Date:   Wed, 14 Nov 2018 16:31:11 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     paulmck@...ux.ibm.com
Cc:     Kyungtae Kim <kt0755@...il.com>, josh@...htriplett.org,
        Steven Rostedt <rostedt@...dmis.org>,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        Byoungyoung Lee <lifeasageek@...il.com>,
        DaeRyong Jeong <threeearcat@...il.com>,
        syzkaller@...glegroups.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: KMSAN: uninit-value in rcu_accelerate_cbs / KMSAN: uninit-value
 in rcu_process_callbacks

On Wed, Nov 14, 2018 at 4:09 PM Paul E. McKenney <paulmck@...ux.ibm.com> wrote:
>
> On Wed, Nov 14, 2018 at 04:03:33AM -0500, Kyungtae Kim wrote:
> > We report two crashes in v4.19-rc8 (4.20-rc1 as well, I guess):
> > (Unfortunately, there is no repro for those.)
> >
> > The two crashes seem to share the same issue.
> > In both cases, (uninitialized) memory access violation occurs
> > when "rdp->cblist" is about to be accessed (kernel/rcu/tree.c:2838,1728).
> > I guess those are freed before the use, but I still haven't figured
> > out the reason why.
> > I'm looking forward to some help.
First of all, I'd avoid reporting KMSAN bugs without clear reproducers.
The tool is still in beta and may still give false positives due to
either missed initialization or rare memory corruptions.

> You lost me on this one.  In both cases, rdp references a per-CPU
> variable that is implicitly initialized to all zeroes, due to being
> (sort of) a C-language global.
>
> If a callback is queued early, then the following lines in __call_rcu()
> will make an honest list of that field because of the :
>
>                 if (rcu_segcblist_empty(&rdp->cblist))
>                         rcu_segcblist_init(&rdp->cblist);
>
> Otherwise, when rcu_init() is invoked during early boot, we have this
> in rcu_init_percpu_data(), which is called from rcutree_prepare_cpu()
> which is called from rcu_init(), which is called from start_kernel():
>
>         if (rcu_segcblist_empty(&rdp->cblist) && /* No early-boot CBs? */
>             !init_nocb_callback_list(rdp))
>                 rcu_segcblist_init(&rdp->cblist);  /* Re-enable callbacks. */
>
> So either init_nocb_callback_list() initializes the alternative callback
> lists for a no-CBs CPU or rcu_segcblist_init() again makes an honest
> list of that field.
>
> My guess is that your tool is missing the
>
>         rdp = this_cpu_ptr(rsp->rda);
>
> in the __call_rcu() case, and also missing the
>
>         struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
>
> Note that the ->rda field is explicitly compile-time initialized to
> the base address of the per-CPU variable, which is rcu_preempt_data,
> rcu_bh_data, or rcu_sched_data, depending on which RCU flavor is at hand.
> (In v4.20-rc1, these are all merged into a single flavor to rule them all.)
>
> Alternatively, your tool might be missing the implicit initialization
> of per-CPU variables.
This used to be fine, but after rebasing to v4.20-rc2 I also started
seeing strange reports on per-CPU variables. Taking a look.
> Or maybe I am missing something.  If so, please let me know what it is.
