CVE-2024-39486

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

drm/drm_file: Fix pid refcounting race

maarten.lankhorst@linux.intel.com, Maxime Ripard mripard@kernel.org, Thomas Zimmermann tzimmermann@suse.de

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, , 1) mutex_unlock(&dev->filelist_mutex) begin drm_file_update_pid mutex_lock(&dev->filelist_mutex) rcu_replace_pointer(filp->pid, , 1) mutex_unlock(&dev->filelist_mutex) get_pid() synchronize_rcu() put_pid() *** pid B reaches refcount 0 and is freed here *** get_pid() *** UAF *** synchronize_rcu() put_pid()

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 tasks 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.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/0acce2a5c619ef1abdee783d7fea5eac78ce4844
• https://git.kernel.org/stable/c/16682588ead4a593cf1aebb33b36df4d1e9e4ffa
• https://git.kernel.org/stable/c/4f2a129b33a2054e62273edd5a051c34c08d96e9