cve/published/2024/CVE-2024-50140.mbox

From bippy-5f407fcff5a0 Mon Sep 17 00:00:00 2001
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: <linux-cve-announce@vger.kernel.org>
Reply-to: <cve@kernel.org>, <linux-kernel@vger.kernel.org>
Subject: CVE-2024-50140: sched/core: Disable page allocation in task_tick_mm_cid()

Description
===========

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 can't sleep while holding
it. IOW, we can't 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.

The Linux kernel CVE team has assigned CVE-2024-50140 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.4 with commit 223baf9d17f25e2608dbdff7232c095c1e612268 and fixed in 6.6.59 with commit 509c29d0d26f68a6f6d0a05cb1a89725237e2b87
	Issue introduced in 6.4 with commit 223baf9d17f25e2608dbdff7232c095c1e612268 and fixed in 6.11.6 with commit ce0241ef83eed55f675376e8a3605d23de53d875
	Issue introduced in 6.4 with commit 223baf9d17f25e2608dbdff7232c095c1e612268 and fixed in 6.12 with commit 73ab05aa46b02d96509cb029a8d04fca7bbde8c7

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-50140
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	include/linux/task_work.h
	kernel/sched/core.c
	kernel/task_work.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/509c29d0d26f68a6f6d0a05cb1a89725237e2b87
	https://git.kernel.org/stable/c/ce0241ef83eed55f675376e8a3605d23de53d875
	https://git.kernel.org/stable/c/73ab05aa46b02d96509cb029a8d04fca7bbde8c7