[PATCH v8 09/12] sysfs: fix deadlock race with module removal

Luis Chamberlain
2021-09-27T16:38:29Z

When driver sysfs attributes use a lock also used on module removal we
can race to deadlock. This happens when for instance a sysfs file on
a driver is used, then at the same time we have module removal call
trigger. The module removal call code holds a lock, and then the
driver's sysfs file entry waits for the same lock. While holding the
lock the module removal tries to remove the sysfs entries, but these
cannot be removed yet as one is waiting for a lock. This won't complete
as the lock is already held. Likewise module removal cannot complete,
and so we deadlock.

This can now be easily reproducible with our sysfs selftest as follows:

./tools/testing/selftests/sysfs/sysfs.sh -t 0027

This uses a local driver lock. Test 0028 can also be used, that uses
the rtnl_lock():

./tools/testing/selftests/sysfs/sysfs.sh -t 0028

To fix this we extend the struct kernfs_node with a module reference
and use the try_module_get() after kernfs_get_active() is called. As
documented in the prior patch, we now know that once kernfs_get_active()
is called the module is implicitly guarded to exist and cannot be removed.
This is because the module is the one in charge of removing the same
sysfs file it created, and removal of sysfs files on module exit will wait
until they don't have any active references. By using a try_module_get()
after kernfs_get_active() we yield to let module removal trump calls to
process a sysfs operation, while also preventing module removal if a sysfs
operation is in already progress. This prevents the deadlock.

This deadlock was first reported with the zram driver, however the live
patching folks have acknowledged they have observed this as well with
live patching, when a live patch is removed. I was then able to
reproduce easily by creating a dedicated selftest for it.

A sketch of how this can happen follows, consider foo a local mutex
part of a driver, and used on the driver's module exit routine and
on one of its sysfs ops:

foo.c:
static DEFINE_MUTEX(foo);
static ssize_t foo_store(struct device *dev,
struct device_attribute *attr,
const char *buf, size_t count)
{
...
mutex_lock(&foo);
...
mutex_lock(&foo);
...
}
static DEVICE_ATTR_RW(foo);
...
void foo_exit(void)
{
mutex_lock(&foo);
...
mutex_unlock(&foo);
}
module_exit(foo_exit);

And this can lead to this condition:

CPU A CPU B
foo_store()
foo_exit()
mutex_lock(&foo)
mutex_lock(&foo)
del_gendisk(some_struct->disk);
device_del()
device_remove_groups()

In this situation foo_store() is waiting for the mutex foo to
become unlocked, but that won't happen until module removal is complete.
But module removal won't complete until the sysfs file being poked at
completes which is waiting for a lock already held.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
arch/x86/kernel/cpu/resctrl/rdtgroup.c | 4 +-
fs/kernfs/dir.c | 44 ++++++++++++++++++----
fs/kernfs/file.c | 6 ++-
fs/kernfs/kernfs-internal.h | 3 +-
fs/kernfs/symlink.c | 3 +-
fs/sysfs/dir.c | 2 +-
fs/sysfs/file.c | 6 ++-
fs/sysfs/group.c | 3 +-
include/linux/kernfs.h | 14 ++++---
include/linux/sysfs.h | 52 ++++++++++++++++++++------
kernel/cgroup/cgroup.c | 2 +-
11 files changed, 105 insertions(+), 34 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index b57b3db9a6a7..4edf3b37fd2c 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -209,7 +209,7 @@ static int rdtgroup_add_file(struct kernfs_node *parent_kn, struct rftype *rft)

kn = __kernfs_create_file(parent_kn, rft->name, rft->mode,
GLOBAL_ROOT_UID, GLOBAL_ROOT_GID,
- 0, rft->kf_ops, rft, NULL, NULL);
+ 0, rft->kf_ops, rft, NULL, NULL, NULL);
if (IS_ERR(kn))
return PTR_ERR(kn);

@@ -2482,7 +2482,7 @@ static int mon_addfile(struct kernfs_node *parent_kn, const char *name,

kn = __kernfs_create_file(parent_kn, name, 0444,
GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, 0,
- &kf_mondata_ops, priv, NULL, NULL);
+ &kf_mondata_ops, priv, NULL, NULL, NULL);
if (IS_ERR(kn))
return PTR_ERR(kn);

