linux-kernel - Re: BUG: unable to handle kernel NULL pointer dereference in rcu

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Date:   Mon, 27 Feb 2023 10:48:48 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Zhouyi Zhou <zhouzhouyi@...il.com>,
        Sanan Hasanov <sanan.hasanov@...ghts.ucf.edu>,
        paulmck@...nel.org, frederic@...nel.org, quic_neeraju@...cinc.com,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller@...glegroups.com,
        contact@...zz.com
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in rcu_core

Hey Steve,

On Mon, Feb 27, 2023 at 10:33 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Mon, 27 Feb 2023 08:15:26 -0500
> Joel Fernandes <joel@...lfernandes.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 ;-) ]

Ok I will try. The thing is, I have not figured out yet how to
plaintext-reply from my iPhone without having it wrap :-(

> 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.

Yes certainly, the rcu_head is allocated on the caller side so it
could have been trampled while the callback was still in flight.

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

Good point..

Thanks,

 - Joel