Hi Tetsuo,
I might sound like a broken record, but we need to reduce the locking
complexity, not make it worse and fall down the locking cliff. I did
send a suggested series this morning, in which you found a bunch of
bugs. I'd appreciate it if you could use your superior skills to
actually help explain the issue and arrive at a common solution that
actually simplifies things instead of steaming down the locking cliff
full speed. Thank you very much.
On Wed, Aug 18, 2021 at 08:07:32PM +0900, Tetsuo Handa wrote:
> syzbot is reporting circular locking problem at __loop_clr_fd() [1], for
> commit a160c6159d4a0cf8 ("block: add an optional probe callback to
> major_names") is calling the module's probe function with major_names_lock
> held.
>
> When copying content of /proc/devices to another file via sendfile(),
>
> sb_writers#$N => &p->lock => major_names_lock
>
> dependency is recorded.
>
> When loop_process_work() from WQ context performs a write request,
>
> (wq_completion)loop$M => (work_completion)&lo->rootcg_work =>
> sb_writers#$N
>
> dependency is recorded.
>
> When flush_workqueue() from drain_workqueue() from destroy_workqueue()
> from __loop_clr_fd() from blkdev_put() from blkdev_close() from __fput()
> is called,
>
> &disk->open_mutex => &lo->lo_mutex => (wq_completion)loop$M
>
> dependency is recorded.
>
> When loop_control_remove() from loop_control_ioctl(LOOP_CTL_REMOVE) is
> called,
>
> loop_ctl_mutex => &lo->lo_mutex
>
> dependency is recorded.
>
> As a result, lockdep thinks that there is
>
> loop_ctl_mutex => &lo->lo_mutex => (wq_completion)loop$M =>
> (work_completion)&lo->rootcg_work => sb_writers#$N => &p->lock =>
> major_names_lock
>
> dependency chain.
>
> Then, if loop_add() from loop_probe() from blk_request_module() from
> blkdev_get_no_open() from blkdev_get_by_dev() from blkdev_open() from
> do_dentry_open() from path_openat() from do_filp_open() is called,
>
> major_names_lock => loop_ctl_mutex
>
> dependency is appended to the dependency chain.
>
> There would be two approaches for breaking this circular dependency.
> One is to kill loop_ctl_mutex => &lo->lo_mutex chain. The other is to
> kill major_names_lock => loop_ctl_mutex chain. This patch implements
> the latter, due to the following reasons.
>
> (1) sb_writers#$N => &p->lock => major_names_lock chain is unavoidable
>
> (2) this patch can also fix similar problem in other modules [2] which
> is caused by calling the probe function with major_names_lock held
>
> (3) I believe that this patch is principally safer than e.g.
> commit bd5c39edad535d9f ("loop: reduce loop_ctl_mutex coverage in
> loop_exit") which waits until the probe function finishes using
> global mutex in order to fix deadlock reproducible by sleep
> injection [3]
>
> This patch adds THIS_MODULE parameter to __register_blkdev() as with
> usb_register(), and drops major_names_lock before calling probe function
> by holding a reference to that module which contains that probe function.
>
> Since cdev uses register_chrdev() and __register_chrdev(), bdev should be
> able to preserve register_blkdev() and __register_blkdev() naming scheme.
> Thus, use register_blkdev_with_probe() as an inline function for
> automagically appending THIS_MODULE parameter.
