>> + probe(devt);
>
> And now you can all probe after it has been freed and/or the module has
> been unloaded. The obviously correct fix is to only hold mtd_table_mutex
> for the actually required critical section:
>
Thank you for the correction, Christoph. I hadn't thought of the
scenario where the module is unloaded. I'll be more conscientious in the
future.
> diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
> index fb8e12d590a1..065d94f9b1fb 100644
> --- a/drivers/mtd/mtd_blkdevs.c
> +++ b/drivers/mtd/mtd_blkdevs.c
> @@ -529,13 +529,11 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
> register_mtd_user(&blktrans_notifier);
>
>
> - mutex_lock(&mtd_table_mutex);
>
> ret = register_blkdev(tr->major, tr->name);
> if (ret < 0) {
> printk(KERN_WARNING "Unable to register %s block device on major %d: %d\n",
> tr->name, tr->major, ret);
> - mutex_unlock(&mtd_table_mutex);
> return ret;
> }
>
> @@ -545,12 +543,12 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
> tr->blkshift = ffs(tr->blksize) - 1;
>
> INIT_LIST_HEAD(&tr->devs);
> - list_add(&tr->list, &blktrans_majors);
>
> + mutex_lock(&mtd_table_mutex);
> + list_add(&tr->list, &blktrans_majors);
> mtd_for_each_device(mtd)
> if (mtd->type != MTD_ABSENT)
> tr->add_mtd(tr, mtd);
> -
> mutex_unlock(&mtd_table_mutex);
> return 0;
> }
>
This fix passes the Syzkaller repro test on my local machine and on
Syzbot. I can prepare a v2 patch for this. May I include you with the
Co-developed-by: and Signed-off-by: tags? If another tag would be more
appropriate, or if you want to submit the patch yourself, please let me
know.
Best wishes,
Desmond
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