CVE-2024-47679

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

vfs: fix race between evice_inodes() and find_inode()&iput()

Hi, all

Recently I noticed a bug[1] in btrfs, after digged it into

and I believe it'a race in vfs.

Let's assume there's a inode (ie ino 261) with i_count 1 is

called by iput(), and there's a concurrent thread calling

generic_shutdown_super().

cpu0: cpu1:

iput() // i_count is 1

->spin_lock(inode)

->dec i_count to 0

->iput_final() generic_shutdown_super()

->__inode_add_lru() ->evict_inodes()

// cause some reason[2] ->if (atomic_read(inode->i_count)) continue;

// return before // inode 261 passed the above check

// list_lru_add_obj() // and then schedule out

->spin_unlock()

// note here: the inode 261

// was still at sb list and hash list,

// and I_FREEING|I_WILL_FREE was not been set

btrfs_iget()

// after some function calls

->find_inode()

// found the above inode 261

->spin_lock(inode)

// check I_FREEING|I_WILL_FREE

// and passed

->__iget()

->spin_unlock(inode) // schedule back

->spin_lock(inode)

// check (I_NEW|I_FREEING|I_WILL_FREE) flags,

// passed and set I_FREEING

iput() ->spin_unlock(inode)

->spin_lock(inode) ->evict()

// dec i_count to 0

->iput_final()

->spin_unlock()

->evict()

Now, we have two threads simultaneously evicting

the same inode, which may trigger the BUG(inode->i_state & I_CLEAR)

statement both within clear_inode() and iput().

To fix the bug, recheck the inode->i_count after holding i_lock.

Because in the most scenarios, the first check is valid, and

the overhead of spin_lock() can be reduced.

If there is any misunderstanding, please let me know, thanks.

[1]: https://lore.kernel.org/linux-btrfs/[email protected]/

[2]: The reason might be 1. SB_ACTIVE was removed or 2. mapping_shrinkable()

return false when I reproduced the bug.