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Date:   Sat, 29 Aug 2020 17:22:52 -0700
From:   Yonghong Song <yhs@...com>
To:     Alexei Starovoitov <ast@...com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        <davem@...emloft.net>
CC:     <daniel@...earbox.net>, <josef@...icpanda.com>,
        <bpoirier@...e.com>, <akpm@...ux-foundation.org>,
        <hannes@...xchg.org>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 5/5] selftests/bpf: Add sleepable tests



On 8/29/20 3:38 PM, Alexei Starovoitov wrote:
> On 8/29/20 3:13 PM, Yonghong Song wrote:
>>
>> When running selftest, I hit the following kernel warning:
>>
>> [  250.871267] ============================================
>> [  250.871902] WARNING: possible recursive locking detected
>> [  250.872561] 5.9.0-rc1+ #830 Not tainted
>> [  250.873166] --------------------------------------------
>> [  250.873991] true/2053 is trying to acquire lock:
>> [  250.874715] ffff8fc1f9cd2068 (&mm->mmap_lock#2){++++}-{3:3}, at: 
>> __might_fault+0x3e/0x90
>> [  250.875943]
>> [  250.875943] but task is already holding lock:
>> [  250.876688] ffff8fc1f9cd2068 (&mm->mmap_lock#2){++++}-{3:3}, at: 
>> do_mprotect_pkey+0xb5/0x2f0
>> [  250.877978]
>> [  250.877978] other info that might help us debug this:
>> [  250.878797]  Possible unsafe locking scenario:
>> [  250.878797]
>> [  250.879708]        CPU0
>> [  250.880095]        ----
>> [  250.880482]   lock(&mm->mmap_lock#2);
>> [  250.881063]   lock(&mm->mmap_lock#2);
>> [  250.881645]
>> [  250.881645]  *** DEADLOCK ***
>> [  250.881645]
>> [  250.882559]  May be due to missing lock nesting notation
>> [  250.882559]
>> [  250.883613] 2 locks held by true/2053:
>> [  250.884194]  #0: ffff8fc1f9cd2068 (&mm->mmap_lock#2){++++}-{3:3}, 
>> at: do_mprotect_pkey+0xb5/0x2f0
>> [  250.885558]  #1: ffffffffbc47b8a0 
>> (rcu_read_lock_trace){....}-{0:0}, at: 
>> __bpf_prog_enter_sleepable+0x0/0x40
>> [  250.887062]
>> [  250.887062] stack backtrace:
>> [  250.887583] CPU: 1 PID: 2053 Comm: true Not tainted 5.9.0-rc1+ #830
>> [  250.888546] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), 
>> BIOS 1.9.3-1.el7.centos 04/01/2014
>> [  250.889896] Call Trace:
>> [  250.890222]  dump_stack+0x78/0xa0
>> [  250.890644]  __lock_acquire.cold.74+0x209/0x2e3
>> [  250.891350]  lock_acquire+0xba/0x380
>> [  250.891919]  ? __might_fault+0x3e/0x90
>> [  250.892510]  ? __lock_acquire+0x639/0x20c0
>> [  250.893150]  __might_fault+0x68/0x90
>> [  250.893717]  ? __might_fault+0x3e/0x90
>> [  250.894325]  _copy_from_user+0x1e/0xa0
>> [  250.894946]  bpf_copy_from_user+0x22/0x50
>> [  250.895581]  bpf_prog_3717002769f30998_test_int_hook+0x76/0x60c
>> [  250.896446]  ? __bpf_prog_enter_sleepable+0x3c/0x40
>> [  250.897207]  ? __bpf_prog_exit+0xa0/0xa0
>> [  250.897819]  bpf_trampoline_18669+0x29/0x1000
>> [  250.898476]  bpf_lsm_file_mprotect+0x5/0x10
>> [  250.899133]  security_file_mprotect+0x32/0x50
>> [  250.899816]  do_mprotect_pkey+0x18a/0x2f0
>> [  250.900472]  __x64_sys_mprotect+0x1b/0x20
>> [  250.901107]  do_syscall_64+0x33/0x40
>> [  250.901670]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> [  250.902450] RIP: 0033:0x7fd95c141ef7
>> [  250.903014] Code: ff 66 90 b8 0b 00 00 00 0f 05 48 3d 01 f0 ff ff 
>> 73 01 c3 48 8d 0d 21 c2 2
>> 0 00 f7 d8 89 01 48 83 c8 ff c3 b8 0a 00 00 00 0f 05 <48> 3d 01 f0 ff 
>> ff 73 01 c3 48 8d 0d 01
>> c2 20 00 f7 d8 89 01 48 83
>> [  250.905732] RSP: 002b:00007ffd4c291fe8 EFLAGS: 00000246 ORIG_RAX: 
>> 000000000000000a
>> [  250.906773] RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 
>> 00007fd95c141ef7
>> [  250.907866] RDX: 0000000000000000 RSI: 00000000001ff000 RDI: 
>> 00007fd95bf20000
>> [  250.908906] RBP: 00007ffd4c292320 R08: 0000000000000000 R09: 
>> 0000000000000000
>> [  250.909915] R10: 00007ffd4c291ff0 R11: 0000000000000246 R12: 
>> 00007fd95c341000
>> [  250.910919] R13: 00007ffd4c292408 R14: 0000000000000002 R15: 
>> 0000000000000801
>>
>> Could this be an real issue here?
>>
>> do_mprotect_pkey() gets a lock of current->mm->mmap_lock
>> before calling security_file_mprotect(bpf_lsm_file_mprotect).
>> Later on, when do _copy_to_user(), page fault may happen
>> and current->mm->mmap_lock might be acquired again and may
>> have a deadlock here?
> 
> Hmm. It does sound like dead_lock.
> But I don't understand why I don't see this splat.
> I have
> LOCKDEP=y
> DEBUG_ATOMIC_SLEEP=y
> LOCK_DEBUGGING_SUPPORT=y
> KASAN=y
> in my .config and don't see it :(
> Could pls send me your .config?

The config file is attached. In my environment, the warning is only
printed out during the first run of `./test_progs -t lsm`. All later
runs did not have this warning.

> I'll analyze further.
> Thanks for the reporting!

View attachment "lsm.config" of type "text/plain" (128059 bytes)

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