linux-ext4 - ext4 corruption causing a crash

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Date:   Wed, 16 Nov 2016 09:57:33 +0200
From:   Nikolay Borisov <kernel@...p.com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Jan Kara <jack@...e.com>, linux-ext4 <linux-ext4@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>, bigeasy@...utronix.de
Subject: ext4 corruption causing a crash

Hello, 

Recently I saw the following report : http://www.securityfocus.com/archive/1/539661

I've been testing that image and even on latest 4.9-rc4 I'm able to reproduce a crash. 
This seems rather indetermenistic in that it causes double/triple faults which causes 
my qemu guest to completely hang or reboot. However, on 4.9-rc4 with errors=panic 
mount options the crash looks like : 

[   47.172026] BUG: unable to handle kernel NULL pointer dereference at 000000000000090f
[   47.172215] IP: [<ffffffff81090e3f>] update_curr+0x2f/0x210
[   47.172342] PGD 0 [   47.172389] 
[   47.172438] Oops: 0000 [#1] SMP
[   47.172521] Modules linked in:
[   47.172627] CPU: 0 PID: 1108 Comm: mount Tainted: G        W       4.9.0-rc4-clouder1 #30
[   47.172777] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.1-0-g4adadbd-20150316_085822-nilsson.home.kraxel.org 04/01/2014
[   47.173008] task: ffff8800068ec100 task.stack: ffff880006648000
[   47.173008] RIP: 0010:[<ffffffff81090e3f>]  [<ffffffff81090e3f>] update_curr+0x2f/0x210
[   47.173008] RSP: 0018:ffff880007c03d68  EFLAGS: 00010082
[   47.173008] RAX: ffffffffffffffff RBX: ffff880006630000 RCX: 0000000afbabb8be
[   47.173008] RDX: 0000000000000000 RSI: ffff8800068ec100 RDI: ffff880006630000
[   47.173008] RBP: ffff880007c03da8 R08: 0000000000000022 R09: 0000000000000026
[   47.173008] R10: 00000000005acec3 R11: 0000000000000000 R12: 0000000000016300
[   47.173008] R13: ffffffffffffffff R14: ffff880006630000 R15: 0000000000000000
[   47.173008] FS:  00007f08d69e97e0(0000) GS:ffff880007c00000(0000) knlGS:0000000000000000
[   47.173008] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   47.173008] CR2: 000000000000090f CR3: 000000000665e000 CR4: 00000000000006f0
[   47.173008] Stack:
[   47.173008]  fffffffffffffed4 ffffffff820368e0 fffffffffffffed4 ffff8800068ec180
[   47.173008]  0000000000016300 0000000000000000 ffff880006630000 0000000000000000
[   47.173008]  ffff880007c03e18 ffffffff81097c84 ffff880007c03df8 ffff880007c16dc0
[   47.173008] Call Trace:
[   47.173008]  <IRQ> [   47.173008]  [<ffffffff81097c84>] task_tick_fair+0x2a4/0x3f0
[   47.173008]  [<ffffffff81084fa8>] scheduler_tick+0x68/0xe0
[   47.173008]  [<ffffffff810c1c81>] update_process_times+0x51/0x70
[   47.173008]  [<ffffffff810d2bf8>] tick_sched_timer+0x58/0x180
[   47.173008]  [<ffffffff810c3678>] ? __remove_hrtimer+0x58/0x90
[   47.173008]  [<ffffffff810c3f07>] __hrtimer_run_queues+0xd7/0x290
[   47.173008]  [<ffffffff810d2ba0>] ? tick_setup_sched_timer+0x100/0x100
[   47.173008]  [<ffffffff8103b14d>] ? lapic_next_event+0x1d/0x30
[   47.173008]  [<ffffffff810ca54b>] ? ktime_get_update_offsets_now+0x5b/0x120
[   47.173008]  [<ffffffff8106100f>] ? __local_bh_enable+0x3f/0x70
[   47.173008]  [<ffffffff810c4252>] hrtimer_interrupt+0xa2/0x1e0
[   47.173008]  [<ffffffff8103b8d9>] local_apic_timer_interrupt+0x39/0x60
[   47.173008]  [<ffffffff816ac3a1>] smp_apic_timer_interrupt+0x41/0x55
[   47.173008]  [<ffffffff816ab699>] apic_timer_interrupt+0x89/0x90
[   47.173008]  <EOI> [   47.173008]  [<ffffffff8127d834>] ? ext4_calculate_overhead+0x264/0x470
[   47.173008]  [<ffffffff8127d81e>] ? ext4_calculate_overhead+0x24e/0x470
[   47.173008]  [<ffffffff8128b979>] ext4_fill_super+0x2019/0x3270
[   47.173008]  [<ffffffff81344483>] ? pointer+0x2a3/0x400
[   47.173008]  [<ffffffff811d0477>] mount_bdev+0x187/0x1d0
[   47.173008]  [<ffffffff81170a65>] ? __alloc_percpu+0x15/0x20
[   47.173008]  [<ffffffff81289960>] ? ext4_alloc_flex_bg_array+0x110/0x110
[   47.173008]  [<ffffffff8127aec5>] ext4_mount+0x15/0x20
[   47.173008]  [<ffffffff811cf3d3>] mount_fs+0x43/0x170
[   47.173008]  [<ffffffff811ef6d6>] vfs_kern_mount+0x76/0x130
[   47.173008]  [<ffffffff811f05f6>] do_mount+0x226/0xcb0
[   47.173008]  [<ffffffff8116a707>] ? memdup_user+0x57/0x90
[   47.173008]  [<ffffffff811f10fa>] SyS_mount+0x7a/0xc0
[   47.173008]  [<ffffffff816a9dea>] entry_SYSCALL_64_fastpath+0x18/0xa8

Without it I usually get a DF in the PF handler (ouch). However, Sebastian 
reported that on his 4.9-rc4 it doesn't crash: 
http://paste.debian.net/hidden/6fae7231/

PeterZ, ffffffff81090e3f is 
kernel/sched/fair.c:2727 - val >>= local_n / LOAD_AVG_PERIOD; in decay_load
kernel/sched/fair.c:2850
kernel/sched/fair.c:3053

the 2727 line code seems very suspicious in that it doesn't work with 
any global memory hmmm...

On older 4.4.x based kernel the symptoms range from DF/TF to memory 
corruption in the scheduler, to a crash on the last return statement in 
count_overhead. It's still puzzling how come ext4 can cause such a far reaching
memory corruption that scheduler data structures are affected. 

Nikolay





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