CVE-2024-50140

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

sched/core: Disable page allocation in task_tick_mm_cid()

With KASAN and PREEMPT_RT enabled, calling task_work_add() in task_tick_mm_cid() may cause the following splat.

[ 63.696416] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48 [ 63.696416] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 610, name: modprobe [ 63.696416] preempt_count: 10001, expected: 0 [ 63.696416] RCU nest depth: 1, expected: 1

This problem is caused by the following call trace.

sched_tick() [ acquire rq->__lock ] -> task_tick_mm_cid() -> task_work_add() -> __kasan_record_aux_stack() -> kasan_save_stack() -> stack_depot_save_flags() -> alloc_pages_mpol_noprof() -> __alloc_pages_noprof() -> get_page_from_freelist() -> rmqueue() -> rmqueue_pcplist() -> __rmqueue_pcplist() -> rmqueue_bulk() -> rt_spin_lock()

The rq lock is a raw_spinlock_t. We cant sleep while holding it. IOW, we cant call alloc_pages() in stack_depot_save_flags().

The task_tick_mm_cid() function with its task_work_add() call was introduced by commit 223baf9d17f2 (sched: Fix performance regression introduced by mm_cid) in v6.4 kernel.

Fortunately, there is a kasan_record_aux_stack_noalloc() variant that calls stack_depot_save_flags() while not allowing it to allocate new pages. To allow task_tick_mm_cid() to use task_work without page allocation, a new TWAF_NO_ALLOC flag is added to enable calling kasan_record_aux_stack_noalloc() instead of kasan_record_aux_stack() if set. The task_tick_mm_cid() function is modified to add this new flag.

The possible downside is the missing stack trace in a KASAN report due to new page allocation required when task_work_add_noallloc() is called which should be rare.

Affected Software

References

• https://git.kernel.org/stable/c/509c29d0d26f68a6f6d0a05cb1a89725237e2b87
• https://git.kernel.org/stable/c/73ab05aa46b02d96509cb029a8d04fca7bbde8c7
• https://git.kernel.org/stable/c/ce0241ef83eed55f675376e8a3605d23de53d875