vfs: Don't evict inode under the inode lru traversing context

In the Linux kernel, the following vulnerability has been resolved:

vfs: Don't evict inode under the inode lru traversing context

The inode reclaiming process(See function prune_icache_sb) collects all

reclaimable inodes and mark them with I_FREEING flag at first, at that

time, other processes will be stuck if they try getting these inodes

(See function find_inode_fast), then the reclaiming process destroy the

inodes by function dispose_list(). Some filesystems(eg. ext4 with

ea_inode feature, ubifs with xattr) may do inode lookup in the inode

evicting callback function, if the inode lookup is operated under the

inode lru traversing context, deadlock problems may happen.

Case 1: In function ext4_evict_inode(), the ea inode lookup could happen

if ea_inode feature is enabled, the lookup process will be stuck

under the evicting context like this:

1. File A has inode i_reg and an ea inode i_ea

2. getfattr(A, xattr_buf) // i_ea is added into lru // lru->i_ea

3. Then, following three processes running like this:

PA PB

echo 2 > /proc/sys/vm/drop_caches

shrink_slab

prune_dcache_sb

// i_reg is added into lru, lru->i_ea->i_reg

prune_icache_sb

list_lru_walk_one

inode_lru_isolate

i_ea->i_state |= I_FREEING // set inode state

inode_lru_isolate

__iget(i_reg)

spin_unlock(&i_reg->i_lock)

spin_unlock(lru_lock)

rm file A

i_reg->nlink = 0

iput(i_reg) // i_reg->nlink is 0, do evict

ext4_evict_inode

ext4_xattr_delete_inode

ext4_xattr_inode_dec_ref_all

ext4_xattr_inode_iget

ext4_iget(i_ea->i_ino)

iget_locked

find_inode_fast

__wait_on_freeing_inode(i_ea) ----→ AA deadlock

dispose_list // cannot be executed by prune_icache_sb

wake_up_bit(&i_ea->i_state)

Case 2: In deleted inode writing function ubifs_jnl_write_inode(), file

deleting process holds BASEHD's wbuf->io_mutex while getting the

xattr inode, which could race with inode reclaiming process(The

reclaiming process could try locking BASEHD's wbuf->io_mutex in

inode evicting function), then an ABBA deadlock problem would

happen as following:

1. File A has inode ia and a xattr(with inode ixa), regular file B has

inode ib and a xattr.

2. getfattr(A, xattr_buf) // ixa is added into lru // lru->ixa

3. Then, following three processes running like this:

PA PB PC

echo 2 > /proc/sys/vm/drop_caches

shrink_slab

prune_dcache_sb

// ib and ia are added into lru, lru->ixa->ib->ia

prune_icache_sb

list_lru_walk_one

inode_lru_isolate

ixa->i_state |= I_FREEING // set inode state

inode_lru_isolate

__iget(ib)

spin_unlock(&ib->i_lock)

spin_unlock(lru_lock)

rm file B

ib->nlink = 0

rm file A

iput(ia)

ubifs_evict_inode(ia)

ubifs_jnl_delete_inode(ia)

ubifs_jnl_write_inode(ia)

make_reservation(BASEHD) // Lock wbuf->io_mutex

ubifs_iget(ixa->i_ino)

iget_locked

find_inode_fast

__wait_on_freeing_inode(ixa)

| iput(ib) // ib->nlink is 0, do evict

| ubifs_evict_inode

| ubifs_jnl_delete_inode(ib)

↓ ubifs_jnl_write_inode

ABBA deadlock ←-----make_reservation(BASEHD)

dispose_list // cannot be executed by prune_icache_sb

wake_up_bit(&ixa->i_state)

Fix the possible deadlock by using new inode state flag I_LRU_ISOLATING

to pin the inode in memory while inode_lru_isolate(

---truncated---