Re: [dm-devel] [dm-devel resend] dm mpath: fix UAF in multipath_message()

[Mike Snitzer <snitzer@kernel.org>]

On Thu, May 18 2023 at  8:11P -0400,
lilingfeng (A) <lilingfeng3@huawei.com> wrote:

> 
> 在 2022/10/18 3:56, Mike Snitzer 写道:
> > On Mon, Oct 10 2022 at 10:41P -0400,
> > Luo Meng <luomeng@huaweicloud.com> wrote:
> > 
> > > From: Luo Meng <luomeng12@huawei.com>
> > > 
> > > If dm_get_device() create dd in multipath_message(),
> > > and then call table_deps() after dm_put_table_device(),
> > > it will lead to concurrency UAF bugs.
> > > 
> > > One of the concurrency UAF can be shown as below:
> > > 
> > >           (USE)                        |    (FREE)
> > >                                        |  target_message
> > >                                        |    multipath_message
> > >                                        |      dm_put_device
> > >                                        |        dm_put_table_device #
> > >                                        |          kfree(td)  # table_device *td
> > > ioctl # DM_TABLE_DEPS_CMD             |         ...
> > >    table_deps                          |         ...
> > >    dm_get_live_or_inactive_table       |         ...
> > >      retrieve_dep                      |         ...
> > >      list_for_each_entry               |         ...
> > >        deps->dev[count++] =            |         ...
> > >            huge_encode_dev             |         ...
> > >            (dd->dm_dev->bdev->bd_dev)  |        list_del(&dd->list)
> > >                                        |        kfree(dd) # dm_dev_internal
> > > 
> > > The root cause of UAF bugs is that find_device() failed in
> > > dm_get_device() and will create dd and refcount set 1, kfree()
> > > in dm_put_table() is not protected. When td, which there are
> > > still pointers point to, is released, the concurrency UAF bug
> > > will happen.
> > > 
> > > This patch add a flag to determine whether to create a new dd.
> > > 
> > > Fixes: 8215d6ec5fee(dm table: remove unused dm_get_device range parameters)
> > > Signed-off-by: Luo Meng <luomeng12@huawei.com>
> > > ---
> > >   drivers/md/dm-mpath.c         |  2 +-
> > >   drivers/md/dm-table.c         | 43 +++++++++++++++++++++--------------
> > >   include/linux/device-mapper.h |  2 ++
> > >   3 files changed, 29 insertions(+), 18 deletions(-)
> > > 
> > > diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
> > > index 0e325469a252..1f51059520ac 100644
> > > --- a/drivers/md/dm-mpath.c
> > > +++ b/drivers/md/dm-mpath.c
> > > @@ -2001,7 +2001,7 @@ static int multipath_message(struct dm_target *ti, unsigned argc, char **argv,
> > >   		goto out;
> > >   	}
> > > -	r = dm_get_device(ti, argv[1], dm_table_get_mode(ti->table), &dev);
> > > +	r = __dm_get_device(ti, argv[1], dm_table_get_mode(ti->table), &dev, false);
> > >   	if (r) {
> > >   		DMWARN("message: error getting device %s",
> > >   		       argv[1]);
> > > diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> > > index d8034ff0cb24..ad18a41f1608 100644
> > > --- a/drivers/md/dm-table.c
> > > +++ b/drivers/md/dm-table.c
> > > @@ -338,12 +338,8 @@ dev_t dm_get_dev_t(const char *path)
> > >   }
> > >   EXPORT_SYMBOL_GPL(dm_get_dev_t);
> > > -/*
> > > - * Add a device to the list, or just increment the usage count if
> > > - * it's already present.
> > > - */
> > > -int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> > > -		  struct dm_dev **result)
> > > +int __dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> > > +		    struct dm_dev **result, bool create_dd)
> > >   {
> > >   	int r;
> > >   	dev_t dev;
> > > @@ -367,19 +363,21 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> > >   	dd = find_device(&t->devices, dev);
> > >   	if (!dd) {
> > > -		dd = kmalloc(sizeof(*dd), GFP_KERNEL);
> > > -		if (!dd)
> > > -			return -ENOMEM;
> > > -
> > > -		if ((r = dm_get_table_device(t->md, dev, mode, &dd->dm_dev))) {
> > > -			kfree(dd);
> > > -			return r;
> > > -		}
> > > +		if (create_dd) {
> > > +			dd = kmalloc(sizeof(*dd), GFP_KERNEL);
> > > +			if (!dd)
> > > +				return -ENOMEM;
> > > -		refcount_set(&dd->count, 1);
> > > -		list_add(&dd->list, &t->devices);
> > > -		goto out;
> > > +			if ((r = dm_get_table_device(t->md, dev, mode, &dd->dm_dev))) {
> > > +				kfree(dd);
> > > +				return r;
> > > +			}
> > > +			refcount_set(&dd->count, 1);
> > > +			list_add(&dd->list, &t->devices);
> > > +			goto out;
> > > +		} else
> > > +			return -ENODEV;
