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]

cachefiles_ondemand_fd_write_iter
fscache_cookie_state_machine
cachefiles_withdraw_cookie
if (!file) return -ENOBUFS
cachefiles_clean_up_object
cachefiles_unmark_inode_in_use
fput(object->file)
object->file = NULL
// file NULL pointer dereference!
__cachefiles_write(..., file, ...)

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

References

• https://nvd.nist.gov/vuln/detail/CVE-2024-56549
• https://git.kernel.org/stable/c/31ad74b20227ce6b40910ff78b1c604e42975cf1
• https://git.kernel.org/stable/c/9582c7664103c9043e80a78f5c382aa6bdd67418
• https://git.kernel.org/stable/c/f98770440c9bc468e2fd878212ec9526dbe08293