>
>                                                         Thanx, Paul
>
> > Crash log 1
> > =========================================
> > BUG: KMSAN: uninit-value in __rcu_process_callbacks
> > kernel/rcu/tree.c:2838 [inline]
> > BUG: KMSAN: uninit-value in rcu_process_callbacks+0x5ac/0x1cb0
> > kernel/rcu/tree.c:2864
> > CPU: 0 PID: 20 Comm: kauditd Not tainted 4.19.0-rc8+ #18
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> > Call Trace:
> >  <IRQ>
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x305/0x460 lib/dump_stack.c:113
> >  kmsan_report+0x1a2/0x2e0 mm/kmsan/kmsan.c:917
> >  __msan_warning+0x7d/0xe0 mm/kmsan/kmsan_instr.c:500
> >  __rcu_process_callbacks kernel/rcu/tree.c:2838 [inline]
> >  rcu_process_callbacks+0x5ac/0x1cb0 kernel/rcu/tree.c:2864
> >  __do_softirq+0x5ff/0xa55 kernel/softirq.c:292
> >  invoke_softirq kernel/softirq.c:373 [inline]
> >  irq_exit+0x22d/0x270 kernel/softirq.c:414
> >  exiting_irq+0xe/0x10 arch/x86/include/asm/apic.h:536
> >  smp_apic_timer_interrupt+0x64/0x90 arch/x86/kernel/apic/apic.c:1059
> >  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:869
> >  </IRQ>
> > RIP: 0010:finish_lock_switch+0x2b/0x40 kernel/sched/core.c:2578
> > Code: 48 89 e5 53 48 89 fb e8 e3 43 9a 00 8b b8 88 0c 00 00 48 8b 00
> > 48 85 c0 75 12 48 89 df e8 7d 38 9a 00 c6 00 00 c6 03 00 fb 5b <5d> c3
> > e8 de 42 9a 00 eb e7 66 66 66 2e 0f 1f 84 00 00 00 00 00 55
> > RSP: 0018:ffff88010622fca0 EFLAGS: 00000286 ORIG_RAX: ffffffffffffff13
> > RAX: ffff8801105bcc40 RBX: ffff8801061554c0 RCX: ffff8801105bdc40
> > RDX: ffff8801105bdc40 RSI: aaaaaaaaaaaab000 RDI: ffffea00077ec560
> > RBP: ffff88010622fca0 R08: ffffffff7fffffff R09: 0000000000000002
> > R10: 0000000000000000 R11: 0000000000000000 R12: ffff8800751cb880
> > R13: 0000000000000000 R14: ffff880106155db8 R15: ffff88013fcb9c40
> >  finish_task_switch+0xe3/0x270 kernel/sched/core.c:2679
> >  context_switch kernel/sched/core.c:2832 [inline]
> >  __schedule+0x78f/0x8f0 kernel/sched/core.c:3479
> >  schedule+0x1cc/0x300 kernel/sched/core.c:3523
> >  kauditd_thread+0xc64/0xee0 kernel/audit.c:889
> >  kthread+0x5b1/0x5f0 kernel/kthread.c:247
> >  ret_from_fork+0x35/0x40 arch/x86/entry/entry_64.S:416
> >
> > Uninit was created at:
> >  kmsan_save_stack_with_flags mm/kmsan/kmsan.c:255 [inline]
> >  kmsan_internal_alloc_meta_for_pages+0x157/0x730 mm/kmsan/kmsan.c:693
> >  kmsan_alloc_page+0x80/0xe0 mm/kmsan/kmsan_hooks.c:320
> >  __alloc_pages_nodemask+0x128c/0x69b0 mm/page_alloc.c:4416
> >  alloc_pages_current+0x51f/0x760 mm/mempolicy.c:2093
> >  alloc_pages include/linux/gfp.h:511 [inline]
> >  alloc_slab_page mm/slub.c:1459 [inline]
> >  allocate_slab mm/slub.c:1604 [inline]
> >  new_slab+0x552/0x1f30 mm/slub.c:1675
> >  new_slab_objects mm/slub.c:2438 [inline]
> >  ___slab_alloc+0x1414/0x1dd0 mm/slub.c:2590
> >  __slab_alloc mm/slub.c:2630 [inline]
> >  slab_alloc_node mm/slub.c:2693 [inline]
> >  slab_alloc mm/slub.c:2735 [inline]
> >  kmem_cache_alloc+0xc9b/0xda0 mm/slub.c:2740
> >  kmem_cache_zalloc include/linux/slab.h:697 [inline]
> >  avc_alloc_node+0x109/0xb90 security/selinux/avc.c:572
> >  avc_update_node+0x172/0x1ee0 security/selinux/avc.c:859
> >  avc_denied+0x312/0x360 security/selinux/avc.c:1024
> >  avc_has_perm_noaudit+0x733/0x770 security/selinux/avc.c:1155
> >  avc_has_perm+0x172/0x480 security/selinux/avc.c:1184