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index ba581429bf7b..e841201fd11b 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -14,6 +14,7 @@
#include <linux/slab.h>
#include <linux/security.h>
#include <linux/hash.h>
+#include <linux/module.h>

#include "kernfs-internal.h"

@@ -414,12 +415,29 @@ static bool kernfs_unlink_sibling(struct kernfs_node *kn)
*/
struct kernfs_node *kernfs_get_active(struct kernfs_node *kn)
{
+ int v;
+
if (unlikely(!kn))
return NULL;

if (!atomic_inc_unless_negative(&kn->active))
return NULL;

+ /*
+ * If a module created the kernfs_node, the module cannot possibly be
+ * removed if the above atomic_inc_unless_negative() succeeded. So the
+ * try_module_get() below is not to protect the lifetime of the module
+ * as that is already guaranteed. The try_module_get() below is used
+ * to ensure that we don't deadlock in case a kernfs operation and
+ * module removal used a shared lock.
+ */
+ if (!try_module_get(kn->owner)) {
+ v = atomic_dec_return(&kn->active);
+ if (unlikely(v == KN_DEACTIVATED_BIAS))
+ wake_up_all(&kernfs_root(kn)->deactivate_waitq);
+ return NULL;
+ }
+
if (kernfs_lockdep(kn))
rwsem_acquire_read(&kn->dep_map, 0, 1, _RET_IP_);
return kn;
@@ -442,6 +460,13 @@ void kernfs_put_active(struct kernfs_node *kn)
if (kernfs_lockdep(kn))
rwsem_release(&kn->dep_map, _RET_IP_);
v = atomic_dec_return(&kn->active);
+
+ /*
+ * We prevent module exit *until* we know for sure all possible
+ * kernfs ops are done.
+ */
+ module_put(kn->owner);
+
if (likely(v != KN_DEACTIVATED_BIAS))
return;

@@ -572,7 +597,8 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
struct kernfs_node *parent,
const char *name, umode_t mode,
kuid_t uid, kgid_t gid,
- unsigned flags)
+ unsigned flags,
+ struct module *owner)
{
struct kernfs_node *kn;
u32 id_highbits;
@@ -607,6 +633,7 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
kn->name = name;
kn->mode = mode;
kn->flags = flags;
+ kn->owner = owner;

if (!uid_eq(uid, GLOBAL_ROOT_UID) || !gid_eq(gid, GLOBAL_ROOT_GID)) {
struct iattr iattr = {
@@ -640,12 +667,13 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
struct kernfs_node *kernfs_new_node(struct kernfs_node *parent,
const char *name, umode_t mode,
kuid_t uid, kgid_t gid,
- unsigned flags)
+ unsigned flags,
+ struct module *owner)
{
struct kernfs_node *kn;

kn = __kernfs_new_node(kernfs_root(parent), parent,
- name, mode, uid, gid, flags);
+ name, mode, uid, gid, flags, owner);
if (kn) {
kernfs_get(parent);
kn->parent = parent;
@@ -927,7 +955,7 @@ struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,

kn = __kernfs_new_node(root, NULL, "", S_IFDIR | S_IRUGO | S_IXUGO,
GLOBAL_ROOT_UID, GLOBAL_ROOT_GID,
- KERNFS_DIR);
+ KERNFS_DIR, NULL);
if (!kn) {
idr_destroy(&root->ino_idr);
kfree(root);
@@ -969,20 +997,22 @@ void kernfs_destroy_root(struct kernfs_root *root)
* @gid: gid of the new directory
* @priv: opaque data associated with the new directory
* @ns: optional namespace tag of the directory
+ * @owner: if set, the module owner of this directory
*
* Returns the created node on success, ERR_PTR() value on failure.
*/
struct kernfs_node *kernfs_create_dir_ns(struct kernfs_node *parent,
const char *name, umode_t mode,
kuid_t uid, kgid_t gid,
- void *priv, const void *ns)
+ void *priv, const void *ns,
+ struct module *owner)
{
struct kernfs_node *kn;
int rc;

/* allocate */
kn = kernfs_new_node(parent, name, mode | S_IFDIR,
- uid, gid, KERNFS_DIR);
+ uid, gid, KERNFS_DIR, owner);
if (!kn)
return ERR_PTR(-ENOMEM);

