CVE-2024-40899 cachefiles: fix slab-use-after-free in cachefiles_ondemand_get_fd()
A use-after-free vulnerability in the Linux kernel's cachefiles subsystem allows local attackers to execute arbitrary code or escalate privileges.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2024-40899?
CVE-2024-40899 is a high-severity use after free vulnerability in Linux kernel, published on 12 Jul 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
cachefiles: fix slab-use-after-free in cachefiles_ondemand_get_fd()
We got the following issue in a fuzz test of randomly issuing the restore command:
================================================================== BUG: KASAN: slab-use-after-free in cachefiles_ondemand_daemon_read+0x609/0xab0 Write of size 4 at addr ffff888109164a80 by task ondemand-04-dae/4962
CPU: 11 PID: 4962 Comm: ondemand-04-dae Not tainted 6.8.0-rc7-dirty #542 Call Trace: kasan_report+0x94/0xc0 cachefiles_ondemand_daemon_read+0x609/0xab0 vfs_read+0x169/0xb50 ksys_read+0xf5/0x1e0
Allocated by task 626: __kmalloc+0x1df/0x4b0 cachefiles_ondemand_send_req+0x24d/0x690 cachefiles_create_tmpfile+0x249/0xb30 cachefiles_create_file+0x6f/0x140 cachefiles_look_up_object+0x29c/0xa60 cachefiles_lookup_cookie+0x37d/0xca0 fscache_cookie_state_machine+0x43c/0x1230 [...]
Freed by task 626: kfree+0xf1/0x2c0 cachefiles_ondemand_send_req+0x568/0x690 cachefiles_create_tmpfile+0x249/0xb30 cachefiles_create_file+0x6f/0x140 cachefiles_look_up_object+0x29c/0xa60 cachefiles_lookup_cookie+0x37d/0xca0 fscache_cookie_state_machine+0x43c/0x1230 [...] ==================================================================
Following is the process that triggers the issue:
mount | daemon_thread1 | daemon_thread2 ------------------------------------------------------------ cachefiles_ondemand_init_object cachefiles_ondemand_send_req REQ_A = kzalloc(sizeof(*req) + data_len) wait_for_completion(&REQ_A->done)
cachefiles_daemon_read cachefiles_ondemand_daemon_read REQ_A = cachefiles_ondemand_select_req cachefiles_ondemand_get_fd copy_to_user(_buffer, msg, n) process_open_req(REQ_A) ------ restore ------ cachefiles_ondemand_restore xas_for_each(&xas, req, ULONG_MAX) xas_set_mark(&xas, CACHEFILES_REQ_NEW);
cachefiles_daemon_read cachefiles_ondemand_daemon_read REQ_A = cachefiles_ondemand_select_req
write(devfd, ("copen %u,%llu", msg->msg_id, size)); cachefiles_ondemand_copen xa_erase(&cache->reqs, id) complete(&REQ_A->done) kfree(REQ_A) cachefiles_ondemand_get_fd(REQ_A) fd = get_unused_fd_flags file = anon_inode_getfile fd_install(fd, file) load = (void *)REQ_A->msg.data; load->fd = fd; // load UAF !!!
This issue is caused by issuing a restore command when the daemon is still alive, which results in a request being processed multiple times thus triggering a UAF. So to avoid this problem, add an additional reference count to cachefiles_req, which is held while waiting and reading, and then released when the waiting and reading is over.
Note that since there is only one reference count for waiting, we need to avoid the same request being completed multiple times, so we can only complete the request if it is successfully removed from the xarray.
Weakness: CWE-416
Which versions are affected by CVE-2024-40899?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2024-40899?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2024-40899 being exploited?
There is no known exploitation of CVE-2024-40899 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2024-40899, DEBIAN-CVE-2024-40899, GHSA-8jvx-wqf8-m6ph, UBUNTU-CVE-2024-40899.