> > >   	} else if (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) {
> > >   		r = upgrade_mode(dd, mode, t->md);
> > >   		if (r)
> > > @@ -390,6 +388,17 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> > >   	*result = dd->dm_dev;
> > >   	return 0;
> > >   }
> > > +EXPORT_SYMBOL(__dm_get_device);
> > > +
> > > +/*
> > > + * Add a device to the list, or just increment the usage count if
> > > + * it's already present.
> > > + */
> > > +int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> > > +		  struct dm_dev **result)
> > > +{
> > > +	return __dm_get_device(ti, path, mode, result, true);
> > > +}
> > >   EXPORT_SYMBOL(dm_get_device);
> > >   static int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
> > > diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
> > > index 04c6acf7faaa..ef4031a844b6 100644
> > > --- a/include/linux/device-mapper.h
> > > +++ b/include/linux/device-mapper.h
> > > @@ -178,6 +178,8 @@ dev_t dm_get_dev_t(const char *path);
> > >   int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> > >   		  struct dm_dev **result);
> > >   void dm_put_device(struct dm_target *ti, struct dm_dev *d);
> > > +int __dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> > > +		    struct dm_dev **result, bool create_dd);
> > >   /*
> > >    * Information about a target type
> > > -- 
> > > 2.31.1
> > > 
> > This patch is treating the one multipath case like it is only consumer
> > of dm_get_device() that might have this issue. It feels too focused on
> > an instance where dm_get_device()'s side-effect of creating the device
> > is unwelcome. Feels like you're treating the symptom rather than the
> > problem (e.g. that the "kfree() in dm_put_table() is not protected"?)
> > 
> > Mike
> 
> In other cases, kfree() in dm_put_table() is protected by srcu.
> For example:
>           USE                              FREE
> table_deps                            dev_remove
>  dm_get_live_or_inactive_table         dm_sync_table
>   // lock
>   srcu_read_lock(&md->io_barrier)
>                                         // wait for unlock
>                                         synchronize_srcu(&md->io_barrier)
>   retrieve_deps
>  dm_put_live_table
>   // unlock
>   srcu_read_unlock(&md->io_barrier...)
>                                        dm_table_destroy
>                                         linear_dtr
>                                          dm_put_device
>                                           dm_put_table_device
> 
> However, in multipath_message(), the dm_dev is created and destroyed
> under srcu_read_lock, which means destroying dm_dev in this process
> and using dm_dev in other process will happen at same time since both
> of them hold srcu_read_lock.
> 
> target_message
>  dm_get_live_table // lock
>   multipath_message
>    dm_get_device // created
>     // may be got by other processes
>    dm_put_device // destroyed
>     // may be used by other processes
>  dm_put_live_table // unlock
> 
> We figured out some other solutions:
> 1) Judge refcount of dd under some lock before using dm_dev;
> 2) Get dd from list and use dm_dev under rcu;
> 3) Implement dm_get_device_xxx() with reference to dm_get_device()
> for dm-mpath to avoid creating dd when find_device() failed.
> 
> Compared to solutions above, Luo Meng's patch may be more appropriate.
> Any suggestions will be appreciated.
> 
> Li

[Cc'ing Mikulas so he can take a look at this too.]

The proposed patch papers over the issue, leaving a landmine for some
future DM target.

In addition, the patch header is very muddled about relation between
table_device and dm_dev_internal. And also needs clarification like:
"kfree(td) in dm_put_table_device() is not interlocked with
dm_dev_internal list (table->devices) management"

Also, the commit referenced with the "Fixes:" is bogus.

Wouldn't this change address the UAF (albeit, list_for_each_entry_safe
likely needed in methods like retrieve_deps())?

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 7d208b2b1a19..39e4c9ee8f16 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -434,9 +434,9 @@ void dm_put_device(struct dm_target *ti, struct dm_dev *d)
 		return;
 	}
 	if (refcount_dec_and_test(&dd->count)) {
-		dm_put_table_device(ti->table->md, d);
 		list_del(&dd->list);
 		kfree(dd);
+		dm_put_table_device(ti->table->md, d);
 	}
 }
 EXPORT_SYMBOL(dm_put_device);

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