@@ -1014,7 +1044,7 @@ struct kernfs_node *kernfs_create_empty_dir(struct kernfs_node *parent,

/* allocate */
kn = kernfs_new_node(parent, name, S_IRUGO|S_IXUGO|S_IFDIR,
- GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, KERNFS_DIR);
+ GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, KERNFS_DIR, NULL);
if (!kn)
return ERR_PTR(-ENOMEM);

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 4479c6580333..0e125287e050 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -978,6 +978,7 @@ const struct file_operations kernfs_file_fops = {
* @priv: private data for the file
* @ns: optional namespace tag of the file
* @key: lockdep key for the file's active_ref, %NULL to disable lockdep
+ * @owner: if set, the module owner of the file
*
* Returns the created node on success, ERR_PTR() value on error.
*/
@@ -987,7 +988,8 @@ struct kernfs_node *__kernfs_create_file(struct kernfs_node *parent,
loff_t size,
const struct kernfs_ops *ops,
void *priv, const void *ns,
- struct lock_class_key *key)
+ struct lock_class_key *key,
+ struct module *owner)
{
struct kernfs_node *kn;
unsigned flags;
@@ -996,7 +998,7 @@ struct kernfs_node *__kernfs_create_file(struct kernfs_node *parent,
flags = KERNFS_FILE;

kn = kernfs_new_node(parent, name, (mode & S_IALLUGO) | S_IFREG,
- uid, gid, flags);
+ uid, gid, flags, owner);
if (!kn)
return ERR_PTR(-ENOMEM);

diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index 9e3abf597e2d..6d275d661987 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -134,7 +134,8 @@ int kernfs_add_one(struct kernfs_node *kn);
struct kernfs_node *kernfs_new_node(struct kernfs_node *parent,
const char *name, umode_t mode,
kuid_t uid, kgid_t gid,
- unsigned flags);
+ unsigned flags,
+ struct module *owner);

/*
* file.c
diff --git a/fs/kernfs/symlink.c b/fs/kernfs/symlink.c
index 19a6c71c6ff5..5a053eebee52 100644
--- a/fs/kernfs/symlink.c
+++ b/fs/kernfs/symlink.c
@@ -36,7 +36,8 @@ struct kernfs_node *kernfs_create_link(struct kernfs_node *parent,
gid = target->iattr->ia_gid;
}

- kn = kernfs_new_node(parent, name, S_IFLNK|0777, uid, gid, KERNFS_LINK);
+ kn = kernfs_new_node(parent, name, S_IFLNK|0777, uid, gid, KERNFS_LINK,
+ target->owner);
if (!kn)
return ERR_PTR(-ENOMEM);

diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
index b6b6796e1616..9763c2fde3c7 100644
--- a/fs/sysfs/dir.c
+++ b/fs/sysfs/dir.c
@@ -57,7 +57,7 @@ int sysfs_create_dir_ns(struct kobject *kobj, const void *ns)
kobject_get_ownership(kobj, &uid, &gid);

kn = kernfs_create_dir_ns(parent, kobject_name(kobj), 0755, uid, gid,
- kobj, ns);
+ kobj, ns, NULL);
if (IS_ERR(kn)) {
if (PTR_ERR(kn) == -EEXIST)
sysfs_warn_dup(parent, kobject_name(kobj));
diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 42dcf96881b6..af9e91fd1a92 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -292,7 +292,8 @@ int sysfs_add_file_mode_ns(struct kernfs_node *parent,
#endif

kn = __kernfs_create_file(parent, attr->name, mode & 0777, uid, gid,
- PAGE_SIZE, ops, (void *)attr, ns, key);
+ PAGE_SIZE, ops, (void *)attr, ns, key,
+ attr->owner);
if (IS_ERR(kn)) {
if (PTR_ERR(kn) == -EEXIST)
sysfs_warn_dup(parent, attr->name);
@@ -327,7 +328,8 @@ int sysfs_add_bin_file_mode_ns(struct kernfs_node *parent,
#endif

