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    Subject[PATCH][2.5.73] stack corruption in devfs_lookup
    Date
    Doing concurrent lookups for the same name in devfs with devfsd and modules 
    enabled may result in stack coruption.

    When devfs_lookup needs to call devfsd it arranges for other lookups for the
    same name to wait. It is using local variable as wait queue head. After
    devfsd returns devfs_lookup wakes up all waiters and returns. Unfortunately
    there is no garantee all waiters will actually get chance to run and clean up
    before devfs_lookup returns. so some of them attempt to access already freed
    storage on stack.

    It is trivial to trigger with SMP kernel (I have single-CPU system if it
    matters) doing

    while true
    do
    ls /dev/foo &
    done

    With spinlock debug enabled this results in large number of stacks similar to

    ------------[ cut here ]------------
    kernel BUG at include/asm/spinlock.h:120!
    invalid operand: 0000 [#1]
    CPU: 0
    EIP: 0060:[<c012004c>] Tainted: G S
    EFLAGS: 00010082
    EIP is at remove_wait_queue+0xac/0xc0
    eax: 0000000e ebx: f6617e4c ecx: 00000000 edx: 00000001
    esi: f6747dd0 edi: f6616000 ebp: f6617e10 esp: f6617df0
    ds: 007b es: 007b ss: 0068
    Process ls (pid: 1517, threadinfo=f6616000 task=f6619900)
    Stack: c03eb9d5 c011ffa0 00000286 f6617e24 c0443880 f6747dd0 f6616000 f6617e4c
    f6617e78 c01cb3e6 c04470c0 f6616000 00000246 f6747dcc c1a6f1dc 00000000
    f6619900 c011d4e0 00000000 00000000 f7d4b73c f663d005 f6759828 00000000
    Call Trace:
    [<c011ffa0>] remove_wait_queue+0x0/0xc0
    [<c01cb3e6>] devfs_d_revalidate_wait+0x1d6/0x1f0
    [<c011d4e0>] default_wake_function+0x0/0x30
    [<c011d4e0>] default_wake_function+0x0/0x30
    [<c017201a>] do_lookup+0x5a/0xa0
    [<c017261e>] link_path_walk+0x5be/0xb20
    [<c0148ceb>] kmem_cache_alloc+0x14b/0x190
    [<c01730fe>] __user_walk+0x3e/0x60
    [<c016d13e>] vfs_stat+0x1e/0x60
    [<c0154c5b>] do_brk+0x12b/0x200
    [<c016d7bb>] sys_stat64+0x1b/0x40
    [<c01532e2>] sys_brk+0xf2/0x120
    [<c011a820>] do_page_fault+0x0/0x4c5
    [<c0109919>] sysenter_past_esp+0x52/0x71

    Code: 0f 0b 78 00 6c b0 3e c0 e9 72 ff ff ff 8d b4 26 00 00 00 00
    <6>note: ls[1517] exited with preempt_count 1
    eip: c011ffa0

    without spinlock debug system usually hung dead with reset button as the only
    possibility.

    I was not able to reproduce it on 2.4 on single-CPU system - in 2.4
    devfs_d_revalidate_wait does not attempt to remove itself from wait queue so
    it appears to be safe.

    attached patch is against 2.5.73 but applies to 2.5.74 as well. It makes
    lookup struct be allocated from heap and adds reference counter to free it
    when no more needed.

    regards

    -andrey


    --- linux-2.5.73/fs/devfs/base.c.stack 2003-06-23 19:46:44.000000000 +0400
    +++ linux-2.5.73/fs/devfs/base.c 2003-06-26 21:01:45.000000000 +0400
    @@ -2208,8 +2208,46 @@ struct devfs_lookup_struct
    {
    devfs_handle_t de;
    wait_queue_head_t wait_queue;
    + atomic_t count;
    };

