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Date:   Mon, 27 Feb 2023 10:32:57 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Zhouyi Zhou <zhouzhouyi@gmail.com>,
        Sanan Hasanov <sanan.hasanov@knights.ucf.edu>,
        paulmck@kernel.org, frederic@kernel.org, quic_neeraju@quicinc.com,
        josh@joshtriplett.org, mathieu.desnoyers@efficios.com,
        jiangshanlai@gmail.com, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller@googlegroups.com,
        contact@pgazz.com
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in
 rcu_core
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On Mon, 27 Feb 2023 08:15:26 -0500
Joel Fernandes <joel@joelfernandes.org> wrote:

> >> asm_sysvec_apic_timer_interrupt+0x1a/0x20
> >> RIP: 0010:default_idle+0xf/0x20
> >> Code: 89 07 49 c7 c0 08 00 00 00 4d 29 c8 4c 01 c7 4c 29 c2 e9 76 ff ff ff cc cc cc cc f3 0f 1e fa eb 07 0f 00 2d e3 8a 34 00 fb f4 <fa> c3 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 f3 0f 1e fa 65
> >> RSP: 0018:ffffc9000017fe00 EFLAGS: 00000202
> >> RAX: 0000000000dfbea1 RBX: dffffc0000000000 RCX: ffffffff89b1da9c
> >> RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000000
> >> RBP: 0000000000000007 R08: 0000000000000001 R09: ffff888119fb6c23
> >> R10: ffffed10233f6d84 R11: dffffc0000000000 R12: 0000000000000003
> >> R13: ffff888100833900 R14: ffffffff8e112850 R15: 0000000000000000
> >> default_idle_call+0x67/0xa0
> >> do_idle+0x361/0x440
> >> cpu_startup_entry+0x18/0x20
> >> start_secondary+0x256/0x300
> >> secondary_startup_64_no_verify+0xce/0xdb
> >> </TASK>
> >> Modules linked in:
> >> CR2: 0000000000000000
> >> ---[ end trace 0000000000000000 ]---
> >> RIP: 0010:0x0
> >> Code: Unable to access opcode bytes at 0xffffffffffffffd6.  
> 
> I have seen this exact signature when the processor tries to execute a function that has a NULL address. That causes IP to goto 0 and the exception. Sounds like something corrupted rcu_head (Just a guess).

[ Joel, you need to line wrap your emails ;-) ]

This looks like a call_rcu() was called on something that later got freed
or reused. That is, the bug is not with RCU but with something using RCU.

OR it could be a bug with RCU if the synchronize_rcu() ended before the
grace periods have finished.

-- Steve