kn = __kernfs_create_file(parent, attr->name, mode & 0777, uid, gid,
- battr->size, ops, (void *)attr, ns, key);
+ battr->size, ops, (void *)attr, ns, key,
+ attr->owner);
if (IS_ERR(kn)) {
if (PTR_ERR(kn) == -EEXIST)
sysfs_warn_dup(parent, attr->name);
diff --git a/fs/sysfs/group.c b/fs/sysfs/group.c
index eeb0e3099421..372864d1cb54 100644
--- a/fs/sysfs/group.c
+++ b/fs/sysfs/group.c
@@ -135,7 +135,8 @@ static int internal_create_group(struct kobject *kobj, int update,
} else {
kn = kernfs_create_dir_ns(kobj->sd, grp->name,
S_IRWXU | S_IRUGO | S_IXUGO,
- uid, gid, kobj, NULL);
+ uid, gid, kobj, NULL,
+ grp->owner);
if (IS_ERR(kn)) {
if (PTR_ERR(kn) == -EEXIST)
sysfs_warn_dup(kobj->sd, grp->name);
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index cd968ee2b503..818b00ebd107 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -161,6 +161,7 @@ struct kernfs_node {
unsigned short flags;
umode_t mode;
struct kernfs_iattrs *iattr;
+ struct module *owner;
};

/*
@@ -370,7 +371,8 @@ void kernfs_destroy_root(struct kernfs_root *root);
struct kernfs_node *kernfs_create_dir_ns(struct kernfs_node *parent,
const char *name, umode_t mode,
kuid_t uid, kgid_t gid,
- void *priv, const void *ns);
+ void *priv, const void *ns,
+ struct module *owner);
struct kernfs_node *kernfs_create_empty_dir(struct kernfs_node *parent,
const char *name);
struct kernfs_node *__kernfs_create_file(struct kernfs_node *parent,
@@ -379,7 +381,8 @@ struct kernfs_node *__kernfs_create_file(struct kernfs_node *parent,
loff_t size,
const struct kernfs_ops *ops,
void *priv, const void *ns,
- struct lock_class_key *key);
+ struct lock_class_key *key,
+ struct module *owner);
struct kernfs_node *kernfs_create_link(struct kernfs_node *parent,
const char *name,
struct kernfs_node *target);
@@ -472,14 +475,15 @@ static inline void kernfs_destroy_root(struct kernfs_root *root) { }
static inline struct kernfs_node *
kernfs_create_dir_ns(struct kernfs_node *parent, const char *name,
umode_t mode, kuid_t uid, kgid_t gid,
- void *priv, const void *ns)
+ void *priv, const void *ns, struct module *owner)
{ return ERR_PTR(-ENOSYS); }

static inline struct kernfs_node *
__kernfs_create_file(struct kernfs_node *parent, const char *name,
umode_t mode, kuid_t uid, kgid_t gid,
loff_t size, const struct kernfs_ops *ops,
- void *priv, const void *ns, struct lock_class_key *key)
+ void *priv, const void *ns, struct lock_class_key *key,
+ struct module *owner)
{ return ERR_PTR(-ENOSYS); }

static inline struct kernfs_node *
@@ -566,7 +570,7 @@ kernfs_create_dir(struct kernfs_node *parent, const char *name, umode_t mode,
{
return kernfs_create_dir_ns(parent, name, mode,
GLOBAL_ROOT_UID, GLOBAL_ROOT_GID,
- priv, NULL);
+ priv, NULL, parent->owner);
}

static inline int kernfs_remove_by_name(struct kernfs_node *parent,
diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
index e3f1e8ac1f85..babbabb460dc 100644
--- a/include/linux/sysfs.h
+++ b/include/linux/sysfs.h
@@ -30,6 +30,7 @@ enum kobj_ns_type;
struct attribute {
const char *name;
umode_t mode;
+ struct module *owner;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
bool ignore_lockdep:1;
struct lock_class_key *key;
@@ -80,6 +81,7 @@ do { \
* @attrs: Pointer to NULL terminated list of attributes.
* @bin_attrs: Pointer to NULL terminated list of binary attributes.
* Either attrs or bin_attrs or both must be provided.
+ * @module: If set, module responsible for this attribute group
*/
struct attribute_group {
const char *name;
@@ -89,6 +91,7 @@ struct attribute_group {
struct bin_attribute *, int);
struct attribute **attrs;
struct bin_attribute **bin_attrs;
+ struct module *owner;
};

