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Message-ID: <20190725134051.GC11115@kroah.com>
Date: Thu, 25 Jul 2019 15:40:51 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Prateek Sood <prsood@...eaurora.org>
Cc: Muchun Song <smuchun@...il.com>, rafael@...nel.org,
mojha@...eaurora.org, benh@...nel.crashing.org,
gkohli@...eaurora.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6] driver core: Fix use-after-free and double free on
glue directory
On Thu, Jul 25, 2019 at 11:52:24AM +0530, Prateek Sood wrote:
> On 7/24/19 9:30 PM, Muchun Song wrote:
> > There is a race condition between removing glue directory and adding a new
> > device under the glue directory. It can be reproduced in following test:
> >
> > path 1: Add the child device under glue dir
> > device_add()
> > get_device_parent()
> > mutex_lock(&gdp_mutex);
> > ....
> > /*find parent from glue_dirs.list*/
> > list_for_each_entry(k, &dev->class->p->glue_dirs.list, entry)
> > if (k->parent == parent_kobj) {
> > kobj = kobject_get(k);
> > break;
> > }
> > ....
> > mutex_unlock(&gdp_mutex);
> > ....
> > ....
> > kobject_add()
> > kobject_add_internal()
> > create_dir()
> > sysfs_create_dir_ns()
> > if (kobj->parent)
> > parent = kobj->parent->sd;
> > ....
> > kernfs_create_dir_ns(parent)
> > kernfs_new_node()
> > kernfs_get(parent)
> > ....
> > /* link in */
> > rc = kernfs_add_one(kn);
> > if (!rc)
> > return kn;
> >
> > kernfs_put(kn)
> > ....
> > repeat:
> > kmem_cache_free(kn)
> > kn = parent;
> >
> > if (kn) {
> > if (atomic_dec_and_test(&kn->count))
> > goto repeat;
> > }
> > ....
> >
> > path2: Remove last child device under glue dir
> > device_del()
> > cleanup_glue_dir()
> > mutex_lock(&gdp_mutex);
> > if (!kobject_has_children(glue_dir))
> > kobject_del(glue_dir);
> > kobject_put(glue_dir);
> > mutex_unlock(&gdp_mutex);
> >
> > Before path2 remove last child device under glue dir, If path1 add a new
> > device under glue dir, the glue_dir kobject reference count will be
> > increase to 2 via kobject_get(k) in get_device_parent(). And path1 has
> > been called kernfs_new_node(), but not call kernfs_get(parent).
> > Meanwhile, path2 call kobject_del(glue_dir) beacause 0 is returned by
> > kobject_has_children(). This result in glue_dir->sd is freed and it's
> > reference count will be 0. Then path1 call kernfs_get(parent) will trigger
> > a warning in kernfs_get()(WARN_ON(!atomic_read(&kn->count))) and increase
> > it's reference count to 1. Because glue_dir->sd is freed by path2, the next
> > call kernfs_add_one() by path1 will fail(This is also use-after-free)
> > and call atomic_dec_and_test() to decrease reference count. Because the
> > reference count is decremented to 0, it will also call kmem_cache_free()
> > to free glue_dir->sd again. This will result in double free.
> >
> > In order to avoid this happening, we also should make sure that kernfs_node
> > for glue_dir is released in path2 only when refcount for glue_dir kobj is
> > 1 to fix this race.
> >
> > The following calltrace is captured in kernel 4.14 with the following patch
> > applied:
> >
> > commit 726e41097920 ("drivers: core: Remove glue dirs from sysfs earlier")
> >
> > --------------------------------------------------------------------------
> > [ 3.633703] WARNING: CPU: 4 PID: 513 at .../fs/kernfs/dir.c:494
> > Here is WARN_ON(!atomic_read(&kn->count) in kernfs_get().
> > ....
> > [ 3.633986] Call trace:
> > [ 3.633991] kernfs_create_dir_ns+0xa8/0xb0
> > [ 3.633994] sysfs_create_dir_ns+0x54/0xe8
> > [ 3.634001] kobject_add_internal+0x22c/0x3f0
> > [ 3.634005] kobject_add+0xe4/0x118
> > [ 3.634011] device_add+0x200/0x870
> > [ 3.634017] _request_firmware+0x958/0xc38
> > [ 3.634020] request_firmware_into_buf+0x4c/0x70
> > ....
> > [ 3.634064] kernel BUG at .../mm/slub.c:294!
> > Here is BUG_ON(object == fp) in set_freepointer().
> > ....