    +static struct devfs_lookup_struct *
    +new_devfs_lookup_struct(void)
    +{
    + struct devfs_lookup_struct *p = kmalloc(sizeof(*p), GFP_KERNEL);
    +
    + if (!p)
    + return NULL;
    +
    + init_waitqueue_head (&p->wait_queue);
    + atomic_set(&p->count, 1);
    + return p;
    +}
    +
    +static void
    +get_devfs_lookup_struct(struct devfs_lookup_struct *info)
    +{
    + if (info)
    + atomic_inc(&info->count);
    + else {
    + printk(KERN_ERR "null devfs_lookup_struct pointer\n");
    + dump_stack();
    + }
    +}
    +
    +static void
    +put_devfs_lookup_struct(struct devfs_lookup_struct *info)
    +{
    + if (info) {
    + if (!atomic_dec_and_test(&info->count))
    + return;
    + kfree(info);
    + } else {
    + printk(KERN_ERR "null devfs_lookup_struct pointer\n");
    + dump_stack();
    + }
    +}
    +
    /* XXX: this doesn't handle the case where we got a negative dentry
    but a devfs entry has been registered in the meanwhile */
    static int devfs_d_revalidate_wait (struct dentry *dentry, int flags)
    @@ -2252,11 +2290,13 @@ static int devfs_d_revalidate_wait (stru
    read_lock (&parent->u.dir.lock);
    if (dentry->d_fsdata)
    {
    + get_devfs_lookup_struct(lookup_info);
    set_current_state (TASK_UNINTERRUPTIBLE);
    add_wait_queue (&lookup_info->wait_queue, &wait);
    read_unlock (&parent->u.dir.lock);
    schedule ();
    remove_wait_queue (&lookup_info->wait_queue, &wait);
    + put_devfs_lookup_struct(lookup_info);
    }
    else read_unlock (&parent->u.dir.lock);
    return 1;
    @@ -2268,7 +2308,7 @@ static int devfs_d_revalidate_wait (stru
    static struct dentry *devfs_lookup (struct inode *dir, struct dentry *dentry)
    {
    struct devfs_entry tmp; /* Must stay in scope until devfsd idle again */
    - struct devfs_lookup_struct lookup_info;
    + struct devfs_lookup_struct *lookup_info;
    struct fs_info *fs_info = dir->i_sb->s_fs_info;
    struct devfs_entry *parent, *de;
    struct inode *inode;
    @@ -2285,9 +2325,10 @@ static struct dentry *devfs_lookup (stru
    read_lock (&parent->u.dir.lock);
    de = _devfs_search_dir (parent, dentry->d_name.name, dentry->d_name.len);
    read_unlock (&parent->u.dir.lock);
    - lookup_info.de = de;
    - init_waitqueue_head (&lookup_info.wait_queue);
    - dentry->d_fsdata = &lookup_info;
    + lookup_info = new_devfs_lookup_struct();
    + if (!lookup_info) return ERR_PTR(-ENOMEM);
    + lookup_info->de = de;
    + dentry->d_fsdata = lookup_info;
    if (de == NULL)
    { /* Try with devfsd. For any kind of failure, leave a negative dentry
    so someone else can deal with it (in the case where the sysadmin
    @@ -2297,6 +2338,7 @@ static struct dentry *devfs_lookup (stru
    if (try_modload (parent, fs_info,
    dentry->d_name.name, dentry->d_name.len, &tmp) < 0)
    { /* Lookup event was not queued to devfsd */
    + put_devfs_lookup_struct(lookup_info);
    d_add (dentry, NULL);
    return NULL;
    }
    @@ -2309,7 +2351,7 @@ static struct dentry *devfs_lookup (stru
    up (&dir->i_sem);
    wait_for_devfsd_finished (fs_info); /* If I'm not devfsd, must wait */
    down (&dir->i_sem); /* Grab it again because them's the rules */
    - de = lookup_info.de;
    + de = lookup_info->de;
    /* If someone else has been so kind as to make the inode, we go home
    early */
    if (dentry->d_inode) goto out;
    @@ -2333,10 +2375,11 @@ static struct dentry *devfs_lookup (stru
    de->name, de->inode.ino, inode, de, current->comm);
    d_instantiate (dentry, inode);
    out:
    + write_lock (&parent->u.dir.lock);
    dentry->d_op = &devfs_dops;
    dentry->d_fsdata = NULL;
    - write_lock (&parent->u.dir.lock);
    - wake_up (&lookup_info.wait_queue);
    + wake_up (&lookup_info->wait_queue);
    + put_devfs_lookup_struct(lookup_info);
    write_unlock (&parent->u.dir.lock);
    devfs_put (de);
    return retval;
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