2.7 KiB
CVE-2024-47679
Description
In the Linux kernel, the following vulnerability has been resolved:vfs: fix race between evice_inodes() and find_inode()&iput()Hi, allRecently I noticed a bug[1] in btrfs, after digged it intoand I believe it'a race in vfs.Let's assume there's a inode (ie ino 261) with i_count 1 iscalled by iput(), and there's a concurrent thread callinggeneric_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 setbtrfs_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_FREEINGiput() ->spin_unlock(inode) ->spin_lock(inode) ->evict() // dec i_count to 0 ->iput_final() ->spin_unlock() ->evict()Now, we have two threads simultaneously evictingthe 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, andthe overhead of spin_lock() can be reduced.If there is any misunderstanding, please let me know, thanks.[1]: https://lore.kernel.org/linux-btrfs/000000000000eabe1d0619c48986@google.com/[2]: The reason might be 1. SB_ACTIVE was removed or 2. mapping_shrinkable()return false when I reproduced the bug.
POC
Reference
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