> > [ 3.634346] Call trace:
> > [ 3.634351] kmem_cache_free+0x504/0x6b8
> > [ 3.634355] kernfs_put+0x14c/0x1d8
> > [ 3.634359] kernfs_create_dir_ns+0x88/0xb0
> > [ 3.634362] sysfs_create_dir_ns+0x54/0xe8
> > [ 3.634366] kobject_add_internal+0x22c/0x3f0
> > [ 3.634370] kobject_add+0xe4/0x118
> > [ 3.634374] device_add+0x200/0x870
> > [ 3.634378] _request_firmware+0x958/0xc38
> > [ 3.634381] request_firmware_into_buf+0x4c/0x70
> > --------------------------------------------------------------------------
> >
> > Fixes: 726e41097920 ("drivers: core: Remove glue dirs from sysfs earlier")
> >
> > Signed-off-by: Muchun Song <smuchun@...il.com>
> > Reviewed-by: Mukesh Ojha <mojha@...eaurora.org>
> > ---
> >
> > Change in v6:
> > 1. Remove hardcoding "1 "
> > Change in v5:
> > 1. Revert to the v1 fix.
> > 2. Add some comment to explain why we need do this in
> > cleanup_glue_dir().
> > Change in v4:
> > 1. Add some kerneldoc comment.
> > 2. Remove unlock_if_glue_dir().
> > 3. Rename get_device_parent_locked_if_glue_dir() to
> > get_device_parent_locked.
> > 4. Update commit message.
> > Change in v3:
> > Add change log.
> > Change in v2:
> > Fix device_move() also.
> >
> > drivers/base/core.c | 57 ++++++++++++++++++++++++++++++++++++++++++++-
> > 1 file changed, 56 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > index 4aeaa0c92bda..49bcb01e44cd 100644
> > --- a/drivers/base/core.c
> > +++ b/drivers/base/core.c
> > @@ -1820,12 +1820,67 @@ static inline struct kobject *get_glue_dir(struct device *dev)
> > */
> > static void cleanup_glue_dir(struct device *dev, struct kobject *glue_dir)
> > {
> > + unsigned int ref;
> > +
> > /* see if we live in a "glue" directory */
> > if (!live_in_glue_dir(glue_dir, dev))
> > return;
> >
> > mutex_lock(&gdp_mutex);
> > - if (!kobject_has_children(glue_dir))
> > + /**
> > + * There is a race condition between removing glue directory
> > + * and adding a new device under the glue directory.
> > + *
> > + * path 1: Add the child device under glue dir
> > + * device_add()
> > + * get_device_parent()
> > + * mutex_lock(&gdp_mutex);
> > + * ....
> > + * list_for_each_entry(k, &dev->class->p->glue_dirs.list,
> > + * entry)
> > + * if (k->parent == parent_kobj) {
> > + * kobj = kobject_get(k);
> > + * break;
> > + * }
> > + * ....
> > + * mutex_unlock(&gdp_mutex);
> > + * ....
> > + * ....
> > + * kobject_add()
> > + * kobject_add_internal()
> > + * create_dir()
> > + * ....
> > + * if (kobj->parent)
> > + * parent = kobj->parent->sd;
> > + * ....
> > + * kernfs_create_dir_ns(parent)
> > + * ....
> > + *
> > + * path2: Remove last child device under glue dir
> > + * device_del()
> > + * cleanup_glue_dir()
> > + * ....
> > + * mutex_lock(&gdp_mutex);
> > + * if (!kobject_has_children(glue_dir))
> > + * kobject_del(glue_dir);
> > + * ....
> > + * mutex_unlock(&gdp_mutex);
> > + *
> > + * Before path2 remove last child device under glue dir, if path1 add
> > + * a new device under glue dir, the glue_dir kobject reference count
> > + * will be increase to 2 via kobject_get(k) in get_device_parent().
> > + * And path1 has been called kernfs_create_dir_ns(). Meanwhile,
> > + * path2 call kobject_del(glue_dir) beacause 0 is returned by
> > + * kobject_has_children(). This result in glue_dir->sd is freed.
> > + * Then the path1 will see a stale "empty" but still potentially used
> > + * glue dir around.
> > + *
> > + * In order to avoid this happening, we also should make sure that
> > + * kernfs_node for glue_dir is released in path2 only when refcount
> > + * for glue_dir kobj is 1.
> > + */
> > + ref = kref_read(&glue_dir->kref);
> > + if (!kobject_has_children(glue_dir) && !--ref)
> > kobject_del(glue_dir);
> > kobject_put(glue_dir);
> > mutex_unlock(&gdp_mutex);
> >
>
> I would recommend to describe the issue as done in patch
> https://lkml.org/lkml/2019/5/1/3
>
> It helps in visualizing the race condition.
I agree, that email shows how this can be made much more "obvious".
Muchun, can you fix that up please?
thanks,
greg k-h
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