/*
@@ -100,38 +103,52 @@ struct attribute_group {

#define __ATTR(_name, _mode, _show, _store) { \
.attr = {.name = __stringify(_name), \
- .mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
+ .mode = VERIFY_OCTAL_PERMISSIONS(_mode), \
+ .owner = THIS_MODULE, \
+ }, \
.show = _show, \
.store = _store, \
}

#define __ATTR_PREALLOC(_name, _mode, _show, _store) { \
.attr = {.name = __stringify(_name), \
- .mode = SYSFS_PREALLOC | VERIFY_OCTAL_PERMISSIONS(_mode) },\
+ .mode = SYSFS_PREALLOC | VERIFY_OCTAL_PERMISSIONS(_mode),\
+ .owner = THIS_MODULE, \
+ }, \
.show = _show, \
.store = _store, \
}

#define __ATTR_RO(_name) { \
- .attr = { .name = __stringify(_name), .mode = 0444 }, \
+ .attr = { .name = __stringify(_name), \
+ .mode = 0444, \
+ .owner = THIS_MODULE, \
+ }, \
.show = _name##_show, \
}

#define __ATTR_RO_MODE(_name, _mode) { \
.attr = { .name = __stringify(_name), \
- .mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
+ .mode = VERIFY_OCTAL_PERMISSIONS(_mode), \
+ .owner = THIS_MODULE, \
+ }, \
.show = _name##_show, \
}

#define __ATTR_RW_MODE(_name, _mode) { \
.attr = { .name = __stringify(_name), \
- .mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
+ .mode = VERIFY_OCTAL_PERMISSIONS(_mode), \
+ .owner = THIS_MODULE, \
+ }, \
.show = _name##_show, \
.store = _name##_store, \
}

#define __ATTR_WO(_name) { \
- .attr = { .name = __stringify(_name), .mode = 0200 }, \
+ .attr = { .name = __stringify(_name), \
+ .mode = 0200, \
+ .owner = THIS_MODULE, \
+ }, \
.store = _name##_store, \
}

@@ -141,8 +158,11 @@ struct attribute_group {

#ifdef CONFIG_DEBUG_LOCK_ALLOC
#define __ATTR_IGNORE_LOCKDEP(_name, _mode, _show, _store) { \
- .attr = {.name = __stringify(_name), .mode = _mode, \
- .ignore_lockdep = true }, \
+ .attr = {.name = __stringify(_name), \
+ .mode = _mode, \
+ .ignore_lockdep = true, \
+ .owner = THIS_MODULE, \
+ }, \
.show = _show, \
.store = _store, \
}
@@ -159,6 +179,7 @@ static const struct attribute_group *_name##_groups[] = { \
#define ATTRIBUTE_GROUPS(_name) \
static const struct attribute_group _name##_group = { \
.attrs = _name##_attrs, \
+ .owner = THIS_MODULE, \
}; \
__ATTRIBUTE_GROUPS(_name)

@@ -199,20 +220,29 @@ struct bin_attribute {

/* macros to create static binary attributes easier */
#define __BIN_ATTR(_name, _mode, _read, _write, _size) { \
- .attr = { .name = __stringify(_name), .mode = _mode }, \
+ .attr = { .name = __stringify(_name), \
+ .mode = _mode, \
+ .owner = THIS_MODULE, \
+ }, \
.read = _read, \
.write = _write, \
.size = _size, \
}

#define __BIN_ATTR_RO(_name, _size) { \
- .attr = { .name = __stringify(_name), .mode = 0444 }, \
+ .attr = { .name = __stringify(_name), \
+ .mode = 0444, \
+ .owner = THIS_MODULE, \
+ }, \
.read = _name##_read, \
.size = _size, \
}

#define __BIN_ATTR_WO(_name, _size) { \
- .attr = { .name = __stringify(_name), .mode = 0200 }, \
+ .attr = { .name = __stringify(_name), \
+ .mode = 0200, \
+ .owner = THIS_MODULE, \
+ }, \
.write = _name##_write, \
.size = _size, \
}
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 9e0390000025..c6b0a28f599c 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3975,7 +3975,7 @@ static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
cgroup_file_mode(cft),
GLOBAL_ROOT_UID, GLOBAL_ROOT_GID,
0, cft->kf_ops, cft,
- NULL, key);
+ NULL, key, NULL);
if (IS_ERR(kn))
return PTR_ERR(kn);

--
2.30.2