> >  sock_has_perm security/selinux/hooks.c:4539 [inline]
> >  selinux_socket_sendmsg+0x297/0x360 security/selinux/hooks.c:4875
> >  security_socket_sendmsg+0x127/0x200 security/security.c:1410
> >  sock_sendmsg net/socket.c:628 [inline]
> >  ___sys_sendmsg+0xd5f/0x1290 net/socket.c:2116
> >  __sys_sendmsg net/socket.c:2154 [inline]
> >  __do_sys_sendmsg net/socket.c:2163 [inline]
> >  __se_sys_sendmsg+0x307/0x460 net/socket.c:2161
> >  __x64_sys_sendmsg+0x4a/0x70 net/socket.c:2161
> >  do_syscall_64+0xb8/0x100 arch/x86/entry/common.c:291
> >  entry_SYSCALL_64_after_hwframe+0x63/0xe7
> > =========================================================
> >
> > Crash log 2
> > =========================================================
> > BUG: KMSAN: uninit-value in rcu_accelerate_cbs+0x821/0x990
> > kernel/rcu/tree.c:1728
> > CPU: 0 PID: 10 Comm: rcu_sched Not tainted 4.19.0-rc8+ #18
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x305/0x460 lib/dump_stack.c:113
> >  kmsan_report+0x1a2/0x2e0 mm/kmsan/kmsan.c:917
> >  __msan_warning+0x7d/0xe0 mm/kmsan/kmsan_instr.c:500
> >  rcu_accelerate_cbs+0x821/0x990 kernel/rcu/tree.c:1728
> >  __note_gp_changes+0x2ac/0x9e0 kernel/rcu/tree.c:1807
> >  rcu_gp_cleanup kernel/rcu/tree.c:2109 [inline]
> >  rcu_gp_kthread+0x3019/0x3990 kernel/rcu/tree.c:2236
> >  kthread+0x5b1/0x5f0 kernel/kthread.c:247
> >  ret_from_fork+0x35/0x40 arch/x86/entry/entry_64.S:416
> >
> > Uninit was created at:
> >  kmsan_save_stack_with_flags mm/kmsan/kmsan.c:255 [inline]
> >  kmsan_internal_alloc_meta_for_pages+0x157/0x730 mm/kmsan/kmsan.c:693
> >  kmsan_alloc_page+0x80/0xe0 mm/kmsan/kmsan_hooks.c:320
> >  __alloc_pages_nodemask+0x128c/0x69b0 mm/page_alloc.c:4416
> >  alloc_pages_current+0x51f/0x760 mm/mempolicy.c:2093
> >  alloc_pages include/linux/gfp.h:511 [inline]
> >  alloc_slab_page mm/slub.c:1459 [inline]
> >  allocate_slab mm/slub.c:1604 [inline]
> >  new_slab+0x552/0x1f30 mm/slub.c:1675
> >  new_slab_objects mm/slub.c:2438 [inline]
> >  ___slab_alloc+0x1414/0x1dd0 mm/slub.c:2590
> >  __slab_alloc mm/slub.c:2630 [inline]
> >  slab_alloc_node mm/slub.c:2693 [inline]
> >  slab_alloc mm/slub.c:2735 [inline]
> >  kmem_cache_alloc+0xc9b/0xda0 mm/slub.c:2740
> >  kmem_cache_zalloc include/linux/slab.h:697 [inline]
> >  avc_alloc_node+0x109/0xb90 security/selinux/avc.c:572
> >  avc_insert security/selinux/avc.c:696 [inline]
> >  avc_compute_av+0x31e/0x1050 security/selinux/avc.c:1008
> >  avc_has_perm_noaudit+0x516/0x770 security/selinux/avc.c:1149
> >  avc_has_perm+0x172/0x480 security/selinux/avc.c:1184
> >  selinux_socket_create+0x248/0x3c0 security/selinux/hooks.c:4560
> >  security_socket_create+0x146/0x210 security/security.c:1372
> >  __sock_create+0x26b/0xf30 net/socket.c:1232
> >  sock_create net/socket.c:1317 [inline]
> >  __sys_socket+0x180/0x670 net/socket.c:1347
> >  __do_sys_socket net/socket.c:1356 [inline]
> >  __se_sys_socket+0x8d/0xb0 net/socket.c:1354
> >  __x64_sys_socket+0x4a/0x70 net/socket.c:1354
> >  do_syscall_64+0xb8/0x100 arch/x86/entry/common.c:291
> >  entry_SYSCALL_64_after_hwframe+0x63/0xe7
> > =========================================================
> >
> > Thanks,
> > Kyungtae Kim
> >
>
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Alexander Potapenko
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