Red Hat: CVE-2024-39486: kernel: drm/drm_file: Fix pid refcounting race (Multiple Advisories)

Red Hat: CVE-2024-39486: kernel: drm/drm_file: Fix pid refcounting race (Multiple Advisories)

Severity

5

CVSS

(AV:L/AC:L/Au:S/C:N/I:N/A:C)

Published

07/06/2024

Created

12/06/2024

Added

12/05/2024

Modified

12/05/2024

Description

In the Linux kernel, the following vulnerability has been resolved: drm/drm_file: Fix pid refcounting race <[email protected]>, Maxime Ripard <[email protected]>, Thomas Zimmermann <[email protected]> filp->pid is supposed to be a refcounted pointer; however, before this patch, drm_file_update_pid() only increments the refcount of a struct pid after storing a pointer to it in filp->pid and dropping the dev->filelist_mutex, making the following race possible: process A process B ========= ========= begin drm_file_update_pid mutex_lock(&dev->filelist_mutex) rcu_replace_pointer(filp->pid, <pid B>, 1) mutex_unlock(&dev->filelist_mutex) begin drm_file_update_pid mutex_lock(&dev->filelist_mutex) rcu_replace_pointer(filp->pid, <pid A>, 1) mutex_unlock(&dev->filelist_mutex) get_pid(<pid A>) synchronize_rcu() put_pid(<pid B>) *** pid B reaches refcount 0 and is freed here *** get_pid(<pid B>) *** UAF *** synchronize_rcu() put_pid(<pid A>) As far as I know, this race can only occur with CONFIG_PREEMPT_RCU=y because it requires RCU to detect a quiescent state in code that is not explicitly calling into the scheduler. This race leads to use-after-free of a "struct pid". It is probably somewhat hard to hit because process A has to pass through a synchronize_rcu() operation while process B is between mutex_unlock() and get_pid(). Fix it by ensuring that by the time a pointer to the current task's pid is stored in the file, an extra reference to the pid has been taken. This fix also removes the condition for synchronize_rcu(); I think that optimization is unnecessary complexity, since in that case we would usually have bailed out on the lockless check above.

Solution(s)

• redhat-upgrade-kernel
• redhat-upgrade-kernel-rt

References

• CVE-2024-39486
• RHSA-2024:9315