>
> Link: https://syzkaller.appspot.com/bug?id=7bb10e8b62f83e4d445cdf4c13d69e407e629558 [1]
> Link: https://syzkaller.appspot.com/bug?id=7bd106c28e846d1023d4ca915718b1a0905444cb [2]
> Link: https://lkml.kernel.org/r/c4edf07f-92e1-a350-2743-f0b0234a2b6c@xxxxxxxxxxxxxxxxxxx [3]
> Reported-by: syzbot <syzbot+f61766d5763f9e7a118f@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Reported-by: syzbot <syzbot+6a8a0d93c91e8fbf2e80@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Tested-by: syzbot <syzbot+f61766d5763f9e7a118f@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Fixes: a160c6159d4a0cf8 ("block: add an optional probe callback to major_names")
> ---
> block/genhd.c | 33 ++++++++++++++++++++++++++-------
> include/linux/genhd.h | 11 +++++++++--
> 2 files changed, 35 insertions(+), 9 deletions(-)
>
> diff --git a/block/genhd.c b/block/genhd.c
> index 298ee78c1bda..a3e2e5666457 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -162,6 +162,7 @@ static struct blk_major_name {
> int major;
> char name[16];
> void (*probe)(dev_t devt);
> + struct module *owner;
> } *major_names[BLKDEV_MAJOR_HASH_SIZE];
> static DEFINE_MUTEX(major_names_lock);
>
> @@ -190,7 +191,8 @@ void blkdev_show(struct seq_file *seqf, off_t offset)
> * @major: the requested major device number [1..BLKDEV_MAJOR_MAX-1]. If
> * @major = 0, try to allocate any unused major number.
> * @name: the name of the new block device as a zero terminated string
> - * @probe: allback that is called on access to any minor number of @major
> + * @probe: callback that is called on access to any minor number of @major
> + * @owner: THIS_MODULE if @probe is not NULL, ignored if @probe is NULL.
> *
> * The @name must be unique within the system.
> *
> @@ -207,8 +209,9 @@ void blkdev_show(struct seq_file *seqf, off_t offset)
> *
> * Use register_blkdev instead for any new code.
> */
> +#undef __register_blkdev
> int __register_blkdev(unsigned int major, const char *name,
> - void (*probe)(dev_t devt))
> + void (*probe)(dev_t devt), struct module *owner)
> {
> struct blk_major_name **n, *p;
> int index, ret = 0;
> @@ -248,6 +251,7 @@ int __register_blkdev(unsigned int major, const char *name,
>
> p->major = major;
> p->probe = probe;
> + p->owner = owner;
> strlcpy(p->name, name, sizeof(p->name));
> p->next = NULL;
> index = major_to_index(major);
> @@ -653,14 +657,29 @@ void blk_request_module(dev_t devt)
> {
> unsigned int major = MAJOR(devt);
> struct blk_major_name **n;
> + void (*probe_fn)(dev_t devt);
>
> 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);
> - mutex_unlock(&major_names_lock);
> - return;
> - }
> + if ((*n)->major != major || !(*n)->probe)
> + continue;
> + if (!try_module_get((*n)->owner))
> + break;
> + /*
> + * Calling probe function with major_names_lock held causes
> + * circular locking dependency problem. Thus, call it after
> + * releasing major_names_lock.
> + */
> + probe_fn = (*n)->probe;
> + mutex_unlock(&major_names_lock);
> + /*
> + * Assuming that unregister_blkdev() is called from module's
> + * __exit function, a module refcount taken above allows us
> + * to safely call probe function without major_names_lock held.
> + */
> + probe_fn(devt);
> + module_put((*n)->owner);
> + return;
> }
> mutex_unlock(&major_names_lock);
>
> diff --git a/include/linux/genhd.h b/include/linux/genhd.h
> index 13b34177cc85..2ed856616d47 100644
> --- a/include/linux/genhd.h
> +++ b/include/linux/genhd.h
> @@ -303,9 +303,16 @@ struct gendisk *__blk_alloc_disk(int node);
> void blk_cleanup_disk(struct gendisk *disk);
>
> int __register_blkdev(unsigned int major, const char *name,
> - void (*probe)(dev_t devt));
> + void (*probe)(dev_t devt), struct module *owner);
> +static inline int register_blkdev_with_probe(unsigned int major, const char *name,
> + void (*probe)(dev_t devt), struct module *owner)
> +{
> + return __register_blkdev(major, name, probe, owner);
> +}
> +#define __register_blkdev(major, name, probe) \
> + register_blkdev_with_probe(major, name, probe, THIS_MODULE)
> #define register_blkdev(major, name) \
> - __register_blkdev(major, name, NULL)
> + register_blkdev_with_probe(major, name, NULL, NULL)
> void unregister_blkdev(unsigned int major, const char *name);
>
> bool bdev_check_media_change(struct block_device *bdev);
> --
> 2.25.1
>
---end quoted text---
