RE: futex/call -to plist_for_each_entry_safe with head=NULL

[David Mozes <david.mozes@silk.us>]

Thx Matthew

1) You are probably correct regarding the place the actual crash happened unless something happens in-betweens....
But that what the gdb told us in addition, the RDI shows us the value of 0x00000246. 


Jun 10 20:49:40 c-node04 kernel: [97562.144463] BUG: unable to handle kernel NULL pointer dereference at 0000000000000246
Jun 10 20:49:40 c-node04 kernel: [97562.145450] PGD 2012ee4067 P4D 2012ee4067 PUD 20135a0067 PMD 0
Jun 10 20:49:40 c-node04 kernel: [97562.145450] Oops: 0000 [#1] SMP
Jun 10 20:49:40 c-node04 kernel: [97562.145450] CPU: 36 PID: 12668 Comm: STAR4BLKS0_WORK Kdump: loaded Tainted: G        W  OE     4.19.149-KM6 #1
Jun 10 20:49:40 c-node04 kram: rpoll(0x7fe624135b90, 85, 50) returning 0 times: 0, 0, 0, 2203, 0 ccount 42
Jun 10 20:49:40 c-node04 kernel: [97562.145450] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090008  12/07/2018
Jun 10 20:49:40 c-node04 kernel: [97562.145450] RIP: 0010:do_futex+0xdf/0xa90
Jun 10 20:49:40 c-node04 kernel: [97562.145450] Code: 08 4c 8d 6d 08 48 8b 3a 48 8d 72 e8 49 39 d5 4c 8d 67 e8 0f 84 89 00 00 00 31 c0 44 89 3c 24 41 89 df 44 89 f3 41 89 c6 eb 16 <49> 8b 7c
24 18 49 8d 54 24 18 4c 89 e6 4c 39 ea 4c 8d 67 e8 74 58
Jun 10 20:49:40 c-node04 kernel: [97562.145450] RSP: 0018:ffff97f6ea8bbdf0 EFLAGS: 00010283
Jun 10 20:49:40 c-node04 kernel: [97562.145450] RAX: 00007f6db1a5d000 RBX: 0000000000000001 RCX: ffffa5530c5f0140
Jun 10 20:49:40 c-node04 kernel: [97562.145450] RDX: ffff97f6e4287d58 RSI: ffff97f6e4287d40 RDI: 0000000000000246
Jun 10 20:49:40 c-node04 kram: rpoll(0x7fe62414a860, 2, 50) returning 0 times: 0, 0, 0, 2191, 0 ccount 277
Jun 10 20:49:40 c-node04 kernel: [97562.145450] RBP: ffffa5530c5bd580 R08: 00007f6db1a5d9c0 R09: 0000000000000001


2) In addition, we got a second crash on the same function a few lines above the previous one 

Jun 12 11:20:43 c-node06 kernel: [91837.319613]  ? pointer+0x137/0x350
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  printk+0x58/0x6f
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  panic+0xce/0x238
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  ? do_futex+0xa3d/0xa90
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  __stack_chk_fail+0x15/0x20
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  do_futex+0xa3d/0xa90
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  ? plist_add+0xc1/0xf0
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  ? plist_add+0xc1/0xf0
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  ? plist_del+0x5f/0xb0
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  __schedule+0x243/0x830
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  ? schedule+0x28/0x80
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  ? exit_to_usermode_loop+0x57/0xe0
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  ? prepare_exit_to_usermode+0x70/0x90
Jun 12 11:20:43 c-node06 kernel: [91837.319613]  ? retint_user+0x8/0x8


(gdb) l *do_futex+0xa3d
0xffffffff8113985d is in do_futex (kernel/futex.c:1742).
1737			if (!(flags & FLAGS_SHARED)) {
1738				cond_resched();
1739				goto retry_private;
1740			}
1741	
1742			put_futex_key(&key2);
1743			put_futex_key(&key1);
1744			cond_resched();
1745			goto retry;
1746		}
(gdb)


Closer to the         double_lock_hb(hb1, hb2) you mention.


Regarding running without proprietary modules, we didn't manage to reproduce, but we are getting half of the  IO load while this problem happens

Thx
David 



-----Original Message-----
From: Matthew Wilcox <willy@infradead.org> 
Sent: Sunday, June 13, 2021 11:04 PM
To: David Mozes <david.mozes@silk.us>
Cc: linux-fsdevel@vger.kernel.org; Thomas Gleixner <tglx@linutronix.de>; Ingo Molnar <mingo@redhat.com>; Peter Zijlstra <peterz@infradead.org>; Darren Hart <dvhart@infradead.org>; linux-kernel@vger.kernel.org
Subject: Re: futex/call -to plist_for_each_entry_safe with head=NULL

On Sun, Jun 13, 2021 at 12:24:52PM +0000, David Mozes wrote:
> Hi *,
> Under a very high load of io traffic, we got the below  BUG trace.
> We can see that:
> plist_for_each_entry_safe(this, next, &hb1->chain, list) {
>                 if (match_futex (&this->key, &key1))
>  
> were called with hb1 = NULL at futex_wake_up function.
> And there is no protection on the code regarding such a scenario.
>  
> The NULL can  be geting from:
> hb1 = hash_futex(&key1);
>  
> How can we protect against such a situation?

Can you reproduce it without loading proprietary modules?

Your analysis doesn't quite make sense:

        hb1 = hash_futex(&key1);
        hb2 = hash_futex(&key2);

retry_private:
        double_lock_hb(hb1, hb2);

If hb1 were NULL, then the oops would come earlier, in double_lock_hb().

> RIP: 0010:do_futex+0xdf/0xa90
>  
> 0xffffffff81138eff is in do_futex (kernel/futex.c:1748).
> 1743                                       put_futex_key(&key1);
> 1744                                       cond_resched();
> 1745                                       goto retry;
> 1746                       }
> 1747      
> 1748                       plist_for_each_entry_safe(this, next, &hb1->chain, list) {
> 1749                                       if (match_futex (&this->key, &key1)) {
> 1750                                                       if (this->pi_state || this->rt_waiter) {
> 1751                                                                       ret = -EINVAL;
> 1752                                                                       goto out_unlock;
> (gdb)
>  
>  
>  
> plist_for_each_entry_safe(this, next, &hb1->chain, list) {
>                 if (match_futex (&this->key, &key1)) {
>  
>  
>  
>  
> This happened in kernel  4.19.149 running on Azure vm
>  
>  
> Thx
> David
> Reply 
> Forward 
> MO
>