proc: Fix proc_sys_prune_dcache to hold a sb reference

Andrei Vagin writes:
FYI: This bug has been reproduced on 4.11.7
> BUG: Dentry ffff895a3dd01240{i=4e7c09a,n=lo}  still in use (1) [unmount of proc proc]
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 13588 at fs/dcache.c:1445 umount_check+0x6e/0x80
> CPU: 1 PID: 13588 Comm: kworker/1:1 Not tainted 4.11.7-200.fc25.x86_64 #1
> Hardware name: CompuLab sbc-flt1/fitlet, BIOS SBCFLT_0.08.04 06/27/2015
> Workqueue: events proc_cleanup_work
> Call Trace:
>  dump_stack+0x63/0x86
>  __warn+0xcb/0xf0
>  warn_slowpath_null+0x1d/0x20
>  umount_check+0x6e/0x80
>  d_walk+0xc6/0x270
>  ? dentry_free+0x80/0x80
>  do_one_tree+0x26/0x40
>  shrink_dcache_for_umount+0x2d/0x90
>  generic_shutdown_super+0x1f/0xf0
>  kill_anon_super+0x12/0x20
>  proc_kill_sb+0x40/0x50
>  deactivate_locked_super+0x43/0x70
>  deactivate_super+0x5a/0x60
>  cleanup_mnt+0x3f/0x90
>  mntput_no_expire+0x13b/0x190
>  kern_unmount+0x3e/0x50
>  pid_ns_release_proc+0x15/0x20
>  proc_cleanup_work+0x15/0x20
>  process_one_work+0x197/0x450
>  worker_thread+0x4e/0x4a0
>  kthread+0x109/0x140
>  ? process_one_work+0x450/0x450
>  ? kthread_park+0x90/0x90
>  ret_from_fork+0x2c/0x40
> ---[ end trace e1c109611e5d0b41 ]---
> VFS: Busy inodes after unmount of proc. Self-destruct in 5 seconds.  Have a nice day...
> BUG: unable to handle kernel NULL pointer dereference at           (null)
> IP: _raw_spin_lock+0xc/0x30
> PGD 0

Fix this by taking a reference to the super block in proc_sys_prune_dcache.

The superblock reference is the core of the fix however the sysctl_inodes
list is converted to a hlist so that hlist_del_init_rcu may be used.  This
allows proc_sys_prune_dache to remove inodes the sysctl_inodes list, while
not causing problems for proc_sys_evict_inode when if it later choses to
remove the inode from the sysctl_inodes list.  Removing inodes from the
sysctl_inodes list allows proc_sys_prune_dcache to have a progress
guarantee, while still being able to drop all locks.  The fact that
head->unregistering is set in start_unregistering ensures that no more
inodes will be added to the the sysctl_inodes list.

Previously the code did a dance where it delayed calling iput until the
next entry in the list was being considered to ensure the inode remained on
the sysctl_inodes list until the next entry was walked to.  The structure
of the loop in this patch does not need that so is much easier to
understand and maintain.

Cc: stable@vger.kernel.org
Reported-by: Andrei Vagin <avagin@gmail.com>
Tested-by: Andrei Vagin <avagin@openvz.org>
Fixes: ace0c791e6c3 ("proc/sysctl: Don't grab i_lock under sysctl_lock.")
Fixes: d6cffbbe9a7e ("proc/sysctl: prune stale dentries during unregistering")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index c5ae09b6c726ab8d8d01043a2d896f752727d0a4..18694598bebfb4781c79cb9c75107f37e54349c4 100644 (file)
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -67,7 +67,7 @@ struct proc_inode {
        struct proc_dir_entry *pde;
        struct ctl_table_header *sysctl;
        struct ctl_table *sysctl_entry;
-       struct list_head sysctl_inodes;
+       struct hlist_node sysctl_inodes;
        const struct proc_ns_operations *ns_ops;
        struct inode vfs_inode;
 };

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 67985a7233c24023e9810e25057e2a28ae7874e0..9bf06e2b12846a9765027cc47d0ef8f6d9bd734a 100644 (file)
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -191,7 +191,7 @@ static void init_header(struct ctl_table_header *head,
        head->set = set;
        head->parent = NULL;
        head->node = node;
-       INIT_LIST_HEAD(&head->inodes);
+       INIT_HLIST_HEAD(&head->inodes);
        if (node) {
                struct ctl_table *entry;
                for (entry = table; entry->procname; entry++, node++)
@@ -261,25 +261,42 @@ static void unuse_table(struct ctl_table_header *p)
                        complete(p->unregistering);
 }
 
-/* called under sysctl_lock */
 static void proc_sys_prune_dcache(struct ctl_table_header *head)
 {
-       struct inode *inode, *prev = NULL;
+       struct inode *inode;
        struct proc_inode *ei;
+       struct hlist_node *node;
+       struct super_block *sb;
 
        rcu_read_lock();
-       list_for_each_entry_rcu(ei, &head->inodes, sysctl_inodes) {
-               inode = igrab(&ei->vfs_inode);
-               if (inode) {
-                       rcu_read_unlock();
-                       iput(prev);
-                       prev = inode;
-                       d_prune_aliases(inode);
+       for (;;) {
+               node = hlist_first_rcu(&head->inodes);
+               if (!node)
+                       break;
+               ei = hlist_entry(node, struct proc_inode, sysctl_inodes);
+               spin_lock(&sysctl_lock);
+               hlist_del_init_rcu(&ei->sysctl_inodes);
+               spin_unlock(&sysctl_lock);
+
+               inode = &ei->vfs_inode;
+               sb = inode->i_sb;
+               if (!atomic_inc_not_zero(&sb->s_active))
+                       continue;
+               inode = igrab(inode);
+               rcu_read_unlock();
+               if (unlikely(!inode)) {
+                       deactivate_super(sb);
                        rcu_read_lock();
+                       continue;
                }
+
+               d_prune_aliases(inode);
+               iput(inode);
+               deactivate_super(sb);
+
+               rcu_read_lock();
        }
        rcu_read_unlock();
-       iput(prev);
 }
 
 /* called under sysctl_lock, will reacquire if has to wait */
@@ -461,7 +478,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
        }
        ei->sysctl = head;
        ei->sysctl_entry = table;
-       list_add_rcu(&ei->sysctl_inodes, &head->inodes);
+       hlist_add_head_rcu(&ei->sysctl_inodes, &head->inodes);
        head->count++;
        spin_unlock(&sysctl_lock);
 
@@ -489,7 +506,7 @@ out:
 void proc_sys_evict_inode(struct inode *inode, struct ctl_table_header *head)
 {
        spin_lock(&sysctl_lock);
-       list_del_rcu(&PROC_I(inode)->sysctl_inodes);
+       hlist_del_init_rcu(&PROC_I(inode)->sysctl_inodes);
        if (!--head->count)
                kfree_rcu(head, rcu);
        spin_unlock(&sysctl_lock);

diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 80d07816def05bf0f7662c46d56387830dec3d57..1c04a26bfd2f017408e4d476f073788c7139fc21 100644 (file)
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -143,7 +143,7 @@ struct ctl_table_header
        struct ctl_table_set *set;
        struct ctl_dir *parent;
        struct ctl_node *node;
-       struct list_head inodes; /* head for proc_inode->sysctl_inodes */
+       struct hlist_head inodes; /* head for proc_inode->sysctl_inodes */
 };
 
 struct ctl_dir {