CVE-2024-56549

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

cachefiles: Fix NULL pointer dereference in object->file

At present, the object->file has the NULL pointer dereference problem in ondemand-mode. The root cause is that the allocated fd and object->file lifetime are inconsistent, and the user-space invocation to anon_fd uses object->file. Following is the process that triggers the issue:

  [write fd]                [umount]

cachefilesondemandfdwriteiter fscachecookiestatemachine cachefileswithdrawcookie if (!file) return -ENOBUFS cachefilescleanupobject cachefilesunmarkinodeinuse fput(object->file) object->file = NULL // file NULL pointer dereference! _cachefileswrite(..., file, ...)

Fix this issue by add an additional reference count to the object->file before write/llseek, and decrement after it finished.