Re: WARNING in apparmor_cred_free

[John Johansen]

On 1/11/19 2:11 PM, Casey Schaufler wrote:
> On 1/11/2019 1:43 AM, syzbot wrote:
>> Hello,
>>
>> syzbot found the following crash on:
>>
>> HEAD commit:ÂÂÂ b808822a75a3 Add linux-next specific files for 20190111
>> git tree:ÂÂÂÂÂÂ linux-next
>> console output: https://syzkaller.appspot.com/x/log.txt?x=179c22f7400000
>> kernel config:Â https://syzkaller.appspot.com/x/.config?x=c052ead0aed5001b
>> dashboard link: https://syzkaller.appspot.com/bug?extid=69ca07954461f189e808
>> compiler:ÂÂÂÂÂÂ gcc (GCC) 9.0.0 20181231 (experimental)
>> syz repro:ÂÂÂÂÂ https://syzkaller.appspot.com/x/repro.syz?x=162d947f400000
>> C reproducer:ÂÂ https://syzkaller.appspot.com/x/repro.c?x=139f6c37400000
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+69ca07954461f189e808@xxxxxxxxxxxxxxxxxxxxxxxxx
>>
>> ------------[ cut here ]------------
>> AppArmor WARN cred_label: ((!blob)):
>> WARNING: CPU: 0 PID: 0 at security/apparmor/include/cred.h:30 cred_label security/apparmor/include/cred.h:30 [inline]
>> WARNING: CPU: 0 PID: 0 at security/apparmor/include/cred.h:30 apparmor_cred_free+0x12f/0x1a0 security/apparmor/lsm.c:62
>> Kernel panic - not syncing: panic_on_warn set ...
>> CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.0.0-rc1-next-20190111 #10
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>> Call Trace:
>> Â<IRQ>
>> Â__dump_stack lib/dump_stack.c:77 [inline]
>> Âdump_stack+0x1db/0x2d0 lib/dump_stack.c:113
>> Âpanic+0x2cb/0x65c kernel/panic.c:214
>> Â__warn.cold+0x20/0x48 kernel/panic.c:571
>> Âreport_bug+0x263/0x2b0 lib/bug.c:186
>> Âfixup_bug arch/x86/kernel/traps.c:178 [inline]
>> Âfixup_bug arch/x86/kernel/traps.c:173 [inline]
>> Âdo_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:271
>> Âdo_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:290
>> Âinvalid_op+0x14/0x20 arch/x86/entry/entry_64.S:973
>> RIP: 0010:cred_label security/apparmor/include/cred.h:30 [inline]
>> RIP: 0010:apparmor_cred_free+0x12f/0x1a0 security/apparmor/lsm.c:62
>> Code: 7c 88 48 c7 c7 00 d0 7c 88 e8 fd 70 f2 fd 0f 0b eb a9 e8 54 3f 29 fe 48 c7 c6 c0 df 7c 88 48 c7 c7 00 d0 7c 88 e8 e1 70 f2 fd <0f> 0b 48 b8 00 00 00 00 00 fc ff df 80 38 00 75 4a 4c 8b 2c 25 00
>> RSP: 0018:ffff8880ae6079f8 EFLAGS: 00010286
>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
>> RDX: 0000000000000100 RSI: ffffffff81687fa6 RDI: 0000000000000006
>> RBP: ffff8880ae607a18 R08: ffffffff8987dec0 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880a86b3100
>> R13: ffff8880a86b3100 R14: ffff8880a86b3188 R15: dffffc0000000000
>> Âsecurity_cred_free+0x4b/0xf0 security/security.c:1490
> 
> The obvious thing to do is put a check in security_cred_free
> for a NULL cred->security, in which case the LSM hooks
> wouldn't get called.

Right, but the question is should we? To my thinking we shouldn't
ever have a cred without cred->security, unless the cred was
allocated but a later step in its construction, say allocating
->security failed.

In which case I'd rather see the cred directly freed and not
call into security_cred_free() as I like being able to detect
corrupt creds.

We certainly can still do the check for security on only live creds
but I would like to understand this particular failure better first

> It's not clear to me how we got a cred
> that doesn't have an allocated security blob.

I have been trying to figure that one out as well.


> 
>> Âput_cred_rcu+0x21f/0x6e0 kernel/cred.c:118
>> Â__rcu_reclaim kernel/rcu/rcu.h:240 [inline]
>> Ârcu_do_batch kernel/rcu/tree.c:2486 [inline]
>> Âinvoke_rcu_callbacks kernel/rcu/tree.c:2799 [inline]
>> Ârcu_core+0xc4a/0x1680 kernel/rcu/tree.c:2780
>> Â__do_softirq+0x30b/0xb11 kernel/softirq.c:292
>> Âinvoke_softirq kernel/softirq.c:373 [inline]
>> Âirq_exit+0x180/0x1d0 kernel/softirq.c:413
>> Âexiting_irq arch/x86/include/asm/apic.h:536 [inline]
>> Âsmp_apic_timer_interrupt+0x1b7/0x760 arch/x86/kernel/apic/apic.c:1062
>> Âapic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:807
>> Â</IRQ>
>> RIP: 0010:native_safe_halt+0x2/0x10 arch/x86/include/asm/irqflags.h:58
>> Code: ff ff ff 48 89 c7 48 89 45 d8 e8 79 6f d0 f9 48 8b 45 d8 e9 ce fe ff ff 48 89 df e8 68 6f d0 f9 eb 82 90 90 90 90 90 90 fb f4 <c3> 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 f4 c3 90 90 90 90 90 90
>> RSP: 0018:ffffffff89807c60 EFLAGS: 00000282 ORIG_RAX: ffffffffffffff13
>> RAX: 1ffffffff1325061 RBX: 1ffffffff1300f8f RCX: 0000000000000000
>> RDX: dffffc0000000000 RSI: 0000000000000001 RDI: ffffffff8987e73c
>> RBP: ffffffff89807d20 R08: ffffffff8987dec0 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
>> R13: ffffffff89807cf8 R14: 0000000000000000 R15: ffffffff899282f8
>> Âarch_cpu_idle+0x10/0x20 arch/x86/kernel/process.c:555
>> Âdefault_idle_call+0x36/0x90 kernel/sched/idle.c:93
>> Âcpuidle_idle_call kernel/sched/idle.c:153 [inline]
>> Âdo_idle+0x386/0x5d0 kernel/sched/idle.c:262
>> Âcpu_startup_entry+0x1b/0x20 kernel/sched/idle.c:353
>> Ârest_init+0x245/0x37b init/main.c:442
>> Âarch_call_rest_init+0xe/0x1b
>> Âstart_kernel+0x882/0x8bd init/main.c:742
>> Âx86_64_start_reservations+0x29/0x2b arch/x86/kernel/head64.c:470
>> Âx86_64_start_kernel+0x77/0x7b arch/x86/kernel/head64.c:451
>> Âsecondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:243
>> Kernel Offset: disabled
>> Rebooting in 86400 seconds..
>>
>>
>> ---
>> This bug is generated by a bot. It may contain errors.
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>> syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxxx
>>
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>>