CVE-2024-47679 Information

Description

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 bug1 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.

[2]: The reason might be 1. SB_ACTIVE was removed or 2. mapping_shrinkable() return false when I reproduced the bug.

CVSS Vector

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H

Reference

https://git.kernel.org/stable/c/47a68c75052a660e4c37de41e321582ec9496195 https://git.kernel.org/stable/c/3721a69403291e2514d13a7c3af50a006ea1153b https://git.kernel.org/stable/c/540fb13120c9eab3ef203f90c00c8e69f37449d1 https://git.kernel.org/stable/c/0eed942bc65de1f93eca7bda51344290f9c573bb https://git.kernel.org/stable/c/0f8a5b6d0dafa4f533ac82e98f8b812073a7c9d1 https://git.kernel.org/stable/c/6c857fb12b9137fee574443385d53914356bbe11 https://git.kernel.org/stable/c/88b1afbf0f6b221f6c5bb66cc80cd3b38d696687

Attack Complexity

HIGH

Privileges Required

LOW

User Interaction Required

LOW

Scope

NONE

Confidentiality Impact

UNCHANGED

Integrity Impact

NONE

Availability Impact

NONE

Base Score

HIGH

Base Severity

4.7