cachefiles: Fix NULL pointer dereference in object->file

Vulnerability Description

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://git.kernel.org/stable/c/d6bba3ece960129a553d4b16f1b00c884dc0993a
• https://git.kernel.org/stable/c/785408bbafcfa24c9fc5b251f03fd0780ce182bd
• https://git.kernel.org/stable/c/f98770440c9bc468e2fd878212ec9526dbe08293
• https://git.kernel.org/stable/c/9582c7664103c9043e80a78f5c382aa6bdd67418
• https://git.kernel.org/stable/c/31ad74b20227ce6b40910ff78b1